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

MasaoFujii · MasaoFujii · commit 0b6aea03843d · 2025-08-06T16:47:20.000+09:00
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
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
@@ -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>
 
