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 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;