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doc: Recommend ANALYZE after ALTER TABLE ... SET EXPRESSION AS.

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ALTER TABLE ... SET EXPRESSION AS removes statistics for the target column,
so running ANALYZE afterward is recommended. But this was previously not
documented, even though a similar recommendation exists for
ALTER TABLE ... SET DATA TYPE, which also clears the column's statistics.
This commit updates the documentation to include the ANALYZE recommendation
for SET EXPRESSION AS.

Since v18, virtual generated columns are supported, and these columns never
have statistics. Therefore, ANALYZE is not needed after SET DATA TYPE or
SET EXPRESSION AS when used on virtual generated columns. This commit also
updates the documentation to clarify that ANALYZE is unnecessary in such cases.

Back-patch the ANALYZE recommendation for SET EXPRESSION AS to v17
where the feature was introduced, and the note about virtual generated
columns to v18 where those columns were added.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/20250804151418.0cf365bd2855d606763443fe@sraoss.co.jp
Backpatch-through: 17
This commit is contained in:
Fujii Masao
2025-08-06 16:47:20 +09:00
parent 9b681e2397
commit 3e65e77f76
1 changed files with 11 additions and 0 deletions
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11
doc/src/sgml/ref/alter_table.sgml
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@@ -210,6 +210,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
When this form is used, the column's statistics are removed,
so running <link linkend="sql-analyze"><command>ANALYZE</command></link>
on the table afterwards is recommended.
For a virtual generated column, <command>ANALYZE</command>
is not necessary because such columns never have statistics.
</para>
</listitem>
</varlistentry>
@@ -271,6 +273,15 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
in a stored generated column is rewritten and all the future changes
will apply the new generation expression.
</para>
<para>
When this form is used on a stored generated column, its statistics
are removed, so running
<link linkend="sql-analyze"><command>ANALYZE</command></link>
on the table afterwards is recommended.
For a virtual generated column, <command>ANALYZE</command>
is not necessary because such columns never have statistics.
</para>
</listitem>
</varlistentry>