intagg 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. int_array_aggregate array_agg 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. int_array_enum 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 many to many table. Such a table usually sits between two indexed tables, for example: CREATE TABLE left_table (id INT PRIMARY KEY, ...); CREATE TABLE right_table (id INT PRIMARY KEY, ...); CREATE TABLE many_to_many(id_left INT REFERENCES left_table, id_right INT REFERENCES right_table); It is typically used like this: SELECT right_table.* FROM right_table JOIN many_to_many ON (right_table.id = many_to_many.id_right) WHERE many_to_many.id_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 many_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 id_left, int_array_aggregate(id_right) AS rights FROM many_to_many GROUP BY id_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 id_left, int_array_enum(rights) FROM summary WHERE id_left = item; The above query using int_array_enum produces the same results as SELECT id_left, id_right FROM many_to_many WHERE id_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 many_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 many_to_many table, which was replaced by: SELECT id_right, count(id_right) FROM ( SELECT id_left, int_array_enum(rights) AS id_right FROM summary JOIN (SELECT id FROM left_table WHERE id = item) AS lefts ON (summary.id_left = lefts.id) ) AS list GROUP BY id_right ORDER BY count DESC;