ALTER TABLE ... SET DATA TYPE removes statistics

[nikolai.berkoff]

The current documentation does not mention that the column statistics are 
removed which I can see they are in src/backend/commands/tablecmds.c

ATExecAlterColumnType
        /*
         * Drop any pg_statistic entry for the column, since it's now wrong type
         */
        RemoveStatistics(RelationGetRelid(rel), attnum);

Although this might be obvious it tripped me up.  For example renaming and SET 
STATISTICS preserves statistics.  Patch attached.

Regards,

Nikolai
diff --git i/doc/src/sgml/ref/alter_table.sgml w/doc/src/sgml/ref/alter_table.sgml
index 8129157..ef59db0 100644
--- i/doc/src/sgml/ref/alter_table.sgml
+++ w/doc/src/sgml/ref/alter_table.sgml
@@ -187,7 +187,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       This form changes the type of a column of a table. Indexes and
       simple table constraints involving the column will be automatically
       converted to use the new column type by reparsing the originally
-      supplied expression.
+      supplied expression. Note this will remove the column's statistics
+      so will normally be followed by an <link linkend="sql-analyze"><command>ANALYZE</command></link> of the table.
       The optional <literal>COLLATE</literal> clause specifies a collation
       for the new column; if omitted, the collation is the default for the
       new column type.

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