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>Appendix F. Additional Supplied Modules</TD
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><H1
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><A
NAME="INTAGG"
>F.17. intagg</A
></H1
><P
> The <TT
CLASS="FILENAME"
>intagg</TT
> module provides an integer aggregator and an
enumerator. <TT
CLASS="FILENAME"
>intagg</TT
> 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.
</P
><DIV
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><H2
CLASS="SECT2"
><A
NAME="AEN148826"
>F.17.1. Functions</A
></H2
><P
> The aggregator is an aggregate function
<CODE
CLASS="FUNCTION"
>int_array_aggregate(integer)</CODE
>
that produces an integer array
containing exactly the integers it is fed.
This is a wrapper around <CODE
CLASS="FUNCTION"
>array_agg</CODE
>,
which does the same thing for any array type.
</P
><P
> The enumerator is a function
<CODE
CLASS="FUNCTION"
>int_array_enum(integer[])</CODE
>
that returns <TT
CLASS="TYPE"
>setof integer</TT
>. 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 <CODE
CLASS="FUNCTION"
>unnest</CODE
>,
which does the same thing for any array type.
</P
></DIV
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><H2
CLASS="SECT2"
><A
NAME="AEN148835"
>F.17.2. Sample Uses</A
></H2
><P
> Many database systems have the notion of a one to many table. Such a table
usually sits between two indexed tables, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>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);</PRE
><P>
It is typically used like this:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right)
WHERE one_to_many.left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>;</PRE
><P>
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.
</P
><P
> Now, this methodology can be cumbersome with a very large number of
entries in the <TT
CLASS="STRUCTNAME"
>one_to_many</TT
> 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.
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE summary AS
SELECT left, int_array_aggregate(right) AS right
FROM one_to_many
GROUP BY left;</PRE
><P>
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
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT left, int_array_enum(right) FROM summary WHERE left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>;</PRE
><P>
The above query using <CODE
CLASS="FUNCTION"
>int_array_enum</CODE
> produces the same results
as
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT left, right FROM one_to_many WHERE left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>;</PRE
><P>
The difference is that the query against the summary table has to get
only one row from the table, whereas the direct query against
<TT
CLASS="STRUCTNAME"
>one_to_many</TT
> must index scan and fetch a row for each entry.
</P
><P
> On one system, an <TT
CLASS="COMMAND"
>EXPLAIN</TT
> showed a query with a cost of 8488 was
reduced to a cost of 329. The original query was a join involving the
<TT
CLASS="STRUCTNAME"
>one_to_many</TT
> table, which was replaced by:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT right, count(right) FROM
( SELECT left, int_array_enum(right) AS right
FROM summary JOIN (SELECT left FROM left_table WHERE left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>) AS lefts
ON (summary.left = lefts.left)
) AS list
GROUP BY right
ORDER BY count DESC;</PRE
><P>
</P
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