doc: PG 13 relnotes: hash_mem_multiplier can restore old behav.

bmomjian · bmomjian · commit e7a6cd5dcf24 · 2020-08-03T17:01:42.000-04:00
Document that hash_mem_multiplier can get query behavior closer to the pre-PG 13 behavior of allowing hashing to use more memory. Reported-by: Peter Geoghegan Discussion: https://postgr.es/m/CAH2-Wzn3kwQm_pe6g2=ki+P7+ZRqH5GvFGn6SWfv_j7UUgcLdQ@mail.gmail.com Backpatch-through: 13 only
diff --git a/doc/src/sgml/release-13.sgml b/doc/src/sgml/release-13.sgml
@@ -649,7 +649,10 @@ Author: Jeff Davis <jdavis@postgresql.org>
 
        <para>
         Previously, hash aggregation was avoided if it was expected to use
-        more than <xref linkend="guc-work-mem"/> memory.
+        more than <xref linkend="guc-work-mem"/> memory.  To reduce the
+        likelihood of using disk storage for hash aggregation and attain
+        behavior similar to previous Postgres releases, increase <xref
+        linkend="guc-hash-mem-multiplier"/>.
        </para>
       </listitem>
 
