In array_agg(), don't create a new context for every group.

Previously, each new array created a new memory context that started
out at 8kB. This is incredibly wasteful when there are lots of small
groups of just a few elements each.

Change initArrayResult() and friends to accept a "subcontext" argument
to indicate whether the caller wants the ArrayBuildState allocated in
a new subcontext or not. If not, it can no longer be released
separately from the rest of the memory context.

Fixes bug report by Frank van Vugt on 2013-10-19.

Tomas Vondra. Reviewed by Ali Akbar, Tom Lane, and me.

src/backend/executor/nodeSubplan.c

@@ -262,7 +262,7 @@ ExecScanSubPlan(SubPlanState *node,
 	/* Initialize ArrayBuildStateAny in caller's context, if needed */
 	if (subLinkType == ARRAY_SUBLINK)
 		astate = initArrayResultAny(subplan->firstColType,
-									CurrentMemoryContext);
+									CurrentMemoryContext, true);
 
 	/*
 	 * We are probably in a short-lived expression-evaluation context. Switch
@@ -964,7 +964,7 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 	/* Initialize ArrayBuildStateAny in caller's context, if needed */
 	if (subLinkType == ARRAY_SUBLINK)
 		astate = initArrayResultAny(subplan->firstColType,
-									CurrentMemoryContext);
+									CurrentMemoryContext, true);
 
 	/*
 	 * Must switch to per-query memory context.

src/backend/utils/adt/array_userfuncs.c

@@ -520,8 +520,13 @@ array_agg_transfn(PG_FUNCTION_ARGS)
 		elog(ERROR, "array_agg_transfn called in non-aggregate context");
 	}
 
-	state = PG_ARGISNULL(0) ? NULL : (ArrayBuildState *) PG_GETARG_POINTER(0);
+	if (PG_ARGISNULL(0))
+		state = initArrayResult(arg1_typeid, aggcontext, false);
+	else
+		state = (ArrayBuildState *) PG_GETARG_POINTER(0);
+
 	elem = PG_ARGISNULL(1) ? (Datum) 0 : PG_GETARG_DATUM(1);
+
 	state = accumArrayResult(state,
 							 elem,
 							 PG_ARGISNULL(1),
@@ -595,7 +600,12 @@ array_agg_array_transfn(PG_FUNCTION_ARGS)
 		elog(ERROR, "array_agg_array_transfn called in non-aggregate context");
 	}
 
-	state = PG_ARGISNULL(0) ? NULL : (ArrayBuildStateArr *) PG_GETARG_POINTER(0);
+
+	if (PG_ARGISNULL(0))
+		state = initArrayResultArr(arg1_typeid, InvalidOid, aggcontext, false);
+	else
+		state = (ArrayBuildStateArr *) PG_GETARG_POINTER(0);
+
 	state = accumArrayResultArr(state,
 								PG_GETARG_DATUM(1),
 								PG_ARGISNULL(1),

src/backend/utils/adt/arrayfuncs.c

@@ -4650,6 +4650,7 @@ array_insert_slice(ArrayType *destArray,
  *
  *	element_type is the array element type (must be a valid array element type)
  *	rcontext is where to keep working state
+ *	subcontext is a flag determining whether to use a separate memory context
  *
  * Note: there are two common schemes for using accumArrayResult().
  * In the older scheme, you start with a NULL ArrayBuildState pointer, and
@@ -4659,24 +4660,39 @@ array_insert_slice(ArrayType *destArray,
  * once per element.  In this scheme you always end with a non-NULL pointer
  * that you can pass to makeArrayResult; you get an empty array if there
  * were no elements.  This is preferred if an empty array is what you want.
+ *
+ * It's possible to choose whether to create a separate memory context for the
+ * array build state, or whether to allocate it directly within rcontext.
+ *
+ * When there are many concurrent small states (e.g. array_agg() using hash
+ * aggregation of many small groups), using a separate memory context for each
+ * one may result in severe memory bloat. In such cases, use the same memory
+ * context to initialize all such array build states, and pass
+ * subcontext=false.
+ *
+ * In cases when the array build states have different lifetimes, using a
+ * single memory context is impractical. Instead, pass subcontext=true so that
+ * the array build states can be freed individually.
  */
 ArrayBuildState *
-initArrayResult(Oid element_type, MemoryContext rcontext)
+initArrayResult(Oid element_type, MemoryContext rcontext, bool subcontext)
 {
 	ArrayBuildState *astate;
-	MemoryContext arr_context;
+	MemoryContext arr_context = rcontext;
 
