feat: Support Spark Expression Decode

docs/source/contributor-guide/spark_expressions_support.md

@@ -517,7 +517,7 @@
 - [ ] collation
 - [x] concat_ws
 - [x] contains
-- [ ] decode
+- [x] decode
 - [ ] elt
 - [ ] encode
 - [x] endswith

docs/source/user-guide/latest/expressions.md

@@ -63,6 +63,7 @@ of expressions that be disabled.
 | Concat          |
 | ConcatWs        |
 | Contains        |
+| Decode          |
 | EndsWith        |
 | InitCap         |
 | Left            |

spark/src/test/resources/sql-tests/expressions/string/decode.sql

@@ -0,0 +1,105 @@
+-- Licensed to the Apache Software Foundation (ASF) under one
+-- or more contributor license agreements.  See the NOTICE file
+-- distributed with this work for additional information
+-- regarding copyright ownership.  The ASF licenses this file
+-- to you 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.
+
+-- Tests for the SQL `decode` function.
+--
+-- Spark's `decode` is overloaded:
+--   * decode(bin, charset)                              -> StringDecode (charset binary->string)
+--   * decode(expr, search, result, ..., [default])      -> CaseWhen with EqualNullSafe branches
+--
+-- The Oracle-style form is implemented in Spark via the RuntimeReplaceable trait, so by the
+-- time Comet sees the plan the wrapper has already been replaced with CaseWhen and Comet
+-- handles it through its existing CaseWhen + EqualNullSafe serde.
+--
+-- The 2-arg charset form lowers to a cast(binary, string) inside Comet's stringDecode
+-- handler, but only when the charset is 'utf-8' (case-insensitive). Other charsets fall
+-- back to Spark JVM execution.
+
+-- ===========================================================================
+-- Charset form: decode(bin, charset) for UTF-8 (the supported native path)
+-- ===========================================================================
+
+statement
+CREATE TABLE test_decode_utf8(b binary) USING parquet
+
+statement
+INSERT INTO test_decode_utf8 VALUES (CAST('hello' AS BINARY)), (CAST('world' AS BINARY)), (CAST('' AS BINARY)),
+    (CAST('café' AS BINARY)), (NULL)
+
+query
+SELECT decode(b, 'utf-8') FROM test_decode_utf8
+
+query
+SELECT decode(b, 'UTF-8') FROM test_decode_utf8
+
+query
+SELECT decode(CAST('hello' AS BINARY), 'utf-8'), decode(CAST('' AS BINARY), 'utf-8'), decode(NULL, 'utf-8')
+
+-- Charset form: non-UTF-8
+
+statement
+CREATE TABLE test_decode_charset_safe(b binary) USING parquet
+
+statement
+INSERT INTO test_decode_charset_safe VALUES (CAST('ab' AS BINARY)), (CAST('abcd' AS BINARY)), (CAST('' AS BINARY)), (NULL)
+
+query expect_fallback(Comet only supports decoding with 'utf-8'.)
+SELECT decode(b, 'UTF-16BE') FROM test_decode_charset_safe
+
+query expect_fallback(Comet only supports decoding with 'utf-8'.)
+SELECT decode(b, 'US-ASCII') FROM test_decode_charset_safe
+
+query expect_fallback(Comet only supports decoding with 'utf-8'.)
+SELECT decode(b, 'ISO-8859-1') FROM test_decode_utf8
+
+-- Oracle-style form: decode(expr, search1, result1, ..., [default])
+
+statement
+CREATE TABLE test_decode_oracle(status string, code int) USING parquet
+
+statement
+INSERT INTO test_decode_oracle VALUES ('A', 1), ('I', 2), ('X', 3), (NULL, 4), ('A', NULL)
+
+query
+SELECT decode(status, 'A', 'Active', 'I', 'Inactive', 'Other') FROM test_decode_oracle
+
+query
+SELECT decode(status, 'A', 'Active', 'I', 'Inactive') FROM test_decode_oracle
+
+query
+SELECT decode(code, 1, 'one', 2, 'two', 3, 'three', 'unknown') FROM test_decode_oracle
+
+query
+SELECT decode(code, 1, 'one', 2, 'two') FROM test_decode_oracle
+
+query
+SELECT decode(status, 'A', 'has-A', NULL, 'is-null', 'other') FROM test_decode_oracle
+
+query
+SELECT decode(status, 'A', 'Active') FROM test_decode_oracle
+
+query
+SELECT decode(2, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 'Other')
+
+query
+SELECT decode(6, 1, 'Southlake', 2, 'San Francisco', 'Other')
+
+query
+SELECT decode(6, 1, 'Southlake', 2, 'San Francisco')
+
+query
+SELECT decode(NULL, 6, 'Spark', NULL, 'SQL', 4, 'rocks')