From 94b77294ac95901f07f1e2a571fad483a7409639 Mon Sep 17 00:00:00 2001
From: Jeff Davis <jeff@j-davis.com>
Date: Mon, 12 Jan 2026 08:58:43 -0800
Subject: [PATCH v1 1/4] ILIKE: use CASEFOLD() rather than LOWER().

For non-C locales, we casefold the entire string before performing
pattern matching with ILIKE. Previously, casefolding was done with the
LOWER() function; now that a proper CASEFOLD() function exists, use
that instead.

CASEFOLD() is better than LOWER() for case-insensitive comparisons in
builtin and ICU locales. For instance, CASEFOLD() transforms a GREEK
SMALL LETTER FINAL SIGMA (U+03C2) into GREEK SMALL LETTER SIGMA
(U+03C3) so that the two characters match in a case-insensitive
comparison; whereas LOWER() does not transform it because it's already
lowercase, so they will not match.
---
 src/backend/utils/adt/like.c               |  8 +++-----
 src/test/regress/expected/collate.utf8.out | 24 ++++++++++++++++++++++
 src/test/regress/sql/collate.utf8.sql      |  6 ++++++
 3 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/src/backend/utils/adt/like.c b/src/backend/utils/adt/like.c
index 2143d8658e8..b04c6cc6661 100644
--- a/src/backend/utils/adt/like.c
+++ b/src/backend/utils/adt/like.c
@@ -190,10 +190,8 @@ Generic_Text_IC_like(text *str, text *pat, Oid collation)
 				 errmsg("nondeterministic collations are not supported for ILIKE")));
 
 	/*
-	 * For efficiency reasons, in the C locale we don't call lower() on the
+	 * For efficiency reasons, in the C locale we don't call casefold() on the
 	 * pattern and text, but instead lowercase each character lazily.
-	 *
-	 * XXX: use casefolding instead?
 	 */
 
 	if (locale->ctype_is_c)
@@ -206,11 +204,11 @@ Generic_Text_IC_like(text *str, text *pat, Oid collation)
 	}
 	else
 	{
-		pat = DatumGetTextPP(DirectFunctionCall1Coll(lower, collation,
+		pat = DatumGetTextPP(DirectFunctionCall1Coll(casefold, collation,
 													 PointerGetDatum(pat)));
 		p = VARDATA_ANY(pat);
 		plen = VARSIZE_ANY_EXHDR(pat);
-		str = DatumGetTextPP(DirectFunctionCall1Coll(lower, collation,
+		str = DatumGetTextPP(DirectFunctionCall1Coll(casefold, collation,
 													 PointerGetDatum(str)));
 		s = VARDATA_ANY(str);
 		slen = VARSIZE_ANY_EXHDR(str);
diff --git a/src/test/regress/expected/collate.utf8.out b/src/test/regress/expected/collate.utf8.out
index 0c3ab5c89b2..3d4292611e2 100644
--- a/src/test/regress/expected/collate.utf8.out
+++ b/src/test/regress/expected/collate.utf8.out
@@ -169,6 +169,18 @@ select casefold('AbCd 123 #$% ıiIİ ẞ ß DŽDždž Σσς' collate PG_C_UTF8);
  abcd 123 #$% ıiiİ ß ß dždždž σσσ
 (1 row)
 
+SELECT U&'ς' ILIKE U&'σ' COLLATE PG_C_UTF8;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT U&'straße' ILIKE U&'STRASSE' COLLATE PG_C_UTF8;
+ ?column? 
+----------
+ f
+(1 row)
+
 --
 -- Test PG_UNICODE_FAST
 --
@@ -338,3 +350,15 @@ select casefold('AbCd 123 #$% ıiIİ ẞ ß DŽDždž Σσς' collate PG_UNICODE_FA
  abcd 123 #$% ıiii̇ ss ss dždždž σσσ
 (1 row)
 
+SELECT U&'ς' ILIKE U&'σ' COLLATE PG_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT U&'straße' ILIKE U&'STRASSE' COLLATE PG_UNICODE_FAST;
+ ?column? 
+----------
+ t
+(1 row)
+
diff --git a/src/test/regress/sql/collate.utf8.sql b/src/test/regress/sql/collate.utf8.sql
index d6d14220ab3..4a5e519cf07 100644
--- a/src/test/regress/sql/collate.utf8.sql
+++ b/src/test/regress/sql/collate.utf8.sql
@@ -85,6 +85,9 @@ SELECT 'δ' ~* '[Γ-Λ]' COLLATE PG_C_UTF8; -- same as above with cases reversed
 -- case folding
 select casefold('AbCd 123 #$% ıiIİ ẞ ß DŽDždž Σσς' collate PG_C_UTF8);
 
+SELECT U&'ς' ILIKE U&'σ' COLLATE PG_C_UTF8;
+SELECT U&'straße' ILIKE U&'STRASSE' COLLATE PG_C_UTF8;
+
 --
 -- Test PG_UNICODE_FAST
 --
@@ -148,3 +151,6 @@ SELECT 'δ' ~* '[Γ-Λ]' COLLATE PG_UNICODE_FAST; -- same as above with cases re
 
 -- case folding
 select casefold('AbCd 123 #$% ıiIİ ẞ ß DŽDždž Σσς' collate PG_UNICODE_FAST);
+
+SELECT U&'ς' ILIKE U&'σ' COLLATE PG_UNICODE_FAST;
+SELECT U&'straße' ILIKE U&'STRASSE' COLLATE PG_UNICODE_FAST;
-- 
2.43.0