Le 21/04/2017 à 17:45, Tom Lane a écrit :
> Christophe Courtois <[email protected]> writes:
>> I've found out that the example in intagg.sgml is wrongly named: the
>> one-to-many table is a many-to-many.
> Well, it'd depend on how it was used. The example clearly intends that
> it be one-to-many, and I'm not sure it still makes sense without that
> restriction. Maybe better to add a unique constraint on
> one_to_many(left)?
Perhaps the whole example can be simplified to get rid of the "left"
table, but I didn't intend to rewrite it.
>> And my colleague Thibaut Madeleine has seen that the "CREATE TABLE
>> right" and "CREATE TABLE left" examples cannot compile due to the
>> reserved words.
> Ouch. Shows you how old this module is :-(
Indeed.
>> I propose the attached patch to fix that.
> Um, the attached file seems empty from here.
Ooops, sorry. It is attached.
--
Christophe Courtois
Consultant Dalibo
http://dalibo.com/ - http://dalibo.org/
--- intagg.sgml.HEAD 2016-11-17 09:41:53.173988630 +0100
+++ intagg.sgml 2017-04-21 16:29:10.421996579 +0200
@@ -54,20 +54,23 @@
<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</>;
+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</>;
</programlisting>
This will return all the items in the right hand table for an entry
@@ -76,7 +79,7 @@
<para>
Now, this methodology can be cumbersome with a very large number of
- entries in the <structname>one_to_many</> table. Often,
+ entries in the <structname>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
@@ -85,9 +88,9 @@
<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 +98,35 @@
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</>;
+SELECT id_left, int_array_enum(rights) FROM summary WHERE id_left = <replaceable>item</>;
</programlisting>
The above query using <function>int_array_enum</> produces the same results
as
<programlisting>
-SELECT left, right FROM one_to_many WHERE left = <replaceable>item</>;
+SELECT id_left, id_right FROM many_to_many WHERE id_left = <replaceable>item</>;
</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</> must index scan and fetch a row for each entry.
+ <structname>many_to_many</> must index scan and fetch a row for each entry.
</para>
<para>
On one system, an <command>EXPLAIN</> 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</> table, which was replaced by:
+ <structname>many_to_many</> 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</>) 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</>) AS lefts
+ ON (summary.id_left = lefts.id)
) AS list
- GROUP BY right
+ GROUP BY id_right
ORDER BY count DESC;
</programlisting>
</para>
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