Allow aggregates to provide estimates of their transition state data size.

Formerly the planner had a hard-wired rule of thumb for guessing the amount
of space consumed by an aggregate function's transition state data.  This
estimate is critical to deciding whether it's OK to use hash aggregation,
and in many situations the built-in estimate isn't very good.  This patch
adds a column to pg_aggregate wherein a per-aggregate estimate can be
provided, overriding the planner's default, and infrastructure for setting
the column via CREATE AGGREGATE.

It may be that additional smarts will be required in future, perhaps even
a per-aggregate estimation function.  But this is already a step forward.

This is extracted from a larger patch to improve the performance of numeric
and int8 aggregates.  I (tgl) thought it was worth reviewing and committing
this infrastructure separately.  In this commit, all built-in aggregates
are given aggtransspace = 0, so no behavior should change.

Hadi Moshayedi, reviewed by Pavel Stehule and Tomas Vondra

Author: tglsfdc

doc/src/sgml/catalogs.sgml

@@ -372,6 +372,13 @@
       <entry><literal><link linkend="catalog-pg-type"><structname>pg_type</structname></link>.oid</literal></entry>
       <entry>Data type of the aggregate function's internal transition (state) data</entry>
      </row>
+     <row>
+      <entry><structfield>aggtransspace</structfield></entry>
+      <entry><type>int4</type></entry>
+      <entry></entry>
+      <entry>Approximate average size (in bytes) of the transition state
+       data, or zero to use a default estimate</entry>
+     </row>
      <row>
       <entry><structfield>agginitval</structfield></entry>
       <entry><type>text</type></entry>

doc/src/sgml/ref/create_aggregate.sgml

@@ -24,6 +24,7 @@ PostgreSQL documentation
 CREATE AGGREGATE <replaceable class="parameter">name</replaceable> ( [ <replaceable class="parameter">argmode</replaceable> ] [ <replaceable class="parameter">arg_name</replaceable> ] <replaceable class="parameter">arg_data_type</replaceable> [ , ... ] ) (
     SFUNC = <replaceable class="PARAMETER">sfunc</replaceable>,
     STYPE = <replaceable class="PARAMETER">state_data_type</replaceable>
+    [ , SSPACE = <replaceable class="PARAMETER">state_data_size</replaceable> ]
     [ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
     [ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ]
     [ , SORTOP = <replaceable class="PARAMETER">sort_operator</replaceable> ]
@@ -35,6 +36,7 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> (
     BASETYPE = <replaceable class="PARAMETER">base_type</replaceable>,
     SFUNC = <replaceable class="PARAMETER">sfunc</replaceable>,
     STYPE = <replaceable class="PARAMETER">state_data_type</replaceable>
+    [ , SSPACE = <replaceable class="PARAMETER">state_data_size</replaceable> ]
     [ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
     [ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ]
     [ , SORTOP = <replaceable class="PARAMETER">sort_operator</replaceable> ]
@@ -264,6 +266,22 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><replaceable class="PARAMETER">state_data_size</replaceable></term>
+    <listitem>
+     <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</>.
+      The planner uses this value to estimate the memory required for a
+      grouped aggregate query.  The planner will consider using hash
+      aggregation for such a query only if the hash table is estimated to fit
+      in <xref linkend="guc-work-mem">; therefore, large values of this
+      parameter discourage use of hash aggregation.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><replaceable class="PARAMETER">ffunc</replaceable></term>
     <listitem>

src/backend/catalog/pg_aggregate.c

@@ -55,6 +55,7 @@ AggregateCreate(const char *aggName,
 				List *aggfinalfnName,
 				List *aggsortopName,
 				Oid aggTransType,
+				int32 aggTransSpace,
 				const char *agginitval)
 {
 	Relation	aggdesc;
@@ -273,6 +274,7 @@ AggregateCreate(const char *aggName,
 	values[Anum_pg_aggregate_aggfinalfn - 1] = ObjectIdGetDatum(finalfn);
 	values[Anum_pg_aggregate_aggsortop - 1] = ObjectIdGetDatum(sortop);
 	values[Anum_pg_aggregate_aggtranstype - 1] = ObjectIdGetDatum(aggTransType);
+	values[Anum_pg_aggregate_aggtransspace - 1] = Int32GetDatum(aggTransSpace);
 	if (agginitval)
 		values[Anum_pg_aggregate_agginitval - 1] = CStringGetTextDatum(agginitval);
 	else

