Allow avoiding tuple copy within tuplesort_gettupleslot().

Add a "copy" argument to make it optional to receive a copy of caller
tuple that is safe to use following a subsequent manipulating of
tuplesort's state.  This is a performance optimization.  Most existing
tuplesort_gettupleslot() callers are made to opt out of copying.
Existing callers that happen to rely on the validity of tuple memory
beyond subsequent manipulations of the tuplesort request their own
copy.

This brings tuplesort_gettupleslot() in line with
tuplestore_gettupleslot().  In the future, a "copy"
tuplesort_getdatum() argument may be added, that similarly allows
callers to opt out of receiving their own copy of tuple.

In passing, clarify assumptions that callers of other tuplesort fetch
routines may make about tuple memory validity, per gripe from Tom
Lane.

Author: Peter Geoghegan
Discussion: CAM3SWZQWZZ_N=DmmL7tKy_OUjGH_5mN=N=A6h7kHyyDvEhg2DA@mail.gmail.com

2 files changed, 20 insertions, 13 deletions

diff --git a/src/backend/utils/adt/orderedsetaggs.c b/src/backend/utils/adt/orderedsetaggs.c
index e462fbd539d..8502fcfc82e 100644
--- a/src/backend/utils/adt/orderedsetaggs.c
+++ b/src/backend/utils/adt/orderedsetaggs.c
@@ -1190,7 +1190,7 @@ hypothetical_rank_common(FunctionCallInfo fcinfo, int flag,
 	tuplesort_performsort(osastate->sortstate);
 
 	/* iterate till we find the hypothetical row */
-	while (tuplesort_gettupleslot(osastate->sortstate, true, slot, NULL))
+	while (tuplesort_gettupleslot(osastate->sortstate, true, true, slot, NULL))
 	{
 		bool		isnull;
 		Datum		d = slot_getattr(slot, nargs + 1, &isnull);
@@ -1353,7 +1353,8 @@ hypothetical_dense_rank_final(PG_FUNCTION_ARGS)
 	slot2 = extraslot;
 
 	/* iterate till we find the hypothetical row */
-	while (tuplesort_gettupleslot(osastate->sortstate, true, slot, &abbrevVal))
+	while (tuplesort_gettupleslot(osastate->sortstate, true, true, slot,
+								  &abbrevVal))
 	{
 		bool		isnull;
 		Datum		d = slot_getattr(slot, nargs + 1, &isnull);
diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
index 65cda52714b..5f62cd5962a 100644
--- a/src/backend/utils/sort/tuplesort.c
+++ b/src/backend/utils/sort/tuplesort.c
@@ -1852,7 +1852,8 @@ tuplesort_performsort(Tuplesortstate *state)
  * Internal routine to fetch the next tuple in either forward or back
  * direction into *stup.  Returns FALSE if no more tuples.
  * Returned tuple belongs to tuplesort memory context, and must not be freed
- * by caller.  Caller should not use tuple following next call here.
+ * by caller.  Note that fetched tuple is stored in memory that may be
+ * recycled by any future fetch.
  */
 static bool
 tuplesort_gettuple_common(Tuplesortstate *state, bool forward,
@@ -2101,12 +2102,15 @@ tuplesort_gettuple_common(Tuplesortstate *state, bool forward,
  * NULL value in leading attribute will set abbreviated value to zeroed
  * representation, which caller may rely on in abbreviated inequality check.
  *
- * The slot receives a copied tuple (sometimes allocated in caller memory
- * context) that will stay valid regardless of future manipulations of the
- * tuplesort's state.
+ * If copy is true, the slot receives a copied tuple that'll that will stay
+ * valid regardless of future manipulations of the tuplesort's state.  Memory
+ * is owned by the caller.  If copy is false, the slot will just receive a
+ * pointer to a tuple held within the tuplesort, which is more efficient, but
+ * only safe for callers that are prepared to have any subsequent manipulation
+ * of the tuplesort's state invalidate slot contents.
  */
 bool
-tuplesort_gettupleslot(Tuplesortstate *state, bool forward,
+tuplesort_gettupleslot(Tuplesortstate *state, bool forward, bool copy,
 					   TupleTableSlot *slot, Datum *abbrev)
 {
 	MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
@@ -2123,8 +2127,10 @@ tuplesort_gettupleslot(Tuplesortstate *state, bool forward,
 		if (state->sortKeys->abbrev_converter && abbrev)
 			*abbrev = stup.datum1;
 
-		stup.tuple = heap_copy_minimal_tuple((MinimalTuple) stup.tuple);
-		ExecStoreMinimalTuple((MinimalTuple) stup.tuple, slot, true);
+		if (copy)
+			stup.tuple = heap_copy_minimal_tuple((MinimalTuple) stup.tuple);
+
+		ExecStoreMinimalTuple((MinimalTuple) stup.tuple, slot, copy);
 		return true;
 	}
 	else
@@ -2137,8 +2143,8 @@ tuplesort_gettupleslot(Tuplesortstate *state, bool forward,
 /*
  * Fetch the next tuple in either forward or back direction.
  * Returns NULL if no more tuples.  Returned tuple belongs to tuplesort memory
- * context, and must not be freed by caller.  Caller should not use tuple
- * following next call here.
+ * context, and must not be freed by caller.  Caller may not rely on tuple
+ * remaining valid after any further manipulation of tuplesort.
  */
 HeapTuple
 tuplesort_getheaptuple(Tuplesortstate *state, bool forward)
@@ -2157,8 +2163,8 @@ tuplesort_getheaptuple(Tuplesortstate *state, bool forward)
 /*
  * Fetch the next index tuple in either forward or back direction.
  * Returns NULL if no more tuples.  Returned tuple belongs to tuplesort memory
- * context, and must not be freed by caller.  Caller should not use tuple
- * following next call here.
+ * context, and must not be freed by caller.  Caller may not rely on tuple
+ * remaining valid after any further manipulation of tuplesort.
  */
 IndexTuple
 tuplesort_getindextuple(Tuplesortstate *state, bool forward)