Re: [HACKERS] WIP: Faster Expression Processing v4 - Mailing list pgsql-hackers

On 2017-03-15 18:48:28 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2017-03-15 18:16:57 -0400, Tom Lane wrote:
> >> [ scratches head... ]  What deforming logic do you think I'm proposing
> >> removing?
> 
> > I thought you were suggesting that we don't do the get_last_attnums (and
> > inlined version in the isSimpleVar case) at execution time anymore,
> > instead relying on logic in the planner to know how much to deform ahead
> > of time.  Then we'd do slot_getsomeattrs in the appropriate places.  But
> > I understood you suggesting to do so only in scan nodes - which doesn't
> > seem sufficient, due to the use of materialized / minimal tuples in
> > other types of nodes.  Did I misunderstand?
> 
> We would need to do it anywhere that we might be projecting from a
> materialized tuple, I suppose.  From the planner's standpoint it would be
> about as easy to do this for all plan nodes as only selected ones.

Yea.


> Anyway the core point here seems to be that skipping ExecProject misses a
> bet because the underlying tuple doesn't get disassembled.  A quick-hack
> way of seeing if this helps might be to do slot_getallattrs in the places
> where we skip ExecProject.
> I'm not sure we'd want that as a permanent
> solution, because it's possible that it extracts columns not actually
> needed, but it should at least move the hotspot in your example case.

Hm, that could hurt pretty badly if we actually only access the first
few columns.

I wonder if we shouldn't essentially get rid of all slot_getattr()
calls, and replace them with one slot_getsomeattrs() and then direct
tts_values/nulls access.  Where we compute the column number is then
essentially a separate discussion.


Looking around most of them seem to access multiple columns, and several
of them probably can't conveniently done via the planner:

src/backend/catalog/partition.c
1635:                   datum = slot_getattr(slot, keycol, &isNull);

acesses all the partitioned columns and has convenient location to
compute column to deform to (RelationGetPartitionDispatchInfo).


src/backend/catalog/index.c
1797:                   iDatum = slot_getattr(slot, keycol, &isNull);

acesses all the partitioned columns and has convenient location to
compute column to deform to (BuildIndexInfo).


src/backend/utils/adt/orderedsetaggs.c
1196:           Datum           d = slot_getattr(slot, nargs + 1, &isnull);
1359:           Datum           d = slot_getattr(slot, nargs + 1, &isnull);

no benefit.


src/backend/executor/nodeMergeAppend.c
246:            datum1 = slot_getattr(s1, attno, &isNull1);
247:            datum2 = slot_getattr(s2, attno, &isNull2);

accesses all columns in the sort key, convenient place to prepare
(ExecInitMergeAppend).


src/backend/executor/execQual.c
594:     * caught inside slot_getattr).  What we have to check for here is the
600:     * Note: we allow a reference to a dropped attribute.  slot_getattr will
637:    return slot_getattr(slot, attnum, isNull);
676:    return slot_getattr(slot, attnum, isNull);
1681:                           return slot_getattr(fcache->funcResultSlot, 1, isNull);

Already converted in proposed expression evaluation patch.


src/backend/executor/nodeSubplan.c
367:                    dvalue = slot_getattr(slot, 1, &disnull);
395:                    prmdata->value = slot_getattr(slot, col, &(prmdata->isnull));
565:                    prmdata->value = slot_getattr(slot, col,
1015:                   dvalue = slot_getattr(slot, 1, &disnull);

Accesses all params, convenient location to compute column number
(ExecInitSubPlan).


src/backend/executor/execCurrent.c
186:            /* Use slot_getattr to catch any possible mistakes */
188:                    DatumGetObjectId(slot_getattr(scanstate->ss_ScanTupleSlot,
193:                    DatumGetPointer(slot_getattr(scanstate->ss_ScanTupleSlot,

Accesses only system columns.


src/backend/executor/nodeSetOp.c
107:    flag = DatumGetInt32(slot_getattr(inputslot,

Not an actual column.


src/backend/executor/nodeGatherMerge.c
678:            datum1 = slot_getattr(s1, attno, &isNull1);
679:            datum2 = slot_getattr(s2, attno, &isNull2);

Same as mergeAppend (huh, why did we copy this code), i.e. beneficial.


src/backend/executor/functions.c
970:            value = slot_getattr(slot, 1, &(fcinfo->isnull));

no benefit.


src/backend/executor/execJunk.c
253:    return slot_getattr(slot, attno, isNull);

no benefit.


src/backend/executor/nodeNestloop.c
136:                            prm->value = slot_getattr(outerTupleSlot,

accesses all future param values, convenient place to compute column
number.


src/backend/executor/execGrouping.c
100:            attr1 = slot_getattr(slot1, att, &isNull1);
102:            attr2 = slot_getattr(slot2, att, &isNull2);
170:            attr1 = slot_getattr(slot1, att, &isNull1);
175:            attr2 = slot_getattr(slot2, att, &isNull2);
501:            attr = slot_getattr(slot, att, &isNull);

accesses all grouped-on values, but only some have a convenient place to
compute column number.


src/backend/access/common/printtup.c
545:            attr = slot_getattr(slot, i + 1, &isnull);

Debug routine.


contrib/postgres_fdw/postgres_fdw.c
3213:                   value = slot_getattr(slot, attnum, &isnull);

Computes all parameter values, convenient-ish place to compute colun
number.

- Andres