diff options
Diffstat (limited to 'src/backend/utils/adt/selfuncs.c')
| -rw-r--r-- | src/backend/utils/adt/selfuncs.c | 25 |
1 files changed, 21 insertions, 4 deletions
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 2d83ac96415..c71e952f8e5 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -4705,6 +4705,7 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata, HeapTuple tup; Datum values[INDEX_MAX_KEYS]; bool isnull[INDEX_MAX_KEYS]; + SnapshotData SnapshotDirty; estate = CreateExecutorState(); econtext = GetPerTupleExprContext(estate); @@ -4727,6 +4728,7 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata, slot = MakeSingleTupleTableSlot(RelationGetDescr(heapRel)); econtext->ecxt_scantuple = slot; get_typlenbyval(vardata->atttype, &typLen, &typByVal); + InitDirtySnapshot(SnapshotDirty); /* set up an IS NOT NULL scan key so that we ignore nulls */ ScanKeyEntryInitialize(&scankeys[0], @@ -4743,8 +4745,23 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata, /* If min is requested ... */ if (min) { - index_scan = index_beginscan(heapRel, indexRel, SnapshotNow, - 1, 0); + /* + * In principle, we should scan the index with our current + * active snapshot, which is the best approximation we've got + * to what the query will see when executed. But that won't + * be exact if a new snap is taken before running the query, + * and it can be very expensive if a lot of uncommitted rows + * exist at the end of the index (because we'll laboriously + * fetch each one and reject it). What seems like a good + * compromise is to use SnapshotDirty. That will accept + * uncommitted rows, and thus avoid fetching multiple heap + * tuples in this scenario. On the other hand, it will reject + * known-dead rows, and thus not give a bogus answer when the + * extreme value has been deleted; that case motivates not + * using SnapshotAny here. + */ + index_scan = index_beginscan(heapRel, indexRel, + &SnapshotDirty, 1, 0); index_rescan(index_scan, scankeys, 1, NULL, 0); /* Fetch first tuple in sortop's direction */ @@ -4775,8 +4792,8 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata, /* If max is requested, and we didn't find the index is empty */ if (max && have_data) { - index_scan = index_beginscan(heapRel, indexRel, SnapshotNow, - 1, 0); + index_scan = index_beginscan(heapRel, indexRel, + &SnapshotDirty, 1, 0); index_rescan(index_scan, scankeys, 1, NULL, 0); /* Fetch first tuple in reverse direction */ |