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Optimize creation of slots for FDW bulk inserts

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Commit b663a41363 introduced bulk inserts for FDW, but the handling of
tuple slots turned out to be problematic for two reasons. Firstly, the
slots were re-created for each individual batch. Secondly, all slots
referenced the same tuple descriptor - with reasonably small batches
this is not an issue, but with large batches this triggers O(N^2)
behavior in the resource owner code.

These two issues work against each other - to reduce the number of times
a slot has to be created/dropped, larger batches are needed. However,
the larger the batch, the more expensive the resource owner gets. For
practical batch sizes (100 - 1000) this would not be a big problem, as
the benefits (latency savings) greatly exceed the resource owner costs.
But for extremely large batches it might be much worse, possibly even
losing with non-batching mode.

Fixed by initializing tuple slots only once (and reusing them across
batches) and by using a new tuple descriptor copy for each slot.

Discussion: https://postgr.es/m/ebbbcc7d-4286-8c28-0272-61b4753af761%40enterprisedb.com
This commit is contained in:
Tomas Vondra 2021-06-11 20:19:48 +02:00
parent 96540f80f8
commit b676ac443b
2 changed files with 37 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

52
src/backend/executor/nodeModifyTable.c
View File

@ -703,16 +703,31 @@ ExecInsert(ModifyTableState *mtstate,
resultRelInfo->ri_BatchSize);
}
resultRelInfo->ri_Slots[resultRelInfo->ri_NumSlots] =
MakeSingleTupleTableSlot(slot->tts_tupleDescriptor,
slot->tts_ops);
ExecCopySlot(resultRelInfo->ri_Slots[resultRelInfo->ri_NumSlots],
slot);
resultRelInfo->ri_PlanSlots[resultRelInfo->ri_NumSlots] =
MakeSingleTupleTableSlot(planSlot->tts_tupleDescriptor,
planSlot->tts_ops);
ExecCopySlot(resultRelInfo->ri_PlanSlots[resultRelInfo->ri_NumSlots],
planSlot);
/*
* Initialize the batch slots. We don't know how many slots will be
* needed, so we initialize them as the batch grows, and we keep
* them across batches. To mitigate an inefficiency in how resource
* owner handles objects with many references (as with many slots
* all referencing the same tuple descriptor) we copy the tuple
* descriptor for each slot.
*/
if (resultRelInfo->ri_NumSlots >= resultRelInfo->ri_NumSlotsInitialized)
{
TupleDesc tdesc = CreateTupleDescCopy(slot->tts_tupleDescriptor);
resultRelInfo->ri_Slots[resultRelInfo->ri_NumSlots] =
MakeSingleTupleTableSlot(tdesc, slot->tts_ops);
ExecCopySlot(resultRelInfo->ri_Slots[resultRelInfo->ri_NumSlots],
slot);
resultRelInfo->ri_PlanSlots[resultRelInfo->ri_NumSlots] =
MakeSingleTupleTableSlot(tdesc, planSlot->tts_ops);
ExecCopySlot(resultRelInfo->ri_PlanSlots[resultRelInfo->ri_NumSlots],
planSlot);
/* remember how many batch slots we initialized */
resultRelInfo->ri_NumSlotsInitialized++;
}
resultRelInfo->ri_NumSlots++;
@ -1034,12 +1049,6 @@ ExecBatchInsert(ModifyTableState *mtstate,
if (canSetTag && numInserted > 0)
estate->es_processed += numInserted;
for (i = 0; i < numSlots; i++)
{
ExecDropSingleTupleTableSlot(slots[i]);
ExecDropSingleTupleTableSlot(planSlots[i]);
}
}
/* ----------------------------------------------------------------
@ -3162,6 +3171,7 @@ ExecEndModifyTable(ModifyTableState *node)
*/
for (i = 0; i < node->mt_nrels; i++)
{
int j;
ResultRelInfo *resultRelInfo = node->resultRelInfo + i;
if (!resultRelInfo->ri_usesFdwDirectModify &&
@ -3169,6 +3179,16 @@ ExecEndModifyTable(ModifyTableState *node)
resultRelInfo->ri_FdwRoutine->EndForeignModify != NULL)
resultRelInfo->ri_FdwRoutine->EndForeignModify(node->ps.state,
resultRelInfo);
/*
* Cleanup the initialized batch slots. This only matters for FDWs with
* batching, but the other cases will have ri_NumSlotsInitialized == 0.
*/
for (j = 0; j < resultRelInfo->ri_NumSlotsInitialized; j++)
{
ExecDropSingleTupleTableSlot(resultRelInfo->ri_Slots[j]);
ExecDropSingleTupleTableSlot(resultRelInfo->ri_PlanSlots[j]);
}
}
/*

1
src/include/nodes/execnodes.h
View File

@ -462,6 +462,7 @@ typedef struct ResultRelInfo
/* batch insert stuff */
int ri_NumSlots; /* number of slots in the array */
int ri_NumSlotsInitialized; /* number of initialized slots */
int ri_BatchSize; /* max slots inserted in a single batch */
TupleTableSlot **ri_Slots; /* input tuples for batch insert */
TupleTableSlot **ri_PlanSlots;