types: fix CASE WHEN wrong result when prepare value is NULL (pingcap#62568)

close pingcap#62564

Author: yihong0618

pkg/types/field_type.go

@@ -194,11 +194,14 @@ func InferParamTypeFromDatum(d *Datum, tp *FieldType) {
 func InferParamTypeFromUnderlyingValue(value any, tp *FieldType) {
 	switch value.(type) {
 	case nil:
-		tp.SetType(mysql.TypeVarString)
-		tp.SetFlen(UnspecifiedLength)
-		tp.SetDecimal(UnspecifiedLength)
-		// Also set the `charset` and `collation` for it, because some function (e.g. `json_object`) will return error
-		// if the argument collation is `binary`.
+		// For NULL parameters, use TypeNull to ensure consistent behavior with literal NULL values
+		// in control flow functions like CASE WHEN. Previously using TypeVarString caused incorrect
+		// type inference in prepared statements (issue #62564).
+		tp.SetType(mysql.TypeNull)
+		tp.SetFlen(0)
+		tp.SetDecimal(0)
+		// Set default charset and collation instead of binary charset to avoid breaking JSON functions
+		// (see PR #54145 which fixed JSON function argument verification issues).
 		tp.SetCharset(mysql.DefaultCharset)
 		tp.SetCollate(mysql.DefaultCollationName)
 	default:

tests/integrationtest/r/planner/core/casetest/integration.result

@@ -1838,3 +1838,13 @@ Update	N/A	root		N/A
     └─TableReader(Probe)	0.00	root		data:Selection
       └─Selection	0.00	cop[tikv]		or(or(and(eq(test.tt.c, 11), eq(test.tt.d, 111)), and(eq(test.tt.c, 22), eq(test.tt.d, 222))), or(and(eq(test.tt.c, 33), eq(test.tt.d, 333)), and(eq(test.tt.c, 44), eq(test.tt.d, 444)))), or(or(eq(test.tt.c, 11), eq(test.tt.c, 22)), or(eq(test.tt.c, 33), eq(test.tt.c, 44))), or(or(eq(test.tt.d, 111), eq(test.tt.d, 222)), or(eq(test.tt.d, 333), eq(test.tt.d, 444)))
         └─TableRangeScan	0.00	cop[tikv]	table:tt	range: decided by [eq(test.tt.c, Column#20) eq(test.tt.d, Column#21)], keep order:false, stats:pseudo
+CREATE TABLE test_62564(c0 DECIMAL);
+INSERT INTO test_62564 VALUES (0);
+SELECT * FROM test_62564 WHERE CAST((CASE test_62564.c0 WHEN test_62564.c0 THEN CAST(test_62564.c0 AS TIME) ELSE NULