Postgres Tree Shootout part 1: Introduction.

I’ve written before about using Adjacency Lists, and even done some performance tests on querying them. Whilst reading a post today, it occurred to me that it might be worthwhile to do a comparison of the various methods of storing hierarchical data in Postgres, and the costs of the same operations on each of those.

This post is just an introduction, with an outline of what I plan to do to run these tests. Please feel free to suggest things that I have missed, or that might be an oversight at my end.


Tree Models

There are four methods of storing the relationships that might form a tree. This analysis will be limited to actual tree, rather than graph structures (no cycles). I plan to detail the data structures in a series of posts, one per method. In each case, where there are multiple ways to store the data, I will attempt to examine each of these.

Adjacency Lists, being the simplest to understand (and the ones I have spent more time on recently), will be discussed first.

Path Enumerations will be next, with a comparison of storing the data using the ltree extension, and using an ARRAY column.

Following this, I’ll make an attempt at using the Closure Table model: where each ancestor-descendant relationship is stored, rather than just the parent-child relationship.

Finally, I’ll have a crack at the Nested Set model. I’m not solidly behind this model for the types of data I’ve had to deal with, but it is a valid mechanism for storing and retrieving this data. Besides, it will be an interesting exercise to implement.

My plan to handle all of these is that all tree manipulation should be “automatic”, that is, adding a node (or removing one, or whatever) should not require explicit updating of the various metadata. This should all be handled by trigger functions on the tables themselves. Whether this turns out to be reasonable we shall see.


Operations

I plan to perform the same set of operations (on the same data, rather than randomly generated data) in all models, and compare the complexity and run-time of the various queries. I’m hoping to cover all of the operations that might be performed on a tree structure, so please add any more to the comments.

The data stored in the table will contain more than one tree: this means we can perform operations which add/remove root nodes/whole trees.

Insertions

  • Insert a single leaf node
  • Insert a single root node (all existing root nodes will then point to this)
  • Insert a single node partway through the tree, with the “replaced” node becoming a child of this (and this one keeps it’s children)
  • Insert a single node partway through the tree, with this node’s parent’s existing children all becoming children of the new node.

Removals

  • Remove a single leaf node
  • Remove a single root node (all children of this are promoted to root nodes)
  • Remove a single node partway through the tree: all children of this node then have their grand-parent as their parent.
  • Remove a subtree (a single non-root node and it’s descendants)
  • Remove a whole tree (a single root node and all descendants)
  • Remove all descendants of a specific node (but not the node itself)

Moves

  • Move a subtree from one parent to another
  • Move a single leaf node to a different parent
  • Move a root node into a tree
  • Make a subtree into a tree (turn a node into a root node).
  • Move all children of a node to a different parent

Fetches

  • Fetch all descendants of a given node
  • Fetch the number of descendants of a given node
  • Fetch descendants of a given node to a given depth
  • Fetch the number of descendants of a given node to a given depth
  • Fetch all ancestors of a given node
  • Fetch the number of ancestors of a given node
  • Fetch ancestors of a given node to a given depth
  • Fetch the number of ancestors of a given node to a given depth

    I don’t think this makes any sense.

  • Fetch all leaf nodes
  • Fetch the number of leaf nodes
  • Fetch all root nodes
  • Fetch the number of root nodes
  • Fetch all non-leaf nodes
  • Fetch the number of non-leaf nodes
  • Fetch all non-root nodes
  • Fetch the number of non-root nodes
  • Fetch all non-root, non-leaf nodes
  • Fetch the number of non-root, non-leaf nodes

Aggregating Ranges in Postgres

Postgres’ range types are very cool. You can use them to store, as you may guess, a value that is a range. There are several included range types, and it’s possible to create your own range type. For now, we’ll just look at using a simple int4range: although everything in principle could be applied to any range type.

Firstly, a quick discussion of how the range types work.

There is a literal form, and then a functional form:

SELECT '(2,17]'::int4range;

  int4range
 -----------
  [3,18)
 (1 row)

SELECT int4range(2, 17, '(]');

 int4range
-----------
 [3,18)
(1 row)

The first thing you’ll notice is that postgres converts them to canonical form. We can provide the bound-types: inclusive or exclusive upper and lower values. This is the same notation you might see if you have done some mathematics.

You can also omit the upper and/or lower bounds to get a range object that goes to ±Infinity. Finally, you can also have empty ranges.

There are several operations that can be performed on a range. The most interesting one from the perspective of this article is the UNION operation.

SELECT '[3,17)'::int4range + '[10,20)';

 ?column?
----------
 [3,20)
(1 row)

You’ll get an error if your union does not result in a contiguous range. There is no way to store a discontinuous range in postgres (but you could store them in an array of int4range[], for instance).

What about if we want to get the aggregate range from a set of ranges?

SELECT value FROM range_test ;
  value
---------
 [2,3)
 [3,4)
 [11,12)
 [5,12)
 [4,5)
(4 rows)

What we’d like to be able to do is something like:

SELECT aggregate_union(value) FROM range_test;
 aggregate_union
-----------------
     '[2,12)'

Obviously, this would be subject to the same limitation on union: we would get an error (or a NULL) if the aggregate range was not continuous.

Perhaps even more useful might be a function that would show us what ranges are missing from a set of ranges. With the same data above, we might see:

SELECT missing_ranges(value) FROM range_test ;                                   missing_ranges
------------------
 {"(,2)","[12,)"}
(1 row)

Indeed, the latter is probably more useful, and, as it turns out, is simpler to perform.

We’ll start with the aggregate_union though, because it’s fun. It’s also the way I worked out the nicer solution for the last problem.

We need to create a postgres AGGREGATE to, well, aggregate the data from columns in a table. A naïve solution might be:

CREATE FUNCTION _aggregate_union(int4range, int4range)
RETURNS int4range AS $$

SELECT COALESCE(
  $1 + $2, $1, $2
);

$$ LANGUAGE SQL;

CREATE AGGREGATE aggregate_union (int4range) (
  sfunc = _aggregate_union,
  stype = int4range
);

This actually works to some degree: but only if the ranges in the query are already sorted, as each iteration of the aggregation function must result in a valid range:

SELECT aggregate_union(value) FROM (SELECT value FROM range_test ORDER BY value) x;
 aggregate_union
-----------------
 [2,12)
(1 row)

However, if the data is not sorted, it will fail.

Instead, we have to either collect all of the items in an array, sort this, and then attempt to aggregate, or, at each step, aggregate as much as possible, and add the current element to the array if we cannot perform a union.

The simpler of these (but will take more memory) is to stick them all into an array, and then sort and apply the union. We only need to define one new function to do it this way:

CREATE OR REPLACE FUNCTION _aggregate_union(int4range[])
RETURNS int4range AS $$

DECLARE
  _range int4range;
  _current int4range;

BEGIN

  _current := (SELECT x FROM unnest($1) x ORDER BY x LIMIT 1);

  FOR _range IN SELECT unnest($1) x ORDER BY x LOOP
    IF _range && _current OR _range -|- _current THEN
      _current := _current + _range;
    ELSE
      RETURN NULL;
    END IF;
  END LOOP;

  RETURN _current;
  END;

$$ LANGUAGE plpgsql;

CREATE AGGREGATE aggregate_union (int4range) (
  stype = int4range[],
  sfunc = array_append,
  finalfunc = _aggregate_union
);

Lets test it out:

SELECT aggregate_union(value) FROM range_test;
 aggregate_union
-----------------
 [2,12)
(1 row)

Bingo.

But what about the other case? What if we care more about what data is missing?

After spending way too many hours on playing around with this, I hit on the idea of using a window function, lead to get the data from the “next” row in the query.

SELECT
  lower(value),
  upper(value),
  lead(lower(value)) OVER (ORDER BY lower(value) NULLS FIRST)
FROM
  range_test
ORDER BY value;

This gets us most of the way. We can see the ones with upper >= lead indicate there is no gap between the ranges, so we can filter. However, we need to do this with a subquery to be able to get access to the columns correctly:

SELECT upper, lead FROM (
  SELECT
    lower(value),
    upper(value),
    lead(lower(value)) OVER (ORDER BY lower(value) NULLS FIRST)
  FROM
    range_test
  ORDER BY value
) x WHERE upper < lead OR (lead IS NULL AND upper IS NOT NULL);

We can aggregate these into an array, and then prefix with an element if our first object is not infinite-lower-bounded.

CREATE OR REPLACE FUNCTION missing_ranges(int4range[])
RETURNS int4range[] AS $$

DECLARE
  _range int4range;
  _missing int4range[];

BEGIN
  _missing := (SELECT
    array_agg(int4range(upper, lead, '[)'))
    FROM (
      SELECT lower(x), upper(x), lead(lower(x)) OVER (ORDER BY lower(x) NULLS FIRST)
      FROM unnest($1) x ORDER BY lower NULLS FIRST
    ) x
    WHERE upper < lead OR (lead IS NULL AND upper IS NOT NULL)
  );

  _range := (SELECT x FROM unnest($1) x ORDER BY x LIMIT 1);

  IF NOT lower_inf(_range) THEN
    _missing := array_prepend(int4range(NULL, lower(_range), '[)'), _missing);
  END IF;

  RETURN _missing;
END;

$$ LANGUAGE plpgsql;

CREATE AGGREGATE missing_ranges (int4range) (
  sfunc = array_append,
  stype = int4range[],
  finalfunc = missing_ranges
);

All too easy.