 	/* Make a temporary context to hold all the junk */
-	arr_context = AllocSetContextCreate(rcontext,
-										"accumArrayResult",
-										ALLOCSET_DEFAULT_MINSIZE,
-										ALLOCSET_DEFAULT_INITSIZE,
-										ALLOCSET_DEFAULT_MAXSIZE);
+	if (subcontext)
+		arr_context = AllocSetContextCreate(rcontext,
+											"accumArrayResult",
+											ALLOCSET_DEFAULT_MINSIZE,
+											ALLOCSET_DEFAULT_INITSIZE,
+											ALLOCSET_DEFAULT_MAXSIZE);
 
 	astate = (ArrayBuildState *)
 		MemoryContextAlloc(arr_context, sizeof(ArrayBuildState));
 	astate->mcontext = arr_context;
-	astate->alen = 64;			/* arbitrary starting array size */
+	astate->private_cxt = subcontext;
+	astate->alen = (subcontext ? 64 : 8);	/* arbitrary starting array size */
 	astate->dvalues = (Datum *)
 		MemoryContextAlloc(arr_context, astate->alen * sizeof(Datum));
 	astate->dnulls = (bool *)
@@ -4710,7 +4726,7 @@ accumArrayResult(ArrayBuildState *astate,
 	if (astate == NULL)
 	{
 		/* First time through --- initialize */
-		astate = initArrayResult(element_type, rcontext);
+		astate = initArrayResult(element_type, rcontext, true);
 	}
 	else
 	{
@@ -4757,6 +4773,9 @@ accumArrayResult(ArrayBuildState *astate,
 /*
  * makeArrayResult - produce 1-D final result of accumArrayResult
  *
+ * Note: only releases astate if it was initialized within a separate memory
+ * context (i.e. using subcontext=true when calling initArrayResult).
+ *
  *	astate is working state (must not be NULL)
  *	rcontext is where to construct result
  */
@@ -4773,7 +4792,8 @@ makeArrayResult(ArrayBuildState *astate,
 	dims[0] = astate->nelems;
 	lbs[0] = 1;
 
-	return makeMdArrayResult(astate, ndims, dims, lbs, rcontext, true);
+	return makeMdArrayResult(astate, ndims, dims, lbs, rcontext,
+							 astate->private_cxt);
 }
 
 /*
@@ -4782,6 +4802,11 @@ makeArrayResult(ArrayBuildState *astate,
  * beware: no check that specified dimensions match the number of values
  * accumulated.
  *
+ * Note: if the astate was not initialized within a separate memory context
+ * (that is, initArrayResult was called with subcontext=false), then using
+ * release=true is illegal. Instead, release astate along with the rest of its
+ * context when appropriate.
+ *
  *	astate is working state (must not be NULL)
  *	rcontext is where to construct result
  *	release is true if okay to release working state
@@ -4814,7 +4839,10 @@ makeMdArrayResult(ArrayBuildState *astate,
 
 	/* Clean up all the junk */
 	if (release)
+	{
+		Assert(astate->private_cxt);
 		MemoryContextDelete(astate->mcontext);
+	}
 
