feat(expr): implement casefold(text) function (PG18)

e2e_test/batch/functions/casefold.slt.part

@@ -0,0 +1,57 @@
+query T
+SELECT casefold('Hello World');
+----
+hello world
+
+query T
+SELECT casefold('HELLO RUST');
+----
+hello rust
+
+# ß folds to ss (full case folding)
+query T
+SELECT casefold('Straße');
+----
+strasse
+
+# Empty string
+query T
+SELECT casefold('');
+----
+(empty)
+
+# NULL input
+query T
+SELECT casefold(NULL::varchar);
+----
+NULL
+
+# ASCII-only input
+query T
+SELECT casefold('ABC123xyz');
+----
+abc123xyz
+
+# Already lowercase
+query T
+SELECT casefold('already lowercase');
+----
+already lowercase
+
+# Mixed with numbers and symbols
+query T
+SELECT casefold('Test@123#ABC');
+----
+test@123#abc
+
+# Latin ligatures: fi folds to fi
+query T
+SELECT casefold('find');
+----
+find
+
+# Capital sharp S also folds to ss
+query T
+SELECT casefold('GROẞE');
+----
+grosse

proto/expr.proto

@@ -169,6 +169,7 @@ message ExprNode {
     TRIM_SCALE = 278;
     GAMMA = 288;
     LGAMMA = 289;
+    CASEFOLD = 291;
 
     // Boolean comparison
     IS_TRUE = 301;

src/expr/impl/Cargo.toml

@@ -66,6 +66,7 @@ tokio = { version = "0.2", package = "madsim-tokio", features = ["time"] }
 # For arrow-udf-runtime/remote
 tonic = { version = "0.12.3", optional = true }
 tracing = "0.1"
+unicode-casefold = "0.2"
 zstd = { version = "0.13", default-features = false, optional = true }
 
 [target.'cfg(not(madsim))'.dependencies]

src/expr/impl/src/scalar/casefold.rs

@@ -0,0 +1,69 @@
+// Copyright 2023 RisingWave Labs
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use risingwave_expr::function;
+use unicode_casefold::UnicodeCaseFold;
+
+/// Unicode case folding for case-insensitive comparison.
+///
+/// Unlike `lower()`, `casefold()` handles multi-character expansions
+/// (e.g., 'ß' → "ss") and normalizes all case variants to a single form
+/// (e.g., 'Σ', 'σ', 'ς' all fold to 'σ').
+///
+/// ```slt
+/// query T
+/// SELECT casefold('Hello World');
+/// ----
+/// hello world
+///
+/// query T
+/// SELECT casefold('Straße');
+/// ----
+/// strasse
+///
+/// query T
+/// SELECT casefold(NULL::varchar);
+/// ----
+/// NULL
+/// ```
+#[function("casefold(varchar) -> varchar")]
+fn casefold(s: &str, writer: &mut impl std::fmt::Write) {
+    for c in s.case_fold() {
+        writer.write_char(c).unwrap();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_casefold() {
+        let cases = [
+            ("Hello World", "hello world"),
+            ("HELLO RUST", "hello rust"),
+            // ß folds to ss
+            ("Straße", "strasse"),
+            // Final sigma (ς) and capital sigma (Σ) both fold to σ
+            ("ΣΊΣΥΦΟΣ", "σίσυφοσ"),
+            ("", ""),
+        ];
+
+        for (input, expected) in cases {
+            let mut writer = String::new();
+            casefold(input, &mut writer);
+            assert_eq!(writer, expected, "casefold({:?})", input);
+        }
+    }
+}

src/expr/impl/src/scalar/mod.rs

@@ -36,6 +36,7 @@ mod bitwise_op;
 mod bytea_bits;
 mod cardinality;
 mod case;
+mod casefold;
 mod cast;
 mod cmp;
 mod coalesce;

src/frontend/src/binder/expr/function/builtin_scalar.rs

@@ -232,6 +232,7 @@ impl Binder {
                 ("length", raw_call(ExprType::Length)),
                 ("upper", raw_call(ExprType::Upper)),
                 ("lower", raw_call(ExprType::Lower)),
+                ("casefold", raw_call(ExprType::Casefold)),
                 ("trim", raw_call(ExprType::Trim)),
                 ("replace", raw_call(ExprType::Replace)),
                 ("overlay", raw_call(ExprType::Overlay)),

src/frontend/src/expr/pure.rs

@@ -144,6 +144,7 @@ impl ExprVisitor for ImpureAnalyzer {
             | Type::SimilarToEscape
             | Type::Upper
             | Type::Lower
+            | Type::Casefold
             | Type::Trim
             | Type::Replace
             | Type::Position

src/frontend/src/optimizer/plan_expr_visitor/strong.rs

@@ -152,6 +152,7 @@ impl Strong {
             | ExprType::SimilarToEscape
             | ExprType::Upper
             | ExprType::Lower
+            | ExprType::Casefold
             | ExprType::Replace
             | ExprType::Position
             | ExprType::Case