It is possible to rewrite this function just using SQL, but it’s not pretty:

SELECT array_agg(value)
FROM
  (SELECT value
   FROM
     (SELECT int4range(UPPER, lead, '[)') AS value
      FROM
        (SELECT NULL AS LOWER,
                NULL::integer AS UPPER,
                LOWER(a.value) AS lead
         FROM range_test a
         ORDER BY a.value LIMIT 1) w
      WHERE lead IS NOT NULL
      UNION SELECT int4range(UPPER, lead, '[)')
      FROM
        (SELECT LOWER(b.value),
                UPPER(b.value),
                lead(LOWER(b.value)) OVER (ORDER BY LOWER(b.value) NULLS FIRST)
         FROM range_test b
         ORDER BY b.value) x
      WHERE UPPER IS NOT NULL
        AND (LEAD IS NULL OR UPPER < lead)
      ) y
   ORDER BY value) z;

I haven’t tried to see which is faster.

Horizontal Partitioning in Postgres

It never surprises me when I find another neat feature of Postgres that makes doing a potentially difficult task simple. Today, I discovered that since 9.0, Postgres has supported a really powerful way to horizontally partition data into separate tables.

For those who haven’t heard of the concept before, horizontal partitioning is where different rows are stored in different tables, depending upon something about the data within the row.

For instance, we could partition audit data into tables based upon the timestamp of the action. Thus, all entries created during October, 2014 would be stored in a table called audit_2014_10, and entries created during March 2011 would be stored in a table called audit_2011_03. And so on. Alternatively, we could have a single table per year, or however we want to partition.

This is called “Range” partitioning.

There are a couple of ways you could think about doing this in a DBMS. You could have a writable view that redirects the writes to the correct table. The problem then is that when you add a new child table, you need to rewrite your view.

Instead, we can use Postgres’ neat table inheritance to handle all of this for you. Indeed, it is discussed in the Postgres documentation.

If we inherit one table from another, and do a query on the parent table, we will also get the rows that match the query from all child tables. That obviates the need for a view that uses UNION ALL or similar to fetch the data.

I’m going to use a toy example here, that just contains a single column.

CREATE TABLE "data" ("value" TIMESTAMPTZ);

CREATE TABLE "data_2014" (
  CHECK (
    "value" >= '2014-01-01' AND "value" < '2015-01-01')
) INHERITS ("data");

CREATE TABLE "data_2015" (
  CHECK (
    "value" >= '2015-01-01' AND "value" < '2016-01-01')
) INHERITS ("data");

This, however, is only part of the picture. Any data that is added to either of the child tables (or indeed the parent table, but we won’t be doing that), will be returned when we query the parent table.

But we want to ensure that data is partitioned out nicely. For that, we can use a trigger.

A naïve trigger may look something like:

CREATE OR REPLACE FUNCTION data_insert_trigger()
RETURNS TRIGGER AS $$

BEGIN

  IF (NEW.value >= '2014-01-01' AND NEW.value < '2015-01-01') THEN
    INSERT INTO data_2014 VALUES (NEW.*);
  ELSIF (NEW.value >= '2015-01-01' AND NEW.value < '2016-01-01') THEN
    INSERT INTO data_2015 VALUES (NEW.*);
  ELSE
    RAISE EXCEPTION 'Date out of range. Please fix the data_insert_trigger() function.';
  END IF;

  RETURN NULL;
END;

$$ LANGUAGE plpgsql;

As you can see by the ELSE clause, we will actually need to do maintainence on this function as we start to get data that falls outside of our existing ranges. We will also need to create a new table for those rows.

It would be nice if we could automatically create tables that are missing, and handle any arbitrary values.

CREATE OR REPLACE FUNCTION data_insert_trigger()
RETURNS TRIGGER AS $$

DECLARE
  table_name text;
  year integer;
  start text;
  finish text;

BEGIN
  year := date_part('year', NEW.value);
  table_name := 'data_' || year;
  start := year || '-01-01';
  finish := (year + 1) || '-01-01';

  PERFORM 1 FROM pg_tables WHERE tablename = table_name;

  IF NOT FOUND THEN
    EXECUTE
      'CREATE TABLE '
      || quote_ident(table_name)
      || ' (CHECK ("value" >= '
      || quote_literal(start)
      || ' AND "value" < '
      || quote_literal(finish)
      || ')) INHERITS (data)';
  END IF;

  EXECUTE
    'INSERT INTO '
    || quote_ident(table_name)
    || ' VALUES ($1.*)'
  USING NEW;

  RETURN NULL;
END;

$$ LANGUAGE plpgsql;

CREATE TRIGGER data_insert_trigger
BEFORE INSERT ON data
FOR EACH ROW EXECUTE PROCEDURE data_insert_trigger();

You would also want to create any indexes on the child tables, as this is where they need to be, rather than the parent table.

This function is pretty neat: it first stores what the table name should be in a variable, as well as the two bounds for this table (start and finish). Then, we see if that table exists, and if not, create it. Finally, we then insert the values into the correct child table. I’m not sure I’d recommend using it as-is: it’s quite possibly subject to a race condition if two new records came in at the same time.

The one thing that was concerning me was that DDL changes to the parent table would not propagate to the child tables: however this turned out to not be an issue at all. Since I mostly use Django, I want as little hard stuff that would require custom migration operations.

ALTER TABLE data ADD COLUMN confirmed BOOLEAN DEFAULT false;

The other thing worth noting is that Postgres will do a really good job of limiting the tables that are accessed to those that contain the relevant data:

INSERT INTO data VALUES
  ('2014-01-06 09:00:00'),
  ('2015-01-09 12:00:00'),
  ('2016-02-22 15:39:00');

EXPLAIN
SELECT * FROM data
WHERE value > '2014-01-01'
AND value < '2014-07-01';
                                      QUERY PLAN
---------------------------------------------------------------------------------------
 Append  (cost=0.00..42.10 rows=12 width=8)
   ->  Seq Scan on data  (cost=0.00..0.00 rows=1 width=8)
         Filter: ((value > '2014-01-01 00:00:00'::timestamp with time zone) AND
                  (value < '2014-07-01 00:00:00'::timestamp with time zone))
   ->  Seq Scan on data_2014  (cost=0.00..42.10 rows=11 width=8)
         Filter: ((value > '2014-01-01 00:00:00'::timestamp with time zone) AND
                  (value < '2014-07-01 00:00:00'::timestamp with time zone))
 Planning time: 0.292 ms
(6 rows)

We see that only the data_2014 table is hit by the query. If your constraints do something like cast to DATE, then this may not happen. This was causing me some concern earlier, but letting Postgres coerce the data types fixed it.

However, you can’t use a tstzrange to query if you want these constraints to help the query planner:

-- Actually hits every table.
SELECT * FROM data WHERE value <@ '[2014-01-01, 2014-07-01)'::tstzrange;

It’s worth noting that if you change a value that would cause that row to belong in a different partition, this will fail.

There are moves afoot to have this feature more tightly integrated into Postgres, perhaps using a syntax like:

CREATE TABLE data (value TIMESTAMPTZ)
PARTITION BY RANGE (value)
(PARTITION data_2014 VALUES LESS THAN '2015-01-01');

CREATE PARTITION data_2015 ON data VALUES LESS THAN '2016-01-01';

It’s not clear to me how you actually define the range. It also seems counter-productive to have to manually create the partition tables.

There are also tools that might be useful to handle the heavy lifting, like pg_partman. I haven’t used this, but it looks interesting.

Adding JSON(B) operators to PostgreSQL

This is a followup/replacement post for this older post. In it, I discussed a way to use PL/python to perform function (and in turn operations) on JSON data. Don’t do that, it’s way too slow.

It’s actually possible, using raw SQL (and some handy functions) to perform some of the operations that are “missing” from the JSON(B) datatypes in Postgres.

In all cases, I’ll work with just JSONB as the input/output formats. In practice, when I first wrote these, I wrote up to four versions of each (JSON+JSON, JSON+JSONB, JSONB+JSON, JSONB+JSONB). I believe it’s possible to write polymorphic versions of these functions, but I’m not that familiar with them just yet.

We’ll start with concatenation: basically joining two JSONB objects into one. This is a good one to start with, as operands must be either JSON or JSONB, so all forms of this function are the same, just with different functions or casting of operands.

Before I begin, I’ll mention a post I saw on Michael Paquier’s excellent blog: Manipulating jsonb data by abusing of key uniqueness. In it, Michael uses the json_object_agg function to build up a JSON object from a query:

CREATE FUNCTION "json_append" (jsonb, jsonb) RETURNS jsonb AS $$

WITH json_union AS
(
  SELECT * FROM jsonb_each($1)
  UNION ALL
  SELECT * FROM jsonb_each($2)
)
SELECT json_object_agg(key, value) FROM json_union;

$$ LANGUAGE SQL;

This seems like a good idea, however it actually performs around two orders of magnitude slower than just iterating over the objects and building them up using string_agg and ||:

CREATE FUNCTION "json_concatenate" (jsonb, jsonb) RETURNS jsonb AS $$

SELECT ('{' || string_add(to_json("key")::text || ':' ||"value", ',') || '}')::jsonb
FROM (
  SELECT * FROM jsonb_each($1) UNION ALL SELECT * jsonb_each($2)
);

$$ LANGUAGE SQL;

CREATE OPERATOR || (
  LEFTARG = jsonb,
  RIGHTARG = jsonb,
  PROCEDURE = jsonb_concatenate
);

It seems that this is not just because of the Common Table Expression (although, using a CTE does make the second function perform just as poorly).

=# SELECT * FROM BENCHMARK(10000,
  'jsonb_concatenate(''{"a": 1, "b":2}''::jsonb, ''{"a":2}''::jsonb)',
  'jsonb_append(''{"a": 1, "b":2}''::jsonb, ''{"a":2}''::jsonb)'
  );
           code              |  runtime   | corrected
-----------------------------+------------+------------
 [Control]                   | 0.00550699 |          0
 jsonb_concatenate(...)      | 0.00652981 | 0.00102282
 jsonb_append(...)           |   0.484099 |   0.478592

My attempt at reimplementing the json_object_agg aggregate function in SQL proved even slower. Not at all surprising.

