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Doc: clarify partial-index example.

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Jonathan Katz

Discussion: https://postgr.es/m/155432280882.722.12392985690846288230@wrigleys.postgresql.org
This commit is contained in:
Tom Lane 2019-04-03 18:28:18 -04:00
parent ea569d64ac
commit af052daec5
1 changed files with 5 additions and 3 deletions
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8
doc/src/sgml/indices.sgml
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@ -814,18 +814,20 @@ SELECT *
FROM access_log
WHERE url = '/index.html' AND client_ip = inet '212.78.10.32';
</programlisting>
A query that cannot use this index is:
Here the query's IP address is covered by the partial index. The
following query cannot use the partial index, as it uses an IP address
that is excluded from the index:
<programlisting>
SELECT *
FROM access_log
WHERE client_ip = inet '192.168.100.23';
WHERE url = '/index.html' AND client_ip = inet '192.168.100.23';
</programlisting>
</para>
<para>
Observe that this kind of partial index requires that the common
values be predetermined, so such partial indexes are best used for
data distributions that do not change. The indexes can be recreated
data distributions that do not change. Such indexes can be recreated
occasionally to adjust for new data distributions, but this adds
maintenance effort.
</para>