On 8/4/26 14:29, Damil Shahzad wrote:
Tom's concern still seems important. Scanning the whole MCV list every
time would cost more in the common case, and it only changes the
result when the comparison operator or collation is different from the
equality used to build the statistics. Before this can move forward, I
think we need a stronger reason for that tradeoff.
+1
A cheaper way to get some benefit here without touching that tradeoff:
when there are no MCV matches, var_eq_const does a second full pass over
MCV list just to compute `sumcommon` - but that branch is only reached
after the first loop has already scanned every entry. So `sumcommon` can
be accumulated inline in that same scan, and the separate summing loop
dropped. The match case is unaffected; only the no-match path gets
faster, by skipping a redundant second traversal.
I attached patch with these changes. What do you think?
--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC,
https://tantorlabs.com
From 1c526c9472f147c318f61c6b21447941c853de46 Mon Sep 17 00:00:00 2001
From: Evdokimov Ilia <[email protected]>
Date: Wed, 5 Aug 2026 10:20:35 +0300
Subject: [PATCH v1] Merge MCV match and sum loops in var_eq_cons
---
src/backend/utils/adt/selfuncs.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 2b4e6acf9a7..6213510d642 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -413,6 +413,7 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
AttStatsSlot sslot;
bool match = false;
int i;
+ double sumcommon = 0.0;
/*
* Is the constant "=" to any of the column's most common values?
@@ -450,6 +451,8 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
{
Datum fresult;
+ sumcommon += sslot.numbers[i];
+
if (varonleft)
fcinfo->args[0].value = sslot.values[i];
else
@@ -484,11 +487,8 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
* of the common values. Its selectivity cannot be more than
* this:
*/
- double sumcommon = 0.0;
double otherdistinct;
- for (i = 0; i < sslot.nnumbers; i++)
- sumcommon += sslot.numbers[i];
selec = 1.0 - sumcommon - nullfrac;
CLAMP_PROBABILITY(selec);
--
2.34.1