The next one we will tackle is the - operator from Hstore.

hstore - text     : delete key from left operand
hstore - text[]   : delete keys from left operand
hstore - hstore   : delete matching pairs from left operand

We can reimplement these for JSON: first as functions, and then create operators using those if we want. What is interesting is that we use the same construction pattern for our output JSONB object, however, I don’t seem to be able to figure out how to extract this out into another function.

CREATE OR REPLACE FUNCTION "jsonb_subtract"(
  "json" jsonb,
  "remove" TEXT
)
  RETURNS jsonb
  LANGUAGE sql
  IMMUTABLE
  STRICT
AS $function$
SELECT CASE WHEN "json" ? "remove" THEN COALESCE(
  (SELECT ('{' || string_agg(to_json("key")::text || ':' || "value", ',') || '}')
     FROM jsonb_each("json") -- Until this function is added!
    WHERE "key" <> "remove"),
  '{}'
)::jsonb
ELSE "json"
END
$function$;

CREATE OPERATOR - (
  LEFTARG = jsonb,
  RIGHTARG = text,
  PROCEDURE = jsonb_subtract
);

You’ll notice that there’s a test for if the key to remove is in the object first: this should be much faster in the situation where it doesn’t appear, as we then don’t need to recreate the object.

The other forms are quite similar, but the WHERE clause varies, and the initial test varies or is removed:

CREATE OR REPLACE FUNCTION "jsonb_subtract"(
  "json" jsonb,
  "keys" TEXT[]
)
  RETURNS jsonb
  LANGUAGE sql
  IMMUTABLE
  STRICT
AS $function$
SELECT CASE WHEN "json" ?| "keys" THEN COALESCE(
  (SELECT ('{' || string_agg(to_json("key")::text || ':' || "value", ',') || '}')
     FROM jsonb_each("json")
    WHERE "key" <> ALL ("keys")),
  '{}'
)::jsonb
ELSE "json"
END
$function$;

CREATE OPERATOR - (
  LEFTARG = jsonb,
  RIGHTARG = text[],
  PROCEDURE = jsonb_subtract
);

CREATE OR REPLACE FUNCTION "jsonb_subtract"(
  "json" jsonb,
  "remove" jsonb
)
  RETURNS jsonb
  LANGUAGE sql
  IMMUTABLE
  STRICT
AS $function$
SELECT COALESCE(
  (
    SELECT ('{' || string_agg(to_json("key")::text || ':' || "value", ',') || '}')
    FROM jsonb_each("json")
    WHERE NOT
      ('{' || to_json("key")::text || ':' || "value" || '}')::jsonb <@ "remove"
      -- Note: updated using code from http://8kb.co.uk/blog/2015/01/16/wanting-for-a-hstore-style-delete-operator-in-jsonb/
  ),
  '{}'
)::jsonb
$function$;

CREATE OPERATOR - (
  LEFTARG = jsonb,
  RIGHTARG = jsonb,
  PROCEDURE = jsonb_subtract
);

There is also the #= operator for hstore: but this seems to be record #= hstore, rather than hstore #= hstore. I’m not sure how to implement this, but I can implement a jsonb #= jsonb function:

CREATE OR REPLACE FUNCTION "jsonb_update_only_if_present"(
  "json" jsonb,
  "other" jsonb
)
  RETURNS jsonb
  LANGUAGE sql
  IMMUTABLE
  STRICT
AS $function$

SELECT COALESCE(
  (SELECT ('{' || string_agg(to_json("key")::text || ':' || "value", ',') || '}')
     FROM (SELECT * FROM jsonb_each("json") UNION ALL SELECT * FROM jsonb_each("other")) AS a
     WHERE "json" ? "key"::text
  ),
  '{}'
)::jsonb
$function$;

CREATE OPERATOR #= (
  LEFTARG = jsonb,
  RIGHTARG = jsonb,
  PROCEDURE = jsonb_update_only_if_present
);

Finally, there are operators %% (convert to array of alternating key/value pairs), and %# (convert to 2-dimensional array of keys, values).

I haven’t figured out a way to create these yet either.


So, that’s what I’ve got so far. Obviously, these will be slower than pure C implementations however, we can run some benchmarks against the hstore operators for comparisons.

=# SELECT * FROM BENCHMARK(100000,
   ' ''{"a": 1, "b":2}''::jsonb || ''{"a":2}''::jsonb',
   ' ''a=>1, b=>2''::hstore || ''a=>2''::hstore'
   );
 
                     code                      |  runtime  |  corrected
-----------------------------------------------+-----------+-------------
 [Control]                                     | 0.0837061 |           0
  '{"a": 1, "b":2}'::jsonb || '{"a":2}'::jsonb | 0.0842681 | 0.000561953
  'a=>1, b=>2'::hstore || 'a=>2'::hstore       | 0.0843148 | 0.000608683
(3 rows)

Whoa. That’s actually pretty good!

And similar for subtract:

=# SELECT * FROM BENCHMARK(100000,
  ' ''{"a": 1, "b":2}''::jsonb - ''a''::text ',
  ' ''a=>1, b=>2''::hstore - ''a''::text '
);
                  code                  |  runtime  | corrected
----------------------------------------+-----------+------------
 [Control]                              | 0.0818689 |          0
  '{"a": 1, "b":2}'::jsonb - 'a'::text  |  0.083431 | 0.00156212
  'a=>1, b=>2'::hstore - 'a'::text      |  0.083159 | 0.00129008
(3 rows)

I might have to run these with some more complicated queries, and compare the results.

Using Postgres Composite Types in Django

Note: this post turned out to be far more complicated than I had hoped. I may write another one that deals with a less complicated type!

Postgres comes with a pretty large range of column types, and the ability to use these types in an ARRAY. There’s also JSON(B) and Hstore, which are useful for storing structured (but possibly varying) data. Additionally, there are also a range of, well, range types.

However, sometimes you actually want to store data in a strict column, but that isn’t a simple scalar type, or one of the standard range types. Postgres allows you to define your own composite types.

There is a command CREATE TYPE that can be used to create an arbitrary type. There are four forms: for now we will just look at Composite Types.

We will create a Composite type that represents the opening hours for a store, or more specifically, the default opening hours. For instance, a store may have the following default opening hours:

+------------+--------+---------+
|    Day     |  Open  |  Close  |
+------------+--------+---------+
|  Monday    |  9 am  |  5 pm   |
|  Tuesday   |  9 am  |  5 pm   |
|  Wednesday |  9 am  |  5 pm   |
|  Thursday  |  9 am  |  9 pm   |
|  Friday    |  9 am  |  5 pm   |
|  Saturday  | 10 am  |  5 pm   |
|  Sunday    | 11 am  |  5 pm   |
+------------+--------+---------+

During the Christmas season this store may be open longer (perhaps even 24 hours). There may also be differences at Easter time, or other public holidays, where the store is closed, or closes early.

It would be nice to be able to store the default opening hours for a store, and then, when creating a week, use these to create concrete (TIMESTAMP) values for each day, which could be overridden on any given day.

There are a few ways we could model this. Postgres has no timerange type, so that’s out. We could create a RANGE type, or we could use (start-time, finish-time). But what about when a store is open after midnight, or for 24 hours? Storing this data implicitly is a real pain, because you need to always check to see if the finish time is less than (or equal to) the start time whenever doing anything. Trust me, this is not the best approach.

An alternative I’ve been toying with is (start-time, interval). You could limit it so that the interval’s maximum is '1 day', but not (from what I can tell) when you define the type. Anyway, the syntax for creating this type is:

CREATE TYPE opening_hours AS (
  start time,
  length interval
);

As an aside, every table in the database also has an associated type (of the same name as the table).

Now, we have our type: we can use it in a table:

CREATE TABLE store (
  store_id SERIAL PRIMARY KEY,
  name TEXT
);

CREATE TABLE default_opening_hours (
  store_id INTEGER REFERENCES store (store_id),
  monday opening_hours,
  tuesday opening_hours,
  wednesday opening_hours,
  thursday opening_hours,
  friday opening_hours,
  saturday opening_hours,
  sunday opening_hours
);

An alternative way of storing this information might be to use an array of opening_hours, directly on the store model. We’ll use this one instead, as it’s a little neater (and means we will look at how to use opening_hours[] later too).

CREATE TABLE store (
  store_id SERIAL PRIMARY KEY,
  name TEXT,
  default_opening_hours opening_hours[7]
);

Now, we can put data in there:

INSERT INTO store (name, default_opening_hours) VALUES
(
  'John Martins',
  ARRAY[
    ('09:00', '08:00')::opening_hours,
    ('09:00', '08:00')::opening_hours,
    ('09:00', '08:00')::opening_hours,
    ('09:00', '12:00')::opening_hours,
    ('09:00', '08:00')::opening_hours,
    ('10:00', '07:00')::opening_hours,
    ('11:00', '06:00')::opening_hours
  ]
);

Note how we need to cast all of the values from record to opening_hours.


In practice, we would probably also want to have some type of restriction where the opening time from one day, plus the default open hours is less than or equal to the starting time on the next day. I’m still not sure of the best way to do this in Postgres, but it is possible to do it in Django.