src/backend/commands/aggregatecmds.c

@@ -60,6 +60,7 @@ DefineAggregate(List *name, List *args, bool oldstyle, List *parameters,
 	List	   *sortoperatorName = NIL;
 	TypeName   *baseType = NULL;
 	TypeName   *transType = NULL;
+	int32		transSpace = 0;
 	char	   *initval = NULL;
 	int			numArgs;
 	oidvector  *parameterTypes;
@@ -102,6 +103,8 @@ DefineAggregate(List *name, List *args, bool oldstyle, List *parameters,
 			transType = defGetTypeName(defel);
 		else if (pg_strcasecmp(defel->defname, "stype1") == 0)
 			transType = defGetTypeName(defel);
+		else if (pg_strcasecmp(defel->defname, "sspace") == 0)
+			transSpace = defGetInt32(defel);
 		else if (pg_strcasecmp(defel->defname, "initcond") == 0)
 			initval = defGetString(defel);
 		else if (pg_strcasecmp(defel->defname, "initcond1") == 0)
@@ -248,5 +251,6 @@ DefineAggregate(List *name, List *args, bool oldstyle, List *parameters,
 						   finalfuncName,		/* final function name */
 						   sortoperatorName,	/* sort operator name */
 						   transTypeId, /* transition data type */
+						   transSpace,	/* transition space */
 						   initval);	/* initial condition */
 }

src/backend/commands/define.c

@@ -164,6 +164,30 @@ defGetBoolean(DefElem *def)
 	return false;				/* keep compiler quiet */
 }
 
+/*
+ * Extract an int32 value from a DefElem.
+ */
+int32
+defGetInt32(DefElem *def)
+{
+	if (def->arg == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("%s requires an integer value",
+						def->defname)));
+	switch (nodeTag(def->arg))
+	{
+		case T_Integer:
+			return (int32) intVal(def->arg);
+		default:
+			ereport(ERROR,
+					(errcode(ERRCODE_SYNTAX_ERROR),
+					 errmsg("%s requires an integer value",
+							def->defname)));
+	}
+	return 0;					/* keep compiler quiet */
+}
+
 /*
  * Extract an int64 value from a DefElem.
  */

src/backend/optimizer/util/clauses.c

@@ -461,6 +461,7 @@ count_agg_clauses_walker(Node *node, count_agg_clauses_context *context)
 		Oid			aggtransfn;
 		Oid			aggfinalfn;
 		Oid			aggtranstype;
+		int32		aggtransspace;
 		QualCost	argcosts;
 		Oid		   *inputTypes;
 		int			numArguments;
@@ -478,6 +479,7 @@ count_agg_clauses_walker(Node *node, count_agg_clauses_context *context)
 		aggtransfn = aggform->aggtransfn;
 		aggfinalfn = aggform->aggfinalfn;
 		aggtranstype = aggform->aggtranstype;
+		aggtransspace = aggform->aggtransspace;
 		ReleaseSysCache(aggTuple);
 
 		/* count it */
@@ -541,35 +543,47 @@ count_agg_clauses_walker(Node *node, count_agg_clauses_context *context)
 		 */
 		if (!get_typbyval(aggtranstype))
 		{
-			int32		aggtranstypmod;
 			int32		avgwidth;
 
