Overload index_form_tuple to allow the memory context to be supplied

david-rowley · david-rowley · commit 0229106afab9 · 2022-07-07T08:14:00.000+12:00
40af10b changed things so we make use of a generation memory context for storing tuples to be sorted by tuplesort.c. That change does not play nicely with the changes made in 9f03ca9 (back in 2014). That commit changed things so that index_form_tuple() is called while switched into the tuplestore's tuplecontext. In order to fetch the tuple from the index, index_form_tuple() must do various memory allocations which are unrelated to the storage of the final returned tuple. Although all of these allocations are pfree'd, the fact that we now use a generation context means that the memory for these pfree'd allocations won't be used again by any other allocation due to generation.c's lack of freelists. This could result in sorts used for building indexes exceeding maintenance_work_mem by a very large amount. Here we fix it so we no longer allocate anything apart from the tuple itself into the generation context by adding a new version of index_form_tuple named index_form_tuple_context, which can be called to specify the MemoryContext to allocate the tuple into. Discussion: https://postgr.es/m/CAApHDvrHQkiFRHiGiAS-LMOvJN-eK-s762=tVzBz8ZqUea-a_A@mail.gmail.com Backpatch-through: 15, where 40af10b was added.
diff --git a/src/backend/access/common/indextuple.c b/src/backend/access/common/indextuple.c
@@ -33,20 +33,39 @@
  * ----------------------------------------------------------------
  */
 
+ /* ----------------
+  *		index_form_tuple
+  *
+  *		As index_form_tuple_context, but allocates the returned tuple in the
+  *		CurrentMemoryContext.
+  * ----------------
+  */
+IndexTuple
+index_form_tuple(TupleDesc tupleDescriptor,
+				 Datum *values,
+				 bool *isnull)
+{
+	return index_form_tuple_context(tupleDescriptor, values, isnull,
+									CurrentMemoryContext);
+}
+
 /* ----------------
- *		index_form_tuple
+ *		index_form_tuple_context
  *
  *		This shouldn't leak any memory; otherwise, callers such as
  *		tuplesort_putindextuplevalues() will be very unhappy.
  *
  *		This shouldn't perform external table access provided caller
  *		does not pass values that are stored EXTERNAL.
+ *
+ *		Allocates returned tuple in provided 'context'.
  * ----------------
  */
 IndexTuple
-index_form_tuple(TupleDesc tupleDescriptor,
-				 Datum *values,
-				 bool *isnull)
+index_form_tuple_context(TupleDesc tupleDescriptor,
+						 Datum *values,
+						 bool *isnull,
+						 MemoryContext context)
 {
 	char	   *tp;				/* tuple pointer */
 	IndexTuple	tuple;			/* return tuple */
@@ -143,7 +162,7 @@ index_form_tuple(TupleDesc tupleDescriptor,
 	size = hoff + data_size;
 	size = MAXALIGN(size);		/* be conservative */
 
-	tp = (char *) palloc0(size);
+	tp = (char *) MemoryContextAllocZero(context, size);
 	tuple = (IndexTuple) tp;
 
 	heap_fill_tuple(tupleDescriptor,
diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
@@ -1884,12 +1884,13 @@ tuplesort_putindextuplevalues(Tuplesortstate *state, Relation rel,
 							  ItemPointer self, Datum *values,
 							  bool *isnull)
 {
-	MemoryContext oldcontext = MemoryContextSwitchTo(state->tuplecontext);
+	MemoryContext oldcontext;
 	SortTuple	stup;
 	Datum		original;
 	IndexTuple	tuple;
 
-	stup.tuple = index_form_tuple(RelationGetDescr(rel), values, isnull);
+	stup.tuple = index_form_tuple_context(RelationGetDescr(rel), values,
+										  isnull, state->tuplecontext);
 	tuple = ((IndexTuple) stup.tuple);
 	tuple->t_tid = *self;
 	USEMEM(state, GetMemoryChunkSpace(stup.tuple));
@@ -1899,7 +1900,7 @@ tuplesort_putindextuplevalues(Tuplesortstate *state, Relation rel,
 							 RelationGetDescr(state->indexRel),
 							 &stup.isnull1);
 
-	MemoryContextSwitchTo(state->sortcontext);
+	oldcontext = MemoryContextSwitchTo(state->sortcontext);
 
 	if (!state->sortKeys || !state->sortKeys->abbrev_converter || stup.isnull1)
 	{
diff --git a/src/include/access/itup.h b/src/include/access/itup.h
@@ -150,6 +150,9 @@ typedef IndexAttributeBitMapData * IndexAttributeBitMap;
 /* routines in indextuple.c */
 extern IndexTuple index_form_tuple(TupleDesc tupleDescriptor,
 								   Datum *values, bool *isnull);
+extern IndexTuple index_form_tuple_context(TupleDesc tupleDescriptor,
+										   Datum *values, bool *isnull,
+										   MemoryContext context);
 extern Datum nocache_index_getattr(IndexTuple tup, int attnum,
 								   TupleDesc tupleDesc);
 extern void index_deform_tuple(IndexTuple tup, TupleDesc tupleDescriptor,