Speaking of Django, we want to be able to access this data type there. We can leverage a really nice feature of Psycopg2 to have these values automatically turned into a Python namedtuple. We do this by registering the type within Psycopg2, using the Django cursor.

from django.db import connection
from psycopg2.extras import register_composite

register_composite('opening_hours', connection.cursor().cursor)

But, this is only half of the pattern. We also need to register an adapter so that values going back the other way are also automatically cast into opening_hours.

from django.db import connection
from psycopg2.extras import register_composite
from psycopg2.extensions import register_adapter, adapt, AsIs

# Get a reference to the namedtuple class
OpeningHours = register_composite(
  'opening_hours',
  connection.cursor().cursor,
  globally=True
).type

def adapt_opening_hours(value):
  return AsIs("(%s, %s)::opening_hours" % (
    adapt(value.start).getquoted(),
    adapt(value.length).getquoted()
  ))

register_adapter(OpeningHours, adapt_opening_hours)

Now, we can fetch data from the database, and know that we will get OpeningHours instances, and, when passing an OpeningHours instance back to the database, know it will be converted into the correct type.

Obviously, in order to do this, the type must exist in the database. We did that manually in this case. In a real situation you would want to do that as a database migration. And that is where things get tricky. You can’t run the register_adapter function until the type exists in the database. I did come up with a relatively neat workaround for this when writing a framework for generic Composite fields, where the registration of the composite type attempts to execute, and if it fails, it stores the data for later registration, and then the actual migration operation fires off a signal, which is handled by a listener that actually performs the registration.

The final piece of the puzzle is the Django Field subclass, which is actually not that complicated. In essence, we are relying on Psycopg to handle the adaptation in both directions, so it can be a bare field (perhaps with a formfield method to get a custom form field). In practice, I wrote the generic CompositeField subclass, which uses some metaclass magic to handle the late registration:

from django.db.models import fields
from django.db import connection
from django.dispatch import receiver, Signal

from psycopg2.extras import register_composite
from psycopg2.extensions import register_adapter, adapt, AsIs
from psycopg2 import ProgrammingError


_missing_types = {}

class CompositeMeta(type):
    def __init__(cls, name, bases, clsdict):
        super(CompositeMeta, cls).__init__(name, bases, clsdict)
        cls.register_composite()

    def register_composite(cls):
        db_type = cls().db_type(connection)
        if db_type:
            try:
                cls.python_type = register_composite(
                    db_type,
                    connection.cursor().cursor,
                    globally=True
                ).type
            except ProgrammingError:
                _missing_types[db_type] = cls
            else:
                def adapt_composite(composite):
                    return AsIs("(%s)::%s" % (
                        ", ".join([
                            adapt(getattr(composite, field)).getquoted() for field in composite._fields
                        ]), db_type
                    ))

                register_adapter(cls.python_type, adapt_composite)


class CompositeField(fields.Field):
    __metaclass__ = CompositeMeta
    """
    A handy base class for defining your own composite fields.

    It registers the composite type.
    """


composite_type_created = Signal(providing_args=['name'])

@receiver(composite_type_created)
def register_composite_late(sender, db_type, **kwargs):
    _missing_types.pop(db_type).register_composite()

We also want to have a custom migration operation:

from django.db.migrations.operations.base import Operation

# Or wherever the code above is located.
from .fields.composite import composite_type_created


class CreateCompositeType(Operation):
    def __init__(self, name=None, fields=None):
        self.name = name
        self.fields = fields

    @property
    def reversible(self):
        return True

    def state_forwards(self, app_label, state):
        pass

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        schema_editor.execute('CREATE TYPE %s AS (%s)' % (
            self.name, ", ".join(["%s %s" % field for field in self.fields])
        ))
        composite_type_created.send(sender=self.__class__, db_type=self.name)

    def state_backwards(self, app_label, state):
        pass

    def database_backwards(self, app_label, schema_editor, from_state, to_state):
        schema_editor.execute('DROP TYPE %s' % self.name)

This is a bit manual, however. You need to create your own migration that creates the composite type, and then begin to use the field.

# migrations/XXXX_create_opening_hours.py

class Migration(migrations.Migration):
    dependencies = []

    operations = [
        CreateCompositeType(
            name='opening_hours',
            fields=[
                ('start', 'time'),
                ('length', 'interval')
            ],
        ),
    ]

The place this pattern falls down is that this migration must be manually created: we don’t have any way to automatically create the migration from the Field subclass, which just looks like:

class OpeningHoursField(CompositeField):

    def db_type(self, connection):
        return 'opening_hours'

    def formfield(self, **kwargs):
        defaults = {
            'form_class': OpeningHoursFormField
        }
        defaults.update(**kwargs)
        return super(OpeningHoursField, self).formfield(**defaults)

I think in the future I’ll attempt to use further metaclass magic to allow defining the fields of the Composite type. This could then be used to automatically create a form field (which is a subclass of forms.MultiValueField).

class OpeningHoursField(CompositeField):
    start = models.DateField()
    length = IntervalField()

    def db_type(self, connection):
        return 'opening_hours'

However, in the meantime, we can still get by. I’m not sure it’s going to be possible to inject extra operations into the migration based upon the field types anyway.

Finally, we can use this in a model:

class Store(models.Model):
    store_id = models.AutoField(primary_key=True)
    name = models.CharField(max_length=128)
    default_opening_hours = ArrayField(
        base_field=OpeningHoursField(null=True, blank=True),
        size=7
    )

I’ve used the ArrayField from django.contrib.postgres, purely for illustration purposes.

The CompositeField and associated operation are part of my django-postgres project: once I have worked out some more kinks, I may submit a pull request to django.contrib.postgres, unless someone else beats me to it.

Oh, and a juicy little extra. Above I mentioned something about preventing overlaps. The logic I use in my form is:

from django import forms
from django.utils.translation import string_concat, ugettext_lazy as _

import postgres.forms

from .fields import OpeningHoursFormField
from .models import Store


def finish(obj):
    "Given an OpeningHours value, get the finish time"
    date = datetime.date(1, 1, 1)
    return (datetime.datetime.combine(date, obj.start) + obj.duration).time()


class StoreForm(forms.ModelForm):
    OVERLAPS_PREVIOUS = _('Open hours overlap previous day.')

    default_opening_hours = postgres.forms.SplitArrayField(
        base_field=OpeningHoursFormField(required=False),
        size=7,
    )

    class Meta:
        model = Store

    def clean_default_opening_hours(self):
        opening_hours = self.cleaned_data['default_opening_hours']
        field = self.fields['default_opening_hours']

        # Ensure consecutive days do not overlap.
        errors = []

        for i in range(7):
            today = opening_hours[i]
            if today.start is None or today.duration is None:
                continue

            yesterday = opening_hours[(i + 6) % 7]

            if yesterday.start is None or yesterday.duration is None:
                continue

            if finish(yesterday) <= yesterday.start:
                if today.start < finish(yesterday):
                    errors.append(forms.ValidationError(
                        string_concat(
                          field.error_messages['item_invalid'],
                          self.OVERLAPS_PREVIOUS
                        ),
                        code='item_invalid',
                        params={'nth': i}
                    ))

        if errors:
            raise forms.ValidationError(errors)

        return opening_hours

I’m currently not displaying the duration/length: I dynamically calculate it based on the entered start/finish pair, but that’s getting quite complicated.

Performance testing Adjancency List recursive queries

Yesterday, I wrote up some ideas about doing recursive queries on Adjacency Lists using Postgres. Today, I wrote up some code that allows me to run some tests on larger data sets. It’s worth noting that this is still somewhat “toy” data, but I did see comparable results with a real query.

Firstly, our data structure:

CREATE TABLE node (
  node_id SERIAL PRIMARY KEY,
  parent_id INTEGER,
  FOREIGN KEY (parent_id) REFERENCES node(node_id)
);

Now, we want to be able to populate it with test data. This function will allow you to populate any number of records, with a 10% chance that any given record will be a root (have no parent). If it has a parent, it will be randomly selected from all existing rows. This means that earlier rows have a much higher chance of being a parent, and the first row is overwhemingly likely to have the most descendants (as it has a 90% chance that row 2 will have it as a parent, and therefore any descendants of that will also be descendants of row 1…)

CREATE OR REPLACE FUNCTION populate_nodes(count integer) RETURNS void AS $$
BEGIN
  FOR i IN 2..count LOOP
    IF ((SELECT count(*) FROM node) = 0) or (random() < 0.1) THEN
      INSERT INTO node (parent_id) SELECT NULL;
    ELSE
      INSERT INTO node (parent_id) SELECT node_id FROM node OFFSET random() * (SELECT count(*) FROM node) LIMIT 1;
    END IF;
  END LOOP;
END;
$$ LANGUAGE plpgsql;

-- Let's stick 10k records in there
SELECT populate_nodes(10000);

For now, we want to find all descendants of node 1.

I can think of eleven ways we could write this query:

  1. INNER JOIN with a RECURSIVE VIEW
  2. Implicit CROSS JOIN with RECURSIVE VIEW, filtered using a WHERE clause
  3. Sub-query with a RECURSIVE VIEW
  4. INNER JOIN with a MATERIALIZED VIEW based on the RECURSIVE VIEW
  5. Implicit CROSS JOIN (filtered) with MATERIALIZED VIEW based on RECURSIVE VIEW
  6. Sub-query with MATERIALIZED VIEW based on RECURSIVE VIEW
  7. RECURSIVE CTE, using an INNER JOIN
  8. RECURSIVE CTE, using an implicit CROSS JOIN (filtered)
  9. INNER JOIN with RECURSIVE CTE
  10. Implicit CROSS JOIN (filtered) with RECURSIVE CTE
  11. Subquery that is a RECURSIVE CTE

(Whilst some of these seem similar, we’ll see below how they differ).