-			/*
-			 * If transition state is of same type as first input, assume it's
-			 * the same typmod (same width) as well.  This works for cases
-			 * like MAX/MIN and is probably somewhat reasonable otherwise.
-			 */
-			if (numArguments > 0 && aggtranstype == inputTypes[0])
-				aggtranstypmod = exprTypmod((Node *) linitial(aggref->args));
+			/* Use average width if aggregate definition gave one */
+			if (aggtransspace > 0)
+				avgwidth = aggtransspace;
 			else
-				aggtranstypmod = -1;
+			{
+				/*
+				 * If transition state is of same type as first input, assume
+				 * it's the same typmod (same width) as well.  This works for
+				 * cases like MAX/MIN and is probably somewhat reasonable
+				 * otherwise.
+				 */
+				int32		aggtranstypmod;
 
-			avgwidth = get_typavgwidth(aggtranstype, aggtranstypmod);
-			avgwidth = MAXALIGN(avgwidth);
+				if (numArguments > 0 && aggtranstype == inputTypes[0])
+					aggtranstypmod = exprTypmod((Node *) linitial(aggref->args));
+				else
+					aggtranstypmod = -1;
+
+				avgwidth = get_typavgwidth(aggtranstype, aggtranstypmod);
+			}
 
+			avgwidth = MAXALIGN(avgwidth);
 			costs->transitionSpace += avgwidth + 2 * sizeof(void *);
 		}
 		else if (aggtranstype == INTERNALOID)
 		{
 			/*
 			 * INTERNAL transition type is a special case: although INTERNAL
 			 * is pass-by-value, it's almost certainly being used as a pointer
-			 * to some large data structure.  We assume usage of
+			 * to some large data structure.  The aggregate definition can
+			 * provide an estimate of the size.  If it doesn't, then we assume
 			 * ALLOCSET_DEFAULT_INITSIZE, which is a good guess if the data is
 			 * being kept in a private memory context, as is done by
 			 * array_agg() for instance.
 			 */
-			costs->transitionSpace += ALLOCSET_DEFAULT_INITSIZE;
+			if (aggtransspace > 0)
+				costs->transitionSpace += aggtransspace;
+			else
+				costs->transitionSpace += ALLOCSET_DEFAULT_INITSIZE;
 		}
 
 		/*

src/bin/pg_dump/pg_dump.c

@@ -11521,12 +11521,14 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 	int			i_aggfinalfn;
 	int			i_aggsortop;
 	int			i_aggtranstype;
+	int			i_aggtransspace;
 	int			i_agginitval;
 	int			i_convertok;
 	const char *aggtransfn;
 	const char *aggfinalfn;
 	const char *aggsortop;
 	const char *aggtranstype;
+	const char *aggtransspace;
 	const char *agginitval;
 	bool		convertok;
 
@@ -11544,12 +11546,26 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 	selectSourceSchema(fout, agginfo->aggfn.dobj.namespace->dobj.name);
 
