fix: Preserve multi-column TupleExpr in tuple simplifier

go/test/endtoend/vtgate/queries/misc/misc_test.go

@@ -40,7 +40,7 @@ func start(t *testing.T) (utils.MySQLCompare, func()) {
 	require.NoError(t, err)
 
 	deleteAll := func() {
-		tables := []string{"t1", "tbl", "unq_idx", "nonunq_idx", "tbl_enum_set", "uks.unsharded"}
+		tables := []string{"t1", "tbl", "unq_idx", "nonunq_idx", "tbl_enum_set", "uks.unsharded", "all_types"}
 		for _, table := range tables {
 			_, _ = mcmp.ExecAndIgnore("delete from " + table)
 		}
@@ -827,3 +827,13 @@ func TestTabletTypeRouting(t *testing.T) {
 	_, err = utils.ExecAllowError(t, vtConn, "select * from ks_misc.t1")
 	require.ErrorContains(t, err, "table unknown not found")
 }
+
+// TestJoinMixedCaseExpr tests that join condition with expression from both table having in clause is handled correctly.
+func TestJoinMixedCaseExpr(t *testing.T) {
+	mcmp, closer := start(t)
+	defer closer()
+
+	mcmp.Exec(`insert into all_types(id, int_unsigned) values (1, 1), (2, 2), (3,3), (4,4), (10,5), (20, 6)`)
+	mcmp.Exec(`prepare prep_pk from 'SELECT t1.id from all_types t1 join all_types t2 on t1.int_unsigned = (case when t2.int_unsigned in (1, 2, 3) then 1 when t2.int_unsigned = 4 then 10 else 20 end)'`)
+	mcmp.AssertMatches(`execute prep_pk`, `[[INT64(1)] [INT64(1)] [INT64(1)]]`)
+}

go/vt/vtgate/evalengine/translate_simplify.go

@@ -138,10 +138,24 @@ func simplifyExpr(env *ExpressionEnv, e IR) (IR, error) {
 		if err != nil {
 			return nil, err
 		}
-		return &Literal{inner: simplified}, nil
+		return evalToIR(simplified), nil
 	}
 	if err := e.simplify(env); err != nil {
 		return nil, err
 	}
 	return e, nil
 }
+
+// evalToIR turns an internal eval result into an IR:
+//   - if it’s an evalTuple, it recurses into a TupleExpr
+//   - otherwise it wraps the single value in a Literal
+func evalToIR(simplified eval) IR {
+	if tuple, isTuple := simplified.(*evalTuple); isTuple {
+		te := make(TupleExpr, len(tuple.t))
+		for i, t := range tuple.t {
+			te[i] = evalToIR(t)
+		}
+		return te
+	}
+	return &Literal{inner: simplified}
+}

go/vt/vtgate/planbuilder/testdata/from_cases.json

@@ -4761,5 +4761,47 @@
         "user.user_extra"
       ]
     }
+  },
+  {
+    "comment": "case statement in join condition for evaluation - tuple‐expr simplification in CASE",
+    "query": "select 1 from user u1 join user u2 on u1.col = (case when u2.col = 2 then 0 when u2.col in (1, 2) then 1 else 2 end)",
+    "plan": {
+      "Type": "Join",
+      "QueryType": "SELECT",
+      "Original": "select 1 from user u1 join user u2 on u1.col = (case when u2.col = 2 then 0 when u2.col in (1, 2) then 1 else 2 end)",
+      "Instructions": {
+        "OperatorType": "Join",
+        "Variant": "Join",
+        "JoinColumnIndexes": "L:0",
+        "JoinVars": {
+          "u1_col": 1
+        },
+        "Inputs": [
+          {
+            "OperatorType": "Route",
+            "Variant": "Scatter",
+            "Keyspace": {
+              "Name": "user",
+              "Sharded": true
+            },
+            "FieldQuery": "select 1, u1.col from `user` as u1 where 1 != 1",
+            "Query": "select 1, u1.col from `user` as u1"
+          },
+          {
+            "OperatorType": "Route",
+            "Variant": "Scatter",
+            "Keyspace": {
+              "Name": "user",
+              "Sharded": true
+            },
+            "FieldQuery": "select 1 from `user` as u2 where 1 != 1",
+            "Query": "select 1 from `user` as u2 where :u1_col /* INT16 */ = case when u2.col = 2 then 0 when u2.col in (1, 2) then 1 else 2 end"
+          }
+        ]
+      },
+      "TablesUsed": [
+        "user.user"
+      ]
+    }
   }
 ]