In all cases, the actual query used for the tree will be:

SELECT node_id, ARRAY[]::integer[], FALSE FROM node WHERE parent_id IS NULL
UNION ALL
SELECT n.node_id, t.ancestors || n.parent_id, n.parent_id = ANY(t.ancestors)
FROM node n, node_tree t WHERE n.parent_id = t.node_id AND NOT cycle

It is extremely likely that the MATERIALIZED VIEW versions will be fastest: it is worth noting that in a very write-heavy environment (where you still need 100% up-to-date data), there would be an extra cost with the REFRESH MATERIALIZED VIEW.

As for which other ones will be fast (or fast enough), I would expect the CTE and VIEW versions to be roughly equivalent, as they appear to do the same amount of work. I’m not sure if the last three will perform as well as the others, as it seems that a “root” CTE would perform better than one later down the track.

So, let’s get underway. I wasn’t able to easily use the benchmark function I wanted to use, so I repeated each query five times and took the average.

We need our views:

CREATE RECURSIVE VIEW node_tree (node_id, ancestors, cycle) AS (
  SELECT node_id, ARRAY[]::integer[], FALSE FROM node WHERE parent_id IS NULL
  UNION ALL
  SELECT n.node_id, t.ancestors || n.parent_id, n.parent_id = ANY(t.ancestors)
  FROM node n, node_tree t WHERE n.parent_id = t.node_id AND NOT cycle
);

CREATE MATERIALIZED VIEW node_tree_mat AS (SELECT * FROM node_tree);

Results

So, some results. I’ll show the query, and then the timing (from EXPLAIN ANALYZE).

#1

SELECT * FROM node INNER JOIN node_tree USING (node_id) WHERE 1 = ANY(ancestors);

Average Time: 54.9ms (stddev 1.99)

#2

SELECT * FROM node n, node_tree t WHERE n.node_id = t.node_id AND 1 = ANY(ancestors);

Average Time: 57.2ms (stddev 2.98)

#3

SELECT * FROM node WHERE node_id IN
  (SELECT node_id FROM node_tree WHERE 1 = ANY(ancestors));

Average Time: 58.5ms (stddev 3.67)

#4

SELECT * FROM node INNER JOIN node_tree_mat USING (node_id) WHERE 1 = ANY(ancestors);

Average Time: 12.2ms (stddev 0.80)

#5

SELECT * FROM node n, node_tree_mat t WHERE n.node_id = t.node_id AND 1 = ANY(ancestors);

Average Time: 11.7ms (stddev 0.90)

This is the fastest query, but not significantly more so than #4.

#6

SELECT * FROM node WHERE node_id IN
  (SELECT node_id FROM node_tree_mat WHERE 1 = ANY(ancestors));

Average Time: 24.0ms (stddev 0.41)

Interestingly, this is much slower than using a JOIN.

#7

WITH RECURSIVE node_tree_cte(node_id, ancestors, cycle) AS (
    SELECT node_id, ARRAY[]::integer[], FALSE FROM node WHERE parent_id IS NULL
  UNION ALL
    SELECT n.node_id, t.ancestors || n.parent_id, n.parent_id = ANY(t.ancestors)
    FROM node n, node_tree t WHERE n.parent_id = t.node_id AND NOT cycle
) SELECT n.* FROM node n INNER JOIN node_tree_cte t USING (node_id)
WHERE 1 = ANY(t.ancestors);

Average Time: 97.0ms (stddev 1.87)

Immediately, we see that using a CTE has a performance hit over using a VIEW. Unexpected.

#8

WITH RECURSIVE node_tree_cte(node_id, ancestors, cycle) AS (
    SELECT node_id, ARRAY[]::integer[], FALSE FROM node WHERE parent_id IS NULL
  UNION ALL
    SELECT n.node_id, t.ancestors || n.parent_id, n.parent_id = ANY(t.ancestors)
    FROM node n, node_tree t WHERE n.parent_id = t.node_id AND NOT cycle
) SELECT n.* FROM node n, node_tree_cte t
WHERE n.node_id = t.node_id AND 1 = ANY(t.ancestors);

Average Time: 96.1ms (stddev 1.16)

#9

SELECT * FROM node INNER JOIN (
  WITH RECURSIVE node_tree_cte(node_id, ancestors, cycle) AS (
    SELECT node_id, ARRAY[]::integer[], FALSE FROM node WHERE parent_id IS NULL
    UNION ALL
    SELECT n.node_id, t.ancestors || n.parent_id, n.parent_id = ANY(t.ancestors)
    FROM node n, node_tree t WHERE n.parent_id = t.node_id AND NOT cycle
  ) SELECT node_id FROM node_tree_cte WHERE 1 = ANY(ancestors)
) node_tree_cte USING (node_id);

Average Time: 114.3ms (stddev 4.64)

This is the slowest (but again, only just, and not significantly more than #10).

#10

SELECT * FROM node, (
  WITH RECURSIVE node_tree_cte(node_id, ancestors, cycle) AS (
    SELECT node_id, ARRAY[]::integer[], FALSE FROM node WHERE parent_id IS NULL
    UNION ALL
    SELECT n.node_id, t.ancestors || n.parent_id, n.parent_id = ANY(t.ancestors)
    FROM node n, node_tree t WHERE n.parent_id = t.node_id AND NOT cycle
  ) SELECT node_id FROM node_tree_cte WHERE 1 = ANY(ancestors)
) node_tree_cte WHERE node.node_id = node_tree_cte.node_id;

Average Time: 110.0ms (stddev 1.05)

#11

SELECT * FROM node WHERE node_id IN (
  WITH RECURSIVE node_tree_cte(node_id, ancestors, cycle) AS (
    SELECT node_id, ARRAY[]::integer[], FALSE FROM node WHERE parent_id IS NULL
    UNION ALL
    SELECT n.node_id, t.ancestors || n.parent_id, n.parent_id = ANY(t.ancestors)
    FROM node n, node_tree t WHERE n.parent_id = t.node_id AND NOT cycle
  ) SELECT node_id FROM node_tree_cte WHERE 1 = ANY(ancestors)
);

Average Time: 96.1ms (stddev 5.50)

Discussion

So, it appears that Common Table Expressions are nearly twice as slow as using a RECURSIVE VIEW. I didn’t expect that at all, as I thought they were equivalent. Unsurprisingly, MATERIALIZED VIEW is much faster.

This has some implications for the stuff I was working on: I was using a query of the #11 form (a sub-query that is a WITH RECURSIVE statement), which, as it turns out is just as fast as doing a “root” CTE. However, it’s still far slower than doing a JOIN with a VIEW.

The problem I have now is that there is no way to have the addition of a Field to a Model in django to cause an extra migration operation to be added. One solution would be to manually add a RunSQL operation, but that is messy. I’ll also have to investigate costs of REFRESH MATERIALIZED VIEW.

Long Live Adjacency Lists

I recently wrote about the excellent book SQL Antipatterns, and in it briefly discussed the tree structures. I’ve been thinking about trees in Postgres a fair bit lately, and a discussion on #django gave me further incentive to revisit this topic.

The book discusses four methods of storing a tree in a database.

Adjacency Lists, apart from the inability to grab a full or partial tree easily, are the simplest to understand. The child object stores a reference to it’s parent. Because this is a foreign key, then it always maintains referential integrity. Fetching a parent is simple, as is fetching all children, or siblings. It’s only when you need to fetch an arbitrary depth that things become problematic, unless you use a recursive query. More on that later.

Postgres has an extension called ltree, which provides an implementation of a Path Enumeration, but one thing that really bothers me about this type of structure is the lack of referential integrity. In practice, I’m not sure what having this ltree structure would give you over simply storing the keys in an ARRAY type. Indeed, if Postgres ever gets Foreign Key constraints for ARRAY elements (which there is a patch floating around for), this becomes even more compelling. It also seems to me that restructuring a tree becomes a bit more challenging in a Path Enumeration than an Adjacency List.

Nested Sets are also interesting, and maintain FK integrity, but require potentially rewriting lots of data when any change is made to the tree. They aren’t that appealing to me: perhaps I fail to see any big advantages of this structure.

Finally, Closure Tables are perhaps the most interesting. This stores all ancestor-descendant relationships, rather than just parent-child, which again requires more work when adding or removing. Again, Referential Integrity is preserved, but it seems like there is lots of work to maintain them.

From all of these, there are some significant advantages, in my mind, to using a simple Adjacency List.

  1. Adding a new row never requires you to alter any other rows in the database.
  2. Moving a subtree to a different location only requires a change to one now in the database.
  3. It’s never possible to end up with Referential Integrity errors: the database will prevent you from deleting a parent row whilst it still has children (or, you may set it to CASCADE or SET NULL the children automatically).
  4. It’s conceptually very simple. Everyone understands the parent-child relationship (and all of the relationships that follow, like grand-parents). It’s a similar mental model to how we think about our own families, except we don’t have exactly one parent.

There is really only two things that are hard to do:

  1. Given a node, select all descendants of that node.
  2. Given a node, select all ancestors of that node.

But, as we shall see shortly, it is possible to do these in Postgres using some nice recursive features.

There is another advantage to using an Adjacency List, this time from the perspective of Django. We can do it without needing to install a new package, or subclass or mix-in a new Model:

class Node(models.Model):
    node_id = models.AutoField(primary_key=True)
    parent = models.ForeignKey('self', null=True, blank=True, related_name='children')

That’s it.

Now, using Postgres, it’s possible to build a recursive VIEW that contains the whole tree:

CREATE RECURSIVE VIEW tree (node_id, ancestors) AS (
    SELECT node_id, '{}'::integer[]
    FROM nodes WHERE parent_id IS NULL
  UNION ALL
    SELECT n.node_id, t.ancestors || n.parent_id
    FROM nodes n, tree t
    WHERE n.parent_id = t.node_id
);

We can then query this (replacing %s with the parent node id):

SELECT node_id
FROM nodes INNER JOIN tree USING (node_id)
WHERE %s = ANY(ancestors);

Or, if you want to select for multiple parents:

SELECT node_id
FROM nodes INNER JOIN tree USING (node_id)
WHERE [%s, %s] && ancestors;

This actually performs relatively well, and, if it doesn’t do well enough, we could create a MATERIALIZED VIEW based on the recursive view, and query that instead (refreshing it whenever we need to, perhaps using a trigger).

