| F.27. intagg — integer aggregator and enumerator | ||||
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F.27. intagg — integer aggregator and enumerator#
Theintagg 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.
F.27.1. Functions#
The aggregator is an aggregate functionint_array_aggregate(integer) that produces an integer array containing exactly the integers it is fed. This is a wrapper aroundarray_agg, which does the same thing for any array type.
The enumerator is a functionint_array_enum(integer[]) that returnssetof 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 aroundunnest, which does the same thing for any array type.
F.27.2. Sample Uses#
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 theone_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 usingint_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 againstone_to_many must index scan and fetch a row for each entry.
On one system, anEXPLAIN showed a query with a cost of 8488 was reduced to a cost of 329. The original query was a join involving theone_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;