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Don't read MCV stats needlessly in eqjoinsel().

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eqjoinsel() currently makes use of MCV stats only when we have such
stats for both sides of the clause.  As coded, though, it would
fetch those stats even when they're present for just one side.
This can be a bit expensive with high statistics targets, leading
to wasted effort in common cases such as joining a unique column
to a non-unique column.  So it seems worth the trouble to do a quick
pre-check to confirm that both sides have MCVs before fetching either.

Also, tweak the API spec for get_attstatsslot() to document the
method we're using here.

David Geier, Tomas Vondra, Tom Lane

Discussion: https://postgr.es/m/b9846ca0-5f1c-9b26-5881-aad3f42b07f0@gmail.com
This commit is contained in:
Tom Lane 2022-11-18 11:01:03 -05:00
parent d8678aba2e
commit f84ff0c6d4
2 changed files with 22 additions and 2 deletions
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20
src/backend/utils/adt/selfuncs.c
View File

@ -2261,6 +2261,7 @@ eqjoinsel(PG_FUNCTION_ARGS)
Form_pg_statistic stats2 = NULL;
bool have_mcvs1 = false;
bool have_mcvs2 = false;
bool get_mcv_stats;
bool join_is_reversed;
RelOptInfo *inner_rel;
@ -2275,11 +2276,25 @@ eqjoinsel(PG_FUNCTION_ARGS)
memset(&sslot1, 0, sizeof(sslot1));
memset(&sslot2, 0, sizeof(sslot2));
/*
* There is no use in fetching one side's MCVs if we lack MCVs for the
* other side, so do a quick check to verify that both stats exist.
*/
get_mcv_stats = (HeapTupleIsValid(vardata1.statsTuple) &&
HeapTupleIsValid(vardata2.statsTuple) &&
get_attstatsslot(&sslot1, vardata1.statsTuple,
STATISTIC_KIND_MCV, InvalidOid,
0) &&
get_attstatsslot(&sslot2, vardata2.statsTuple,
STATISTIC_KIND_MCV, InvalidOid,
0));
if (HeapTupleIsValid(vardata1.statsTuple))
{
/* note we allow use of nullfrac regardless of security check */
stats1 = (Form_pg_statistic) GETSTRUCT(vardata1.statsTuple);
if (statistic_proc_security_check(&vardata1, opfuncoid))
if (get_mcv_stats &&
statistic_proc_security_check(&vardata1, opfuncoid))
have_mcvs1 = get_attstatsslot(&sslot1, vardata1.statsTuple,
STATISTIC_KIND_MCV, InvalidOid,
ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS);
@ -2289,7 +2304,8 @@ eqjoinsel(PG_FUNCTION_ARGS)
{
/* note we allow use of nullfrac regardless of security check */
stats2 = (Form_pg_statistic) GETSTRUCT(vardata2.statsTuple);
if (statistic_proc_security_check(&vardata2, opfuncoid))
if (get_mcv_stats &&
statistic_proc_security_check(&vardata2, opfuncoid))
have_mcvs2 = get_attstatsslot(&sslot2, vardata2.statsTuple,
STATISTIC_KIND_MCV, InvalidOid,
ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS);

4
src/backend/utils/cache/lsyscache.c vendored
View File

@ -3183,6 +3183,10 @@ get_attavgwidth(Oid relid, AttrNumber attnum)
*
* If it's desirable to call free_attstatsslot when get_attstatsslot might
* not have been called, memset'ing sslot to zeroes will allow that.
*
* Passing flags=0 can be useful to quickly check if the requested slot type
* exists. In this case no arrays are extracted, so free_attstatsslot need
* not be called.
*/
bool
get_attstatsslot(AttStatsSlot *sslot, HeapTuple statstuple,