CREATE MATERIALIZED VIEW tree_m AS (SELECT * FROM tree);

CREATE FUNCTION refresh_tree_m() RETURNS trigger AS $$
  BEGIN
  REFRESH MATERIALIZED VIEW tree_m;
  END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trig_refresh_tree_m AFTER TRUNCATE OR INSERT OR UPDATE OR DELETE
ON nodes FOR EACH STATEMENT
EXECUTE PROCEDURE refresh_tree_m();

This view is still not perfect though. We can improve it to allow us to limit depth of ancestry:

CREATE RECURSIVE VIEW tree (node_id, ancestors, depth) AS (
    SELECT node_id, '{}'::integer[], 0
    FROM nodes WHERE parent_id IS NULL
  UNION ALL
    SELECT n.node_id, t.ancestors || n.parent_id, t.depth + 1
    FROM nodes n, tree t
    WHERE n.parent_id = t.node_id
);

SELECT node_id FROM nodes INNER JOIN tree USING (node_id)
WHERE %s = ANY(ancestors) AND depth < %s;

This is pretty good now, but if we have cycles in our tree (yes, this makes it technically no longer a tree, but a graph, of which a tree is a restricted kind), this query will run forever. There’s a pretty neat trick to prevent cycles:

CREATE RECURSIVE VIEW tree (node_id, ancestors, depth, cycle) AS (
    SELECT node_id, '{}'::integer[], 0, FALSE
    FROM nodes WHERE parent_id IS NULL
  UNION ALL
    SELECT
      n.node_id, t.ancestors || n.parent_id, t.depth + 1,
      n.parent_id = ANY(t.ancestors)
    FROM nodes n, tree t
    WHERE n.parent_id = t.node_id
    AND NOT t.cycle
);

You don’t need to use the cycle column outside of the view.

The query used for the view can be repurposed into a Common Table Expression, which is basically a way of defining a view that only exists for the query we are executing (but will itself only be executed once, even if it’s referred to lots of times):

WITH RECURSIVE tree (node_id, ancestors, depth, cycle) AS (
    SELECT node_id, '{}'::integer[], 0, FALSE
    FROM nodes WHERE parent_id IS NULL
  UNION ALL
    SELECT
      n.node_id, t.ancestors || n.parent_id, t.depth + 1,
      n.parent_id = ANY(t.ancestors)
    FROM nodes n, tree t
    WHERE n.parent_id = t.node_id
    AND NOT t.cycle
) SELECT n.* FROM nodes n INNER JOIN tree USING (node_id)
WHERE %s = ANY(ancestors);

You can see that this syntax basically defines the view before running the real query.


Looking at it from the perspective of Django, we would like to be able to spell a query something like:

Node.objects.filter(parent__recursive=node)
Node.objects.filter(parent__recursive__in=nodes)
Node.objects.filter(children__recursive__contains=node)

The problem we have with using the CTE immediately above is that we don’t have access to the full query at the time we are dealing with the filter. We could define the view prior to running the query (perhaps in a migration), but this means it’s more than just a simple field: although with the new migrations framework, we could make it so that makemigrations automatically adds a migration operation to create the recursive view.

The other solution is to still use a recursive CTE, but use it as a subquery. I’m still investigating if this will have poor performance characteristics.

Here is an implementation of doing just that:

from django.db import models

SQL = """
WITH RECURSIVE "tree" ("{pk}", "related", "cycle") AS (
    SELECT "{pk}", ARRAY[]::integer[], FALSE
    FROM "{table}" WHERE "{fk}" IS NULL
  UNION ALL
    SELECT a."{pk}", b."related" || a."{fk}", a."{fk}" = ANY(b."related")
    FROM "tree" b, "{table}" a
    WHERE a."{fk}" = b."{pk}" AND NOT b."cycle"
) {query}
"""


class RecursiveRelation(models.ForeignKey):
    def __init__(self, *args, **kwargs):
        super(RecursiveRelation, self).__init__('self', *args, **kwargs)

    def get_lookup_constraint(self, constraint_class, alias, targets, sources, lookups,
                              raw_value):
        if lookups[0] == 'recursive':
            # With a recursive query, we want to build up a subquery that creates
            # the simplest possible tree we can deal with.
            data = {
                'fk': self.get_attname(),
                'pk': self.related_fields[0][1].get_attname(),
                'table': self.model._meta.db_table
            }
            if lookups[-1] == 'in':
                if targets[0] == self:
                    raw_value = ForeignKeyRecursiveInLookup(raw_value, **data)
                else:
                    raw_value = ForeignKeyRecursiveReverseInLookup(raw_value, **data)
            else:
                if targets[0] == self:
                    raw_value = ForeignKeyRecursiveLookup(raw_value, **data)
                else:
                    raw_value = ForeignKeyRecursiveReverseLookup(raw_value, **data)

            # Rewrite some variables so we get correct behaviour.

            # This makes the query based on the original table, not the joined version,
            # which was skipping a level of relation. It still joins the table, however,
            # which can't be great for performance
            alias = self.model._meta.db_table
            # This sets the correct lookup type, removing the recursive bit.
            lookups = lookups[1:] or ['exact']

        return super(RecursiveRelation, self).get_lookup_constraint(
            constraint_class, alias, targets, sources, lookups, raw_value
        )


class ForeignKeyRecursiveLookup(object):
    query = 'SELECT "{pk}" FROM "tree" WHERE %s = ANY("related")'

    def __init__(self, value, **kwargs):
        self.value = value
        self.data = kwargs

    def get_compiler(self, *args, **kwargs):
        return self

    def as_subquery_condition(self, alias, columns, qn):
        sql = SQL.format(
            query=self.query.format(**self.data),
            **self.data
        )
        return '%s.%s IN (%s)' % (qn(alias), qn(self.data['pk']), sql), [self.value]


class ForeignKeyRecursiveInLookup(ForeignKeyRecursiveLookup):
    query = 'SELECT "{pk}" FROM "tree" WHERE %s && "related"'


class ForeignKeyRecursiveReverseLookup(ForeignKeyRecursiveLookup):
    query = 'SELECT unnest("related") FROM "tree" WHERE "{pk}" = %s'


class ForeignKeyRecursiveReverseInLookup(ForeignKeyRecursiveLookup):
    query = 'SELECT unnest("related") FROM "tree" WHERE "{pk}" IN %s'

If we were to use an existing view (created using a migration), then the structure would be largely the same: simply the SQL constant would be simpler:

SQL = 'SELECT {pk} FROM "{table}_{fk}_tree" WHERE {where}'

But then we would need some sort of name mangling for the view: I’ve suggested <tablename>_<fk-name>-tree.

I went into this exercise thinking it would be simple: just write a Lookup (or Transform), but it seems that Foreign Keys in django have a fair bit of special casing. There’s also a bit of lax code around the names of lookups: I may polish it up at some stage.

For now, though, you use it as:

class Node(models.Model):
    node_id = models.AutoField(primary_key=True)
    parent = RecursiveRelation(null=True, blank=True, related_name='children')

Postgres VIEW meet Django Model

Postgres VIEWs are a nice way to store a subset of a table in a way that can itself be queried, or perhaps slightly or radically changing the shape of your table. It has a fairly simple syntax:

CREATE VIEW "foo" AS
SELECT "bar", "baz", "qux"
FROM "corge"
WHERE "grault" IS NULL;

You may use any valid SELECT query as the source of a VIEW, including one that contains UNION or UNION ALL. You can use this form to create a view that takes two similarly formatted tables and combines them into one logical table. Note that for a UNION to work, the columns (and column types) must be identical between the two parts of the query. A UNION will do extra work to ensure all rows are unique: UNION ALL may perform better, especially if you know your rows will be unique (or you need duplicates).

By default, a Postgres VIEW is dynamic, and read-only. With the use of the CREATE MATERIALIZED VIEW form, it’s possible to have a cached copy stored on disk, which requires an UPDATE MATERIALIZED VIEW "viewname" in order to cause an update.

It’s also possible to create a writeable VIEW, but I’m not going to discuss those now.


There is a feature of Django that makes in really simple to use a VIEW as the read-only source of a Django Model subclass: managed = False.

Given the VIEW defined above, we can write a Model that will allow us to query it:

from django.db import models

class Foo(models.Model):
    bar = models.CharField()
    baz = models.CharField()
    qux = models.CharField()

    class Meta:
      managed = False

Psycopg2 also has the ability to automatically convert values as it fetches them, so you don’t even really need to set the fields as the correct type: but you will probably want to where possible, as an aid to code readability.

In my case, I was returning a two-dimensional ARRAY of TIMESTAMPTZ, but didn’t want to have to include the full code for an ArrayField. So, I just defined it as a CharField, and psycopg2 just gave me the type of object I actually wanted anyway.

There is one little catch, and the code above will not quite work.

Django requires a primary key, even though in this case it makes no sense. You could define any field as a primary key, include a relevant key field from the parent model, or even a dummy value that is the same on every row. Relying on the same psycopg2 trick as above, you could use <tablename>-<id> so as to ensure uniqueness, even though that is not normally a valid value for a Django AutoField.

You probably need to be a little careful here, as if you are doing comparisons, Django will test __class__ and pk for testing equality, so you could hurt yourself if you aren’t careful.

