From 3e71ec0c22617756a8ca927dbf956d3ac0034478 Mon Sep 17 00:00:00 2001
From: "ZizhuanLiu(X-MAN)" <44973863@qq.com>
Date: Thu, 20 Aug 2026 14:43:56 +0800
Subject: [PATCH v4] Fix var_eq_const: sum selectivity of all matching MCV When
 the column statistics have deterministic collation and the constant has
 non-deterministic collation. discussion:
 https://www.postgresql.org/message-id/flat/tencent_A15A9D89A86F2E4B086ABA462578B9B64307@qq.com
 commitfest: https://commitfest.postgresql.org/patch/7075/

---
 src/backend/utils/adt/selfuncs.c | 47 +++++++++++++++++++++++---------
 1 file changed, 34 insertions(+), 13 deletions(-)

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index e27ec9e..178296d 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?
@@ -427,6 +428,7 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
 		{
 			LOCAL_FCINFO(fcinfo, 2);
 			FmgrInfo	eqproc;
+			bool		scan_all_mcv_values = false;
 
 			fmgr_info(opfuncoid, &eqproc);
 
@@ -446,6 +448,27 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
 			else
 				fcinfo->args[0].value = constval;
 
+			if (collation != sslot.stacoll && OidIsValid(collation))
+			{
+				pg_locale_t const_locale = NULL;
+
+				const_locale = pg_newlocale_from_collation(collation);
+				if (!const_locale->deterministic)
+				{
+					pg_locale_t sta_locale = NULL;
+
+					if (OidIsValid(sslot.stacoll))
+						sta_locale = pg_newlocale_from_collation(sslot.stacoll);
+					/*
+					 * Full MCV scanning is only justified and beneficial when the column
+					 * statistics use a deterministic collation, while the comparison uses
+					 * a nondeterministic collation.
+					 */
+					scan_all_mcv_values = !sta_locale || sta_locale->deterministic;
+				}
+			}
+			selec = 0.0;
+
 			for (i = 0; i < sslot.nvalues; i++)
 			{
 				Datum		fresult;
@@ -459,8 +482,17 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
 				if (!fcinfo->isnull && DatumGetBool(fresult))
 				{
 					match = true;
-					break;
+					if (scan_all_mcv_values == false)
+					{
+						selec = sslot.numbers[i];
+						break;
+					}
+					else
+						/* Scan all statistical entries and accumulate selectivity for matching items. */
+						selec += sslot.numbers[i];
 				}
+
+				sumcommon += sslot.numbers[i];
 			}
 		}
 		else
@@ -469,26 +501,15 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
 			i = 0;				/* keep compiler quiet */
 		}
 
-		if (match)
-		{
-			/*
-			 * Constant is "=" to this common value.  We know selectivity
-			 * exactly (or as exactly as ANALYZE could calculate it, anyway).
-			 */
-			selec = sslot.numbers[i];
-		}
-		else
+		if (match == false)
 		{
 			/*
 			 * Comparison is against a constant that is neither NULL nor any
 			 * 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);
 
-- 
2.43.0