132 lines
4.1 KiB
Plaintext
132 lines
4.1 KiB
Plaintext
<!-- doc/src/sgml/intagg.sgml -->
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<sect1 id="intagg" xreflabel="intagg">
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<title>intagg — integer aggregator and enumerator</title>
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<indexterm zone="intagg">
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<primary>intagg</primary>
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</indexterm>
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<para>
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The <filename>intagg</filename> module provides an integer aggregator and an
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enumerator. <filename>intagg</filename> is now obsolete, because there
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are built-in functions that provide a superset of its capabilities.
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However, the module is still provided as a compatibility wrapper around
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the built-in functions.
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</para>
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<sect2 id="intagg-functions">
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<title>Functions</title>
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<indexterm>
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<primary>int_array_aggregate</primary>
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</indexterm>
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<indexterm>
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<primary>array_agg</primary>
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</indexterm>
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<para>
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The aggregator is an aggregate function
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<function>int_array_aggregate(integer)</function>
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that produces an integer array
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containing exactly the integers it is fed.
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This is a wrapper around <function>array_agg</function>,
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which does the same thing for any array type.
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</para>
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<indexterm>
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<primary>int_array_enum</primary>
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</indexterm>
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<para>
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The enumerator is a function
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<function>int_array_enum(integer[])</function>
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that returns <type>setof integer</type>. It is essentially the reverse
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operation of the aggregator: given an array of integers, expand it
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into a set of rows. This is a wrapper around <function>unnest</function>,
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which does the same thing for any array type.
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</para>
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</sect2>
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<sect2 id="intagg-samples">
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<title>Sample Uses</title>
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<para>
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Many database systems have the notion of a one to many table. Such a table
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usually sits between two indexed tables, for example:
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<programlisting>
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CREATE TABLE left (id INT PRIMARY KEY, ...);
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CREATE TABLE right (id INT PRIMARY KEY, ...);
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CREATE TABLE one_to_many(left INT REFERENCES left, right INT REFERENCES right);
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</programlisting>
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It is typically used like this:
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<programlisting>
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SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right)
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WHERE one_to_many.left = <replaceable>item</replaceable>;
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</programlisting>
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This will return all the items in the right hand table for an entry
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in the left hand table. This is a very common construct in SQL.
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</para>
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<para>
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Now, this methodology can be cumbersome with a very large number of
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entries in the <structname>one_to_many</structname> table. Often,
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a join like this would result in an index scan
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and a fetch for each right hand entry in the table for a particular
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left hand entry. If you have a very dynamic system, there is not much you
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can do. However, if you have some data which is fairly static, you can
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create a summary table with the aggregator.
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<programlisting>
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CREATE TABLE summary AS
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SELECT left, int_array_aggregate(right) AS right
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FROM one_to_many
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GROUP BY left;
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</programlisting>
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This will create a table with one row per left item, and an array
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of right items. Now this is pretty useless without some way of using
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the array; that's why there is an array enumerator. You can do
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<programlisting>
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SELECT left, int_array_enum(right) FROM summary WHERE left = <replaceable>item</replaceable>;
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</programlisting>
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The above query using <function>int_array_enum</function> produces the same results
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as
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<programlisting>
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SELECT left, right FROM one_to_many WHERE left = <replaceable>item</replaceable>;
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</programlisting>
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The difference is that the query against the summary table has to get
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only one row from the table, whereas the direct query against
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<structname>one_to_many</structname> must index scan and fetch a row for each entry.
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</para>
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<para>
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On one system, an <command>EXPLAIN</command> showed a query with a cost of 8488 was
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reduced to a cost of 329. The original query was a join involving the
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<structname>one_to_many</structname> table, which was replaced by:
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<programlisting>
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SELECT right, count(right) FROM
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( SELECT left, int_array_enum(right) AS right
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FROM summary JOIN (SELECT left FROM left_table WHERE left = <replaceable>item</replaceable>) AS lefts
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ON (summary.left = lefts.left)
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) AS list
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GROUP BY right
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ORDER BY count DESC;
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</programlisting>
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</para>
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</sect2>
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</sect1>
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