You may also want to prevent write access at the Django level. Overriding save() and delete() on the Model class would be a good start, as well as writing a custom Manager/QuerySet that does the same. You could raise an exception that makes sense, like NotImplemented, or just leave it as a database error.

Avoiding SQL Antipatterns using Django (and Postgres)

The book SQL Antipatterns is one of my favourite books. I took the opportunity to reread it on a trip to Xerocon in Sydney, and as usual it enlightened me to thing I am probably doing in my database interactions.

So, I’m going to look at these Antipatterns, and discuss how you can avoid them when using Django. This post is intended to be read with each chapter of the book. I’ve used the section headings, but instead of the chapter headings, I’ve used the Antipattern headings. They are still in the same order, though.

It seems the printed version of this book is on sale now: I’m tempted to buy a few extra copies for gifts. Ahem, cow-orkers.

Logical Database Design Antipatterns

Format Comma-Separated Lists

This one is pretty simple: use a relation instead of a Comma Separated field. In the cases described in the book, a ManyToManyField is in fact simpler than a Comma Separated field. Django gets a gold star here, both in ease of use, but also in documentation about relations.

However, there may be times when a relation is overkill, and a real array is better. For instance, when storing data related to which days of the week are affected by a certain condition, it may make sense to store it in this way.

But we can do better than a simple Comma Separated field. Storing the data in a Postgres Array means we can rely on the database to validate the data, and allows searching. Similarly, we could store it in JSON, too.

I’ve maintained a JSONField for Django, although it’s not easily queryable. However, an ArrayField is coming in Django 1.8. There are alternatives already available if you need to use one now. I’ve got a project to mostly backport the django.contrib.postgres features to 1.7: django-postgres.

Things like JSON, Array and Hstore are a better solution than storing other-delimitered values in a straight text column too. With Django 1.7, it became possible to have lookups, which can leverage the DBMS’ ability to query these datatypes.

Always Depend on One’s Parent

Read chapter online.

Straight into a trickier one! And, Django’s documentation points out how to create his type of relation, but does not call out the possible issues. This book is worth it for this section alone.

So, how do we deal with trees in Django?

We can use django-mptt. This gives us (from what I can see) the “Nested Sets” pattern outlined in the book, but under the name “Modified Preorder Tree Traversal”.

I’m quite interested in the idea of using a Closure Table, and there are a couple of projects with quite different approaches to this:

  • django-ctt: uses a Model class you inherit from.
  • django-ct: better documented, but uses an unusual pattern of a pseudo-manager-thing.

Knowing me, I’m probably going to spend some time building a not-complete implementation at some point.

Update: Whilst I haven’t built an implementation of a Closure Table, I did implement recursive queries for an Adjacency List.

One Size Fits All

Using a field id for all tables by default is probably one of the biggest mistakes I think Django makes. And, as we shall see, we can’t yet avoid them, for at least a subset of situations.

Indeed, Django can use any single column for the primary key, and doesn’t require the use of a key column of name id. So, in my mind, it would have been better to use the <tablename>_id, as suggested in the book. Especially since you may also access the primary key attribute using the pk shortcut.

class Foo(models.Model):
    foo_id = models.AutoField(primary_key=True)

However, it’s not currently possible to do composite primary keys (but may be soon), which makes doing the best thing for a plain ManyToManyField possible: indeed, you don’t control that table anyway, and if you remove the id column (and create a proper primary key), things don’t work. In practice, you can just ignore this issue, since you (mostly) don’t deal with this table, or the objects from it.

So, assuming we are changing the id column into the name suggested in the book, what does that give us?

Nothing, until we actually need to write raw SQL code, and specifically code that joins multiple tables.

Then, we are able to use a slightly less verbose way of defining the join, and not worry about duplicate columns named id:

SELECT * FROM foo_foo JOIN foo_bar USING (foo_id);

I’m still not sure if it’s actually worthwhile doing this or not. I’m going to start doing it, just to see whether there are any drawbacks (already found one in some of my own code, that hard-coded an id field), or any great benefits.

Leave out the Constraints

Within Django, it’s more work to create relations without the relevant constraints, and it’s not possible to create a table without a primary key, so we can just pass this one by with a big:

smile and wave, boys

Use a Generic Attribute Table

Again, it’s possible to create this type of a monstrosity in Django, but not easy. A better solution, if your table’s requirements change is to use migrations (included in Django 1.7), or a more flexible store, like JSON or Hstore. This also has the added advantage of being a column, rather than a related table, which means you can fetch it in one go, simply. Similarly, with Postgres 9.3, you can do all sorts of querying, and even more in 9.4.

Document or key stores are no substitute for proper attributes, but they do have their uses.

The other solution is to use Model inheritance, which Django does well. You can choose either abstract or concrete table inheritance, and with something like django-model-utils, even get some nice features like fetching only the subtypes when fetching a queryset of superclass models.

Use Dual-Purpose Foreign Key

Unfortunately, Django comes with a built-in way to do this: so-called Generic Relations.

Using this, it’s possible to have an association from a given model instance to any other object of any other model class.

“You may find that this antipattern is unavoidable if you use an object-relational programming framework […]. Such a framework may mitigate the risks introduced by Polymorphic Associations by encapsulating application logic to maintain referential integrity. If you choose a mature and reputable framework, then you have some confidence that its designers have written the code to implement the association without error.”

I guess we’ll just have to rely on the fact Django is a mature and reputable framework.

In all reality, I’ve used this type of relation once: for notifications that need to be able to refer to any given object. It’s also possible to use, say, a tagging app that had generic relations. But, I’m struggling to think of too many situations where it would be better than a proper relation.

I’ve also come across it in django-reversion, and running queries against objects from it is a pain in the arse.

Create Multiple Columns

Interestingly, the example for this Antipattern is the example I just used above: tags. And, this type of situation should be done in a better way: a proper relation, or perhaps an Array type. It all depends how good your database is at querying arrays. django.contrib.postgres makes this rather easy:

class Post(models.Model):
    name = models.CharField(...)
    tags = ArrayField(models.CharField(...), blank=True)

Post.objects.filter(tags__contains=['foo'])

What may not be so easy is getting all of the tags in use. This may be possible: I just haven’t thought of a way to do this yet. A nice syntax might be:

Post.objects.aggregate(All('tags'))

The SQL you might be able to use to get this could look like:

SELECT
  array_agg(distinct t) AS tags
FROM (
  SELECT unnest(tags) FROM posts
) t;

I’m not sure if there’s a better way to get this data.

Clone Tables or Columns

I can’t actually see that doing this in Django would be easy, or likely. It’s gotten me interested in some method of seamlessly doing Horizontal Partitioning as a method of archiving old data, and perhaps moving it to a different database. Specifically, moving old audit data into a separate store may become necessary at some point.

Partitioning using a multi-tenancy approach using Postgres’ schemata is another of my interests, and I’ve been working on a django-specific way to do this: django-boardinghouse. Note, this is a partial-segmentation approach, where some tables are shared, but others are per-schema.

Physical Database Design Antipatterns

Use FLOAT Data Type

Just don’t.

There’s a DecimalField, and no reason not to use it.

Specify Values in the Column Definition

The example the book uses is to define check constraints on a given question. Django’s approach is a bit different: the valid choices are defined in the column definition, but can be changed in code at any time. Any existing values that are no longer valid are fine, but any attempt to save an object will require it to have one of the newly valid choices.

This is both better and worse than the problem described in the book. There’s no way (short of a migration) to change the existing data, but maybe that’s actually just better.

Again, the best solution is just to use a related field, but in some cases this is indeed overkill: specifically if values are unlikely to change.

Assume You Must Use Files

I’m still 50-50 on this one. Basically, storing binary files in your database (a) makes the database much bigger, which means it takes longer to back it up (and restore it), and (b) means that it’s harder to do things like use the web server, rather than the application server, to serve static files (even those user-supplied, that must be authenticated).

The main disadvantage, of not having backups, is purely an operations issue.

The secondary disadvantage: the lack of transactionality is also easily solved: don’t delete files (unless necessary), and don’t overwrite them. If you really must, then use a Postres NOTIFY delete-file <filepath> or similar, and have a listener that handles that.

The other disadvantage, about SQL privilidges is mostly moot under Django anyway, as you are always running as the one database user.

Using Indexes Without a Plan

Indexes are fairly tangiential to an ORM: I’m going to pass over this one without too much comment. I’ve been doing a fair bit of index-level optimisations on my production database lately, in an effort to improve performance. Mostly, it’s better to optimise the query, as the likely targets for indexes probably already have them.

Query Antipatterns

Use Null as an Ordinary Value, or Vice Versa.

Python has it’s own None type/value, and using it in queries basically converts it into NULL. Django is a little annoying how at times it stores empty strings instead of NULL in string fields. I was playing around with making these into proper NULLs, but it seemed to create other problems.

At least there is no established pattern to use other values instead of NULL.

Reference Non-grouped Columns

Since I’m dealing with Postgres, I understand this one is not much of an issue. Your query will fail if you build it wrong. Which should be the way databases work.

Sort Data Randomly

Read this chapter online.

The problem of how to fetch a single random instance from a Model comes up every now and then on IRC, indeed, it did again last weekend. Unsurprisingly, I provided a link to this chapter.