 	return PointerGetDatum(result);
 }
@@ -4831,26 +4859,42 @@ makeMdArrayResult(ArrayBuildState *astate,
  * initArrayResultArr - initialize an empty ArrayBuildStateArr
  *
  *	array_type is the array type (must be a valid varlena array type)
- *	element_type is the type of the array's elements
+ *	element_type is the type of the array's elements (lookup if InvalidOid)
  *	rcontext is where to keep working state
+ *	subcontext is a flag determining whether to use a separate memory context
  */
 ArrayBuildStateArr *
-initArrayResultArr(Oid array_type, Oid element_type, MemoryContext rcontext)
+initArrayResultArr(Oid array_type, Oid element_type, MemoryContext rcontext,
+				   bool subcontext)
 {
 	ArrayBuildStateArr *astate;
-	MemoryContext arr_context;
+	MemoryContext arr_context = rcontext;   /* by default use the parent ctx */
+
+	/* Lookup element type, unless element_type already provided */
+	if (! OidIsValid(element_type))
+	{
+		element_type = get_element_type(array_type);
+
+		if (!OidIsValid(element_type))
+			ereport(ERROR,
+					(errcode(ERRCODE_DATATYPE_MISMATCH),
+					 errmsg("data type %s is not an array type",
+							format_type_be(array_type))));
+	}
 
 	/* Make a temporary context to hold all the junk */
-	arr_context = AllocSetContextCreate(rcontext,
-										"accumArrayResultArr",
-										ALLOCSET_DEFAULT_MINSIZE,
-										ALLOCSET_DEFAULT_INITSIZE,
-										ALLOCSET_DEFAULT_MAXSIZE);
+	if (subcontext)
+		arr_context = AllocSetContextCreate(rcontext,
+											"accumArrayResultArr",
+											ALLOCSET_DEFAULT_MINSIZE,
+											ALLOCSET_DEFAULT_INITSIZE,
+											ALLOCSET_DEFAULT_MAXSIZE);
 
 	/* Note we initialize all fields to zero */
 	astate = (ArrayBuildStateArr *)
 		MemoryContextAllocZero(arr_context, sizeof(ArrayBuildStateArr));
 	astate->mcontext = arr_context;
+	astate->private_cxt = subcontext;
 
 	/* Save relevant datatype information */
 	astate->array_type = array_type;
@@ -4897,21 +4941,9 @@ accumArrayResultArr(ArrayBuildStateArr *astate,
 	arg = DatumGetArrayTypeP(dvalue);
 
 	if (astate == NULL)
-	{
-		/* First time through --- initialize */
-		Oid			element_type = get_element_type(array_type);
-
-		if (!OidIsValid(element_type))
-			ereport(ERROR,
-					(errcode(ERRCODE_DATATYPE_MISMATCH),
-					 errmsg("data type %s is not an array type",
-							format_type_be(array_type))));
-		astate = initArrayResultArr(array_type, element_type, rcontext);
-	}
+		astate = initArrayResultArr(array_type, InvalidOid, rcontext, true);
 	else
-	{
 		Assert(astate->array_type == array_type);
-	}
 
 	oldcontext = MemoryContextSwitchTo(astate->mcontext);
 
@@ -5090,7 +5122,10 @@ makeArrayResultArr(ArrayBuildStateArr *astate,
 
 	/* Clean up all the junk */
 	if (release)
+	{
+		Assert(astate->private_cxt);
 		MemoryContextDelete(astate->mcontext);
+	}
 
 	return PointerGetDatum(result);
 }
@@ -5106,9 +5141,10 @@ makeArrayResultArr(ArrayBuildStateArr *astate,
  *
  *	input_type is the input datatype (either element or array type)
  *	rcontext is where to keep working state
+ *	subcontext is a flag determining whether to use a separate memory context
  */
 ArrayBuildStateAny *
-initArrayResultAny(Oid input_type, MemoryContext rcontext)
+initArrayResultAny(Oid input_type, MemoryContext rcontext, bool subcontext)
 {
 	ArrayBuildStateAny *astate;
 	Oid			element_type = get_element_type(input_type);
@@ -5118,7 +5154,7 @@ initArrayResultAny(Oid input_type, MemoryContext rcontext)
 		/* Array case */
 		ArrayBuildStateArr *arraystate;
 
-		arraystate = initArrayResultArr(input_type, element_type, rcontext);
+		arraystate = initArrayResultArr(input_type, InvalidOid, rcontext, subcontext);
 		astate = (ArrayBuildStateAny *)
 			MemoryContextAlloc(arraystate->mcontext,
 							   sizeof(ArrayBuildStateAny));
@@ -5133,7 +5169,7 @@ initArrayResultAny(Oid input_type, MemoryContext rcontext)
 		/* Let's just check that we have a type that can be put into arrays */
 		Assert(OidIsValid(get_array_type(input_type)));
 
