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Commitf84ff0c

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Don't read MCV stats needlessly in eqjoinsel().
eqjoinsel() currently makes use of MCV stats only when we have suchstats for both sides of the clause. As coded, though, it wouldfetch those stats even when they're present for just one side.This can be a bit expensive with high statistics targets, leadingto wasted effort in common cases such as joining a unique columnto a non-unique column. So it seems worth the trouble to do a quickpre-check to confirm that both sides have MCVs before fetching either.Also, tweak the API spec for get_attstatsslot() to document themethod we're using here.David Geier, Tomas Vondra, Tom LaneDiscussion:https://postgr.es/m/b9846ca0-5f1c-9b26-5881-aad3f42b07f0@gmail.com
1 parentd8678ab commitf84ff0c

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2 files changed

+22
-2
lines changed

2 files changed

+22
-2
lines changed

‎src/backend/utils/adt/selfuncs.c

Lines changed: 18 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -2261,6 +2261,7 @@ eqjoinsel(PG_FUNCTION_ARGS)
22612261
Form_pg_statisticstats2=NULL;
22622262
boolhave_mcvs1= false;
22632263
boolhave_mcvs2= false;
2264+
boolget_mcv_stats;
22642265
booljoin_is_reversed;
22652266
RelOptInfo*inner_rel;
22662267

@@ -2275,11 +2276,25 @@ eqjoinsel(PG_FUNCTION_ARGS)
22752276
memset(&sslot1,0,sizeof(sslot1));
22762277
memset(&sslot2,0,sizeof(sslot2));
22772278

2279+
/*
2280+
* There is no use in fetching one side's MCVs if we lack MCVs for the
2281+
* other side, so do a quick check to verify that both stats exist.
2282+
*/
2283+
get_mcv_stats= (HeapTupleIsValid(vardata1.statsTuple)&&
2284+
HeapTupleIsValid(vardata2.statsTuple)&&
2285+
get_attstatsslot(&sslot1,vardata1.statsTuple,
2286+
STATISTIC_KIND_MCV,InvalidOid,
2287+
0)&&
2288+
get_attstatsslot(&sslot2,vardata2.statsTuple,
2289+
STATISTIC_KIND_MCV,InvalidOid,
2290+
0));
2291+
22782292
if (HeapTupleIsValid(vardata1.statsTuple))
22792293
{
22802294
/* note we allow use of nullfrac regardless of security check */
22812295
stats1= (Form_pg_statistic)GETSTRUCT(vardata1.statsTuple);
2282-
if (statistic_proc_security_check(&vardata1,opfuncoid))
2296+
if (get_mcv_stats&&
2297+
statistic_proc_security_check(&vardata1,opfuncoid))
22832298
have_mcvs1=get_attstatsslot(&sslot1,vardata1.statsTuple,
22842299
STATISTIC_KIND_MCV,InvalidOid,
22852300
ATTSTATSSLOT_VALUES |ATTSTATSSLOT_NUMBERS);
@@ -2289,7 +2304,8 @@ eqjoinsel(PG_FUNCTION_ARGS)
22892304
{
22902305
/* note we allow use of nullfrac regardless of security check */
22912306
stats2= (Form_pg_statistic)GETSTRUCT(vardata2.statsTuple);
2292-
if (statistic_proc_security_check(&vardata2,opfuncoid))
2307+
if (get_mcv_stats&&
2308+
statistic_proc_security_check(&vardata2,opfuncoid))
22932309
have_mcvs2=get_attstatsslot(&sslot2,vardata2.statsTuple,
22942310
STATISTIC_KIND_MCV,InvalidOid,
22952311
ATTSTATSSLOT_VALUES |ATTSTATSSLOT_NUMBERS);

‎src/backend/utils/cache/lsyscache.c

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3183,6 +3183,10 @@ get_attavgwidth(Oid relid, AttrNumber attnum)
31833183
*
31843184
* If it's desirable to call free_attstatsslot when get_attstatsslot might
31853185
* not have been called, memset'ing sslot to zeroes will allow that.
3186+
*
3187+
* Passing flags=0 can be useful to quickly check if the requested slot type
3188+
* exists. In this case no arrays are extracted, so free_attstatsslot need
3189+
* not be called.
31863190
*/
31873191
bool
31883192
get_attstatsslot(AttStatsSlot*sslot,HeapTuplestatstuple,

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