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doc, intagg: fix one-to-many mention to many-to-many 

Reported-by: Christophe Courtois

Discussion: https://postgr.es/m/aa7cfd73-0d8d-596a-b684-39faa479afa5@dalibo.com

Author: Christophe Courtois

Backpatch-through: master
This commit is contained in:
Bruce Momjian 2023-12-07 19:36:52 -05:00
parent 719b342d36
commit 651030a3d7
1 changed files with 23 additions and 19 deletions
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42
doc/src/sgml/intagg.sgml
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@ -54,20 +54,22 @@
<title>Sample Uses</title>
<para>
Many database systems have the notion of a one to many table. Such a table
Many database systems have the notion of a many to many table. Such a table
usually sits between two indexed tables, for example:
<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);
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);
</programlisting>
It is typically used like this:
<programlisting>
SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right)
WHERE one_to_many.left = <replaceable>item</replaceable>;
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 = <replaceable>item</replaceable>;
</programlisting>
This will return all the items in the right hand table for an entry
@ -76,7 +78,7 @@ SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right)
<para>
Now, this methodology can be cumbersome with a very large number of
entries in the <structname>one_to_many</structname> table. Often,
entries in the <structname>many_to_many</structname> 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
@ -85,9 +87,9 @@ SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right)
<programlisting>
CREATE TABLE summary AS
SELECT left, int_array_aggregate(right) AS right
FROM one_to_many
GROUP BY left;
SELECT id_left, int_array_aggregate(id_right) AS rights
FROM many_to_many
GROUP BY id_left;
</programlisting>
This will create a table with one row per left item, and an array
@ -95,33 +97,35 @@ CREATE TABLE summary AS
the array; that's why there is an array enumerator. You can do
<programlisting>
SELECT left, int_array_enum(right) FROM summary WHERE left = <replaceable>item</replaceable>;
SELECT id_left, int_array_enum(rights) FROM summary WHERE id_left = <replaceable>item</replaceable>;
</programlisting>
The above query using <function>int_array_enum</function> produces the same results
as
<programlisting>
SELECT left, right FROM one_to_many WHERE left = <replaceable>item</replaceable>;
SELECT id_left, id_right FROM many_to_many WHERE id_left = <replaceable>item</replaceable>;
</programlisting>
The difference is that the query against the summary table has to get
only one row from the table, whereas the direct query against
<structname>one_to_many</structname> must index scan and fetch a row for each entry.
<structname>many_to_many</structname> must index scan and fetch a row for each entry.
</para>
<para>
On one system, an <command>EXPLAIN</command> showed a query with a cost of 8488 was
reduced to a cost of 329. The original query was a join involving the
<structname>one_to_many</structname> table, which was replaced by:
<structname>many_to_many</structname> table, which was replaced by:
<programlisting>
SELECT right, count(right) FROM
( SELECT left, int_array_enum(right) AS right
FROM summary JOIN (SELECT left FROM left_table WHERE left = <replaceable>item</replaceable>) AS lefts
ON (summary.left = lefts.left)
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 = <replaceable>item</replaceable>) AS lefts
ON (summary.id_left = lefts.id)
) AS list
GROUP BY right
GROUP BY id_right
ORDER BY count DESC;
</programlisting>
</para>