One solution that is presented in the book is to select a single row, using a random offset:

import random
# Note: the initial version of this would fail since queryset.count()
# is the number of elements, randint(a, b) includes the value 'b',
# and queryset[b] would be out of range.
index = random.randint(0, queryset.count() - 1)
instance = queryset.all()[index]

This, converts to the query:

SELECT * FROM "table" LIMIT 1 OFFSET %s;

However, without an ordering, I believe this will still do a complete table seek. Instead, you want to order on a column with an index. Like the primary key:

instance = queryset.order_by('pk')[index]

It does take two queries, but sometimes two queries is better than one. Obviously, if your table was always going to be small, it may be better to do the random ordering:

instance = queryset.order_by('?')[0]

Pattern Matching Predicates

I’m sorry to say Django makes it far too easy to do this:

queryset.filter(foo__contains='bar')

Becomes something like:

SELECT * FROM "table" WHERE "table"."foo" LIKE '%bar%';

In many cases, this will be fine, but as you can imagine, you may get surprising matches, or performance may really suck.

Using Postgres’s full-text search is relatively simple: you can quite easily make a custom field that handles this, and with Django 1.7 or later, you can even create your own lookups:

from django.db import models


class TSVectorField(models.Field):
    def db_type(self, connection):
        return 'tsvector'


class TSVectorMatches(models.lookups.BuiltinLookup):
    lookup_name = 'matches'
    def process_lhs(self, qn, connection, lhs=None):
        lhs = lhs or self.lhs
        return qn.compile(lhs)

    def get_rgs_op(self, connection, rhs):
        return '@@ to_tsquery(%s)' % rhs

TSVectorField.register_lookup(TSVectorMatches)

Then, you are able, on a correctly defined field, able to do:

queryset.filter(foo__matches='bar')

Which roughly translates to:

SELECT * FROM "table" WHERE (foo @@ to_tsquery('bar'));

It’s actually a little more complicated than that, but I have a working prototype at https://bitbucket.org/schinckel/django-postgres/. There is a field class, but also an example within the search sub-app.

Clearly, you’ll want to be creating the right indexes.

Solve a Complex Problem in One Step

By their very nature, ORMs tend to make this a little less easy to do. Because you don’t normally write custom code, this scenario is less common than you might see in a normal SQL access.

However, with Django, it is possible to write over-complicated queries, but also to use things like .raw(), and .extra() to write “Spaghetti Queries”.

However, it is worth noting that with judicious use of these features, you can indeed write queries that perform exceptionally well, indeed, far better than the ORM is able to generate for you. It’s also worth noting that you can write really, really bad queries that take a very long time, just using the ORM (without even doing things like N+1 queries for related objects).

Indeed, the “how to recognize” section of this chapter shows the biggest red flag I have noticed lately: “Just stick another DISTINCT in there”.

I’ve seen, first-hand how a .distinct() can cause a query to take a very long period of time. Removing the need for a distinct by removing the join, and instead using subqueries, caused a query that was taking around 17 seconds with a given data set to suddenly take less than 200ms.

That alone has forced me to reconsider each and every time I use .distinct() in my code (and probably explains why our code that runs queries against django-reversion) performs so horribly.

A Shortcut That Gets You Lost

I’ve used, in my SQL snippets in this post, the shortcut that is mentioned here: SELECT * FROM .... Luckily, Django doesn’t use this shortcut, and instead lists out every column it expects to see.

This has a really nice side-effect: if your database tables have not been migrated to add that new column, then whenever you try to run any queries against that table, you will have an error. Which is much more likely to happen immediately, rather than at 3am when that column is first actually used.

Application Development Antipatterns

Store Password in Plain Text

There is no, I repeat, no reason you should ever be doing this. It’s a cardinal sin, and Django has a great authentication and authorisation framework, that you can extend however you need it.

As noted in the legitimate uses section: if you are accessing a third-party system, you may need to store the password in a readable format. In this case, something like Oauth, if available, may make things a little safer.

Execute Unverified Input As Code

Read this chapter online.

Most of the risks of SQL Injection are mitigated when you use an ORM like Django’s. Of course, if you write .raw() or .extra() queries that don’t properly escape user-provided data, then you may still be at risk. .extra() in particular has arguments that allow you to pass an iterable of parameters, which will then be correctly escaped as they are added to the query.

Filling in the Corners

Educate your manager if (s)he thinks it’s a bad thing to have non-contiguous primary keys. Transaction rollbacks, deleted objects: there’s all sorts of reasons why there may be gaps.

Making Bricks Without Straw

It goes without saying that you should have error handling within your python code.

Make SQL a Second-Class Citizen

This is kind-of the point of an ORM: to remove from you the need to deal with creating complex queries in raw SQL.

Your Django models are the documentation of your table structure, or documentation can be generated from them. Your migrations files show the changes that have been made over time. Naturally, both of these will be stored in your Source Code Management system.

Clearly, as soon as you are doing anything in raw SQL, then you should follow the practices you do with the rest of your code.

Testing in-database is something I am a little bit interested in. As I move more code into the database (often for performance reasons, sometimes because it’s just fun), it would be nice to have tests for these functions. I have a long list of things in my Reading List about Postgres Unit Testing. Perhaps I’ll get around to them at some point. Integrating these with the Django test runner would be really neat.

The Model Is an Active Record

Django’s use of the Active Record is slightly different to Rails. In Rails, the column types in the database control what attributes are on the model, but in Django, the python object is the master. I think this is more meaningful, because it means that everything you need to know about an object is in the model definition: you don’t need to follow the migrations to see what attributes you have.

I do like the concept of a Domain Model: it’s an approach I’ve lightly tried in the past. Perhaps it is an avenue I’ll push down further at some point. In some ways, Django’s Form classes allow you to encapsulate this, but mostly business logic still lives on our Model classes.

Summary

So, how did Django do?

Pretty good, I’d say. The ones that were less successful either don’t really matter most of the time (primary key column is always called id, choices defined in the model), or you don’t really need to use them (Generic Relations, searching using LIKE %foo%, using raw SQL).

We do fall down a bit with files stored in the database, and fat models, but I would argue that those patterns work just fine, at least for me right now.

In-Database Audit Trail

I’ve been thinking about audit trails, object versioning and the like a bit lately. We have situations where it’s actually important to be able to know what changes were made, when, and by whom.

The tool we are currently using in Django for this is django-reversion, but I’m not that happy with it. Part of the problem is that it adds a fair amount of overhead into each request, which we have noticed, and also that querying the data it produces is not simple, unless you are just looking for changes made to a specific object.

For our case, we want to know all the changes made to a set of objects (that may not all be of the same type), made within a given range, where there is (eventually) a foreign key relationship back to our Company model.

Part of the problem is just that: a custom solution could store the Company relationship explicitly, but also, we really care about what the changes were, rather than what the object looks like after the changes. Well, technically, we want both.

However, it has gotten me thinking about other ways to solve this problem.

In most DBMSs, it’s possible to get a trigger function to run when an update happens to each row, which makes it possible to get access to this information. Indeed, googling “Postgres audit trigger” pops up some hits that are highly relevant. I looked at Audit trigger 91plus, since it’s “improved”. It has some really nice features like storing the audit data in an hstore column, which means you can query it.

However, one place where this type of trigger breaks down is that it’s not generally possible to get the “application user” associated with a request, only the “database user”, which for our system (and most other web applications) is fixed for all access.

One way to get around this might be to, at the start of every database transaction, inject a call that creates a temporary table, with the extra bits of data that you want to log, and then use that in the trigger function.

CREATE TEMP TABLE IF NOT EXISTS
  "_app_user" (user_id integer, ip_address inet);

Then we need to add (or update) the one row that will contain our data. We must ensure that we only ever have one row in this table.

UPDATE _app_user SET user_id=%s, ip_address=%s;
INSERT INTO _app_user (user_id, ip_address)
  SELECT %s, %s WHERE NOT EXISTS (SELECT * FROM _app_user);

This code will ensure that the first statement (UPDATE) will affect all rows in the table (of which there will be at most one), and the second statement (INSERT ... SELECT ... WHERE NOT EXISTS ...) will only create a new row if there are no rows currently in the table.

It’s up to you to then pass the correct data to this. I’m currently looking at doing this using Django middleware, although I suspect this may fall down using the newer transaction handling, as otherwise we could have just ensured our middleware ran after the TransactionMiddleware. It may be possible to do it with a custom database backend, but it needs to somehow get access to the request object (which contains the user, and the ip address). Obviously, you could log other data about the request, too.

The final part of the puzzle is to inject this data into the row that will be used for the audit table entry. I modified the table definition so it included columns for the data I wanted: app_user_id and app_ip_address.

Then, inside the actual trigger function, after the audit_row object has been created, but before it is written to the table, we inject the data we want.

We need to be a little careful, as it’s possible the table does not exist:

BEGIN
  PERFORM 
    n.nspname, c.relname 
  FROM
    pg_catalog.pg_class c 
  LEFT JOIN 
    pg_catalog.pg_namespace n
  ON n.oid = c.relnamespace
  WHERE
    n.nspname like 'pg_temp_%' 
  AND
    c.relname = '_app_user';

  IF FOUND THEN
    FOR r IN SELECT * FROM _app_user LIMIT 1 LOOP
      audit_row.app_user_id = r.user_id;
      audit_row.app_ip_address = r.ip_address;
    END LOOP;
    END IF;
END;

This checks to see if the _app_user table exists in any of the valid temporary table namespaces, and if so, grabs the first (and only, from above) entry, using the values to update the row.

This function then works: if there is a temporary table with this name, it uses these fields when creating the audit, if not, it creates the audit row with empty values. This would mean that some audit statements may not contain proper user data, but in the case of Django, it’s possible to make changes outside of the request-response cycle. You could require that a user starting a shell session authenticates with a valid django username+password, but that still leaves management commands. I guess you could have a system account, but leaving these entries blank is like an explicit system user.

I haven’t got any production code using anything like this: I’d still want to test that it works as expected as part of the request, and would want to build up some method of querying it. There’s probably no reason you couldn’t do the table definition as a Django model (and indeed, have the function definition as a migration).