-		scalarstate = initArrayResult(input_type, rcontext);
+		scalarstate = initArrayResult(input_type, rcontext, subcontext);
 		astate = (ArrayBuildStateAny *)
 			MemoryContextAlloc(scalarstate->mcontext,
 							   sizeof(ArrayBuildStateAny));
@@ -5159,7 +5195,7 @@ accumArrayResultAny(ArrayBuildStateAny *astate,
 					MemoryContext rcontext)
 {
 	if (astate == NULL)
-		astate = initArrayResultAny(input_type, rcontext);
+		astate = initArrayResultAny(input_type, rcontext, true);
 
 	if (astate->scalarstate)
 		(void) accumArrayResult(astate->scalarstate,

src/backend/utils/adt/xml.c

@@ -3948,7 +3948,7 @@ xpath(PG_FUNCTION_ARGS)
 	ArrayType  *namespaces = PG_GETARG_ARRAYTYPE_P(2);
 	ArrayBuildState *astate;
 
-	astate = initArrayResult(XMLOID, CurrentMemoryContext);
+	astate = initArrayResult(XMLOID, CurrentMemoryContext, true);
 	xpath_internal(xpath_expr_text, data, namespaces,
 				   NULL, astate);
 	PG_RETURN_ARRAYTYPE_P(makeArrayResult(astate, CurrentMemoryContext));

src/include/utils/array.h

@@ -89,6 +89,7 @@ typedef struct ArrayBuildState
 	int16		typlen;			/* needed info about datatype */
 	bool		typbyval;
 	char		typalign;
+	bool		private_cxt;	/* use private memory context */
 } ArrayBuildState;
 
 /*
@@ -109,6 +110,7 @@ typedef struct ArrayBuildStateArr
 	int			lbs[MAXDIM];
 	Oid			array_type;		/* data type of the arrays */
 	Oid			element_type;	/* data type of the array elements */
+	bool		private_cxt;	/* use private memory context */
 } ArrayBuildStateArr;
 
 /*
@@ -293,7 +295,7 @@ extern void deconstruct_array(ArrayType *array,
 extern bool array_contains_nulls(ArrayType *array);
 
 extern ArrayBuildState *initArrayResult(Oid element_type,
-				MemoryContext rcontext);
+				MemoryContext rcontext, bool subcontext);
 extern ArrayBuildState *accumArrayResult(ArrayBuildState *astate,
 				 Datum dvalue, bool disnull,
 				 Oid element_type,
@@ -304,7 +306,7 @@ extern Datum makeMdArrayResult(ArrayBuildState *astate, int ndims,
 				  int *dims, int *lbs, MemoryContext rcontext, bool release);
 
 extern ArrayBuildStateArr *initArrayResultArr(Oid array_type, Oid element_type,
-				   MemoryContext rcontext);
+				   MemoryContext rcontext, bool subcontext);
 extern ArrayBuildStateArr *accumArrayResultArr(ArrayBuildStateArr *astate,
 					Datum dvalue, bool disnull,
 					Oid array_type,
@@ -313,7 +315,7 @@ extern Datum makeArrayResultArr(ArrayBuildStateArr *astate,
 				   MemoryContext rcontext, bool release);
 
 extern ArrayBuildStateAny *initArrayResultAny(Oid input_type,
-				   MemoryContext rcontext);
+				   MemoryContext rcontext, bool subcontext);
 extern ArrayBuildStateAny *accumArrayResultAny(ArrayBuildStateAny *astate,
 					Datum dvalue, bool disnull,
 					Oid input_type,

src/pl/plperl/plperl.c

@@ -1218,7 +1218,7 @@ plperl_array_to_datum(SV *src, Oid typid, int32 typmod)
 				 errmsg("cannot convert Perl array to non-array type %s",
 						format_type_be(typid))));
 
-	astate = initArrayResult(elemtypid, CurrentMemoryContext);
+	astate = initArrayResult(elemtypid, CurrentMemoryContext, true);
 
 	_sv_to_datum_finfo(elemtypid, &finfo, &typioparam);