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The intagg module provides an integer aggregator and an enumerator. intagg is now obsolete, because there are built-in functions that provide a superset of its capabilities. However, the module is still provided as a compatibility wrapper around the built-in functions.
The aggregator is an aggregate function
int_array_aggregate(integer)
that produces an integer array
containing exactly the integers it is fed.
This is a wrapper around array_agg,
which does the same thing for any array type.
The enumerator is a function
int_array_enum(integer[])
that returns setof integer. It is essentially the reverse
operation of the aggregator: given an array of integers, expand it
into a set of rows. This is a wrapper around unnest,
which does the same thing for any array type.
Many database systems have the notion of a one to many table. Such a table usually sits between two indexed tables, for example:
CREATE TABLE left (id INT PRIMARY KEY, ...); CREATE TABLE right (id INT PRIMARY KEY, ...); CREATE TABLE one_to_many(left INT REFERENCES left, right INT REFERENCES right);
It is typically used like this:
SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right) WHERE one_to_many.left = item;
This will return all the items in the right hand table for an entry in the left hand table. This is a very common construct in SQL.
Now, this methodology can be cumbersome with a very large number of entries in the one_to_many table. Often, a join like this would result in an index scan and a fetch for each right hand entry in the table for a particular left hand entry. If you have a very dynamic system, there is not much you can do. However, if you have some data which is fairly static, you can create a summary table with the aggregator.
CREATE TABLE summary AS SELECT left, int_array_aggregate(right) AS right FROM one_to_many GROUP BY left;
This will create a table with one row per left item, and an array of right items. Now this is pretty useless without some way of using the array; that's why there is an array enumerator. You can do
SELECT left, int_array_enum(right) FROM summary WHERE left = item;
The above query using int_array_enum produces the same results
as
SELECT left, right FROM one_to_many WHERE left = item;
The difference is that the query against the summary table has to get only one row from the table, whereas the direct query against one_to_many must index scan and fetch a row for each entry.
On one system, an EXPLAIN showed a query with a cost of 8488 was reduced to a cost of 329. The original query was a join involving the one_to_many table, which was replaced by:
SELECT right, count(right) FROM
( SELECT left, int_array_enum(right) AS right
FROM summary JOIN (SELECT left FROM left_table WHERE left = item) AS lefts
ON (summary.left = lefts.left)
) AS list
GROUP BY right
ORDER BY count DESC;