Allow negative aggtransspace to indicate unbounded state size

Richard Guo · Richard Guo · commit 185e30426334 · 2025-10-08T17:01:48.000+09:00
This patch reuses the existing aggtransspace in pg_aggregate to signal that an aggregate's transition state can grow unboundedly. If aggtransspace is set to a negative value, it now indicates that the transition state may consume unpredictable or large amounts of memory, such as in aggregates like array_agg or string_agg that accumulate input rows. This information can be used by the planner to avoid applying memory-sensitive optimizations (e.g., eager aggregation) when there is a risk of excessive memory usage during partial aggregation. Bump catalog version. Per idea from Robert Haas, though applied differently than originally suggested. Discussion: https://postgr.es/m/CA+TgmoYbkvYwLa+1vOP7RDY7kO2=A7rppoPusoRXe44VDOGBPg@mail.gmail.com
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
@@ -596,7 +596,10 @@
       </para>
       <para>
        Approximate average size (in bytes) of the transition state
-       data, or zero to use a default estimate
+       data. A positive value provides an estimate; zero means to
+       use a default estimate. A negative value indicates the state
+       data can grow unboundedly in size, such as when the aggregate
+       accumulates input rows (e.g., array_agg, string_agg).
       </para></entry>
      </row>
 
diff --git a/doc/src/sgml/ref/create_aggregate.sgml b/doc/src/sgml/ref/create_aggregate.sgml
@@ -384,9 +384,13 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
      <para>
       The approximate average size (in bytes) of the aggregate's state value.
       If this parameter is omitted or is zero, a default estimate is used
-      based on the <replaceable>state_data_type</replaceable>.
+      based on the <replaceable>state_data_type</replaceable>. If set to a
+      negative value, it indicates the state data can grow unboundedly in
+      size, such as when the aggregate accumulates input rows (e.g.,
+      array_agg, string_agg).
       The planner uses this value to estimate the memory required for a
-      grouped aggregate query.
+      grouped aggregate query and to avoid optimizations that may cause
+      excessive memory usage.
      </para>
     </listitem>
    </varlistentry>
@@ -568,7 +572,8 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
      <para>
       The approximate average size (in bytes) of the aggregate's state
       value, when using moving-aggregate mode.  This works the same as
-      <replaceable>state_data_size</replaceable>.
+      <replaceable>state_data_size</replaceable>, except that negative
+      values are not used to indicate unbounded state size.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202510081
+#define CATALOG_VERSION_NO	202510082
 
 #endif
diff --git a/src/include/catalog/pg_aggregate.dat b/src/include/catalog/pg_aggregate.dat
@@ -558,26 +558,28 @@
   aggfinalfn => 'array_agg_finalfn', aggcombinefn => 'array_agg_combine',
   aggserialfn => 'array_agg_serialize',
   aggdeserialfn => 'array_agg_deserialize', aggfinalextra => 't',
-  aggtranstype => 'internal' },
+  aggtranstype => 'internal', aggtransspace => '-1' },
 { aggfnoid => 'array_agg(anyarray)', aggtransfn => 'array_agg_array_transfn',
   aggfinalfn => 'array_agg_array_finalfn',
   aggcombinefn => 'array_agg_array_combine',
   aggserialfn => 'array_agg_array_serialize',
   aggdeserialfn => 'array_agg_array_deserialize', aggfinalextra => 't',
-  aggtranstype => 'internal' },
+  aggtranstype => 'internal', aggtransspace => '-1' },
 
 # text
 { aggfnoid => 'string_agg(text,text)', aggtransfn => 'string_agg_transfn',
   aggfinalfn => 'string_agg_finalfn', aggcombinefn => 'string_agg_combine',
   aggserialfn => 'string_agg_serialize',
-  aggdeserialfn => 'string_agg_deserialize', aggtranstype => 'internal' },
+  aggdeserialfn => 'string_agg_deserialize',
+  aggtranstype => 'internal', aggtransspace => '-1' },
 
 # bytea
 { aggfnoid => 'string_agg(bytea,bytea)',
   aggtransfn => 'bytea_string_agg_transfn',
   aggfinalfn => 'bytea_string_agg_finalfn',
   aggcombinefn => 'string_agg_combine', aggserialfn => 'string_agg_serialize',
-  aggdeserialfn => 'string_agg_deserialize', aggtranstype => 'internal' },
+  aggdeserialfn => 'string_agg_deserialize',
+  aggtranstype => 'internal', aggtransspace => '-1' },
 
 # range
 { aggfnoid => 'range_intersect_agg(anyrange)',
diff --git a/src/test/regress/expected/opr_sanity.out b/src/test/regress/expected/opr_sanity.out
@@ -1470,7 +1470,7 @@ WHERE aggfnoid = 0 OR aggtransfn = 0 OR
     (aggkind = 'n' AND aggnumdirectargs > 0) OR
     aggfinalmodify NOT IN ('r', 's', 'w') OR
     aggmfinalmodify NOT IN ('r', 's', 'w') OR
-    aggtranstype = 0 OR aggtransspace < 0 OR aggmtransspace < 0;
+    aggtranstype = 0 OR aggmtransspace < 0;
  ctid | aggfnoid 
 ------+----------
 (0 rows)
diff --git a/src/test/regress/sql/opr_sanity.sql b/src/test/regress/sql/opr_sanity.sql
@@ -847,7 +847,7 @@ WHERE aggfnoid = 0 OR aggtransfn = 0 OR
     (aggkind = 'n' AND aggnumdirectargs > 0) OR
     aggfinalmodify NOT IN ('r', 's', 'w') OR
     aggmfinalmodify NOT IN ('r', 's', 'w') OR
-    aggtranstype = 0 OR aggtransspace < 0 OR aggmtransspace < 0;
+    aggtranstype = 0 OR aggmtransspace < 0;
 
 -- Make sure the matching pg_proc entry is sensible, too.
 