 	/* Get aggregate-specific details */
-	if (fout->remoteVersion >= 80400)
+	if (fout->remoteVersion >= 90400)
+	{
+		appendPQExpBuffer(query, "SELECT aggtransfn, "
+						  "aggfinalfn, aggtranstype::pg_catalog.regtype, "
+						  "aggsortop::pg_catalog.regoperator, "
+						  "aggtransspace, agginitval, "
+						  "'t'::boolean AS convertok, "
+						  "pg_catalog.pg_get_function_arguments(p.oid) AS funcargs, "
+						  "pg_catalog.pg_get_function_identity_arguments(p.oid) AS funciargs "
+					  "FROM pg_catalog.pg_aggregate a, pg_catalog.pg_proc p "
+						  "WHERE a.aggfnoid = p.oid "
+						  "AND p.oid = '%u'::pg_catalog.oid",
+						  agginfo->aggfn.dobj.catId.oid);
+	}
+	else if (fout->remoteVersion >= 80400)
 	{
 		appendPQExpBuffer(query, "SELECT aggtransfn, "
 						  "aggfinalfn, aggtranstype::pg_catalog.regtype, "
 						  "aggsortop::pg_catalog.regoperator, "
-						  "agginitval, "
+						  "0 AS aggtransspace, agginitval, "
 						  "'t'::boolean AS convertok, "
 						  "pg_catalog.pg_get_function_arguments(p.oid) AS funcargs, "
 						  "pg_catalog.pg_get_function_identity_arguments(p.oid) AS funciargs "
@@ -11563,7 +11579,7 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 		appendPQExpBuffer(query, "SELECT aggtransfn, "
 						  "aggfinalfn, aggtranstype::pg_catalog.regtype, "
 						  "aggsortop::pg_catalog.regoperator, "
-						  "agginitval, "
+						  "0 AS aggtransspace, agginitval, "
 						  "'t'::boolean AS convertok "
 						  "FROM pg_catalog.pg_aggregate a, pg_catalog.pg_proc p "
 						  "WHERE a.aggfnoid = p.oid "
@@ -11575,7 +11591,7 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 		appendPQExpBuffer(query, "SELECT aggtransfn, "
 						  "aggfinalfn, aggtranstype::pg_catalog.regtype, "
 						  "0 AS aggsortop, "
-						  "agginitval, "
+						  "0 AS aggtransspace, agginitval, "
 						  "'t'::boolean AS convertok "
 					  "FROM pg_catalog.pg_aggregate a, pg_catalog.pg_proc p "
 						  "WHERE a.aggfnoid = p.oid "
@@ -11587,7 +11603,7 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 		appendPQExpBuffer(query, "SELECT aggtransfn, aggfinalfn, "
 						  "format_type(aggtranstype, NULL) AS aggtranstype, "
 						  "0 AS aggsortop, "
-						  "agginitval, "
+						  "0 AS aggtransspace, agginitval, "
 						  "'t'::boolean AS convertok "
 						  "FROM pg_aggregate "
 						  "WHERE oid = '%u'::oid",
@@ -11599,7 +11615,7 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 						  "aggfinalfn, "
 						  "(SELECT typname FROM pg_type WHERE oid = aggtranstype1) AS aggtranstype, "
 						  "0 AS aggsortop, "
-						  "agginitval1 AS agginitval, "
+						  "0 AS aggtransspace, agginitval1 AS agginitval, "
 						  "(aggtransfn2 = 0 and aggtranstype2 = 0 and agginitval2 is null) AS convertok "
 						  "FROM pg_aggregate "
 						  "WHERE oid = '%u'::oid",
@@ -11612,13 +11628,15 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 	i_aggfinalfn = PQfnumber(res, "aggfinalfn");
 	i_aggsortop = PQfnumber(res, "aggsortop");
 	i_aggtranstype = PQfnumber(res, "aggtranstype");
+	i_aggtransspace = PQfnumber(res, "aggtransspace");
 	i_agginitval = PQfnumber(res, "agginitval");
 	i_convertok = PQfnumber(res, "convertok");
 
 	aggtransfn = PQgetvalue(res, 0, i_aggtransfn);
 	aggfinalfn = PQgetvalue(res, 0, i_aggfinalfn);
 	aggsortop = PQgetvalue(res, 0, i_aggsortop);
 	aggtranstype = PQgetvalue(res, 0, i_aggtranstype);
+	aggtransspace = PQgetvalue(res, 0, i_aggtransspace);
 	agginitval = PQgetvalue(res, 0, i_agginitval);
 	convertok = (PQgetvalue(res, 0, i_convertok)[0] == 't');
 
@@ -11672,6 +11690,12 @@ dumpAgg(Archive *fout, AggInfo *agginfo)
 						  fmtId(aggtranstype));
 	}
 
+	if (strcmp(aggtransspace, "0") != 0)
+	{
+		appendPQExpBuffer(details, ",\n    SSPACE = %s",
+						  aggtransspace);
+	}
+
 	if (!PQgetisnull(res, 0, i_agginitval))
 	{
 		appendPQExpBuffer(details, ",\n    INITCOND = ");

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201311081
+#define CATALOG_VERSION_NO	201311161
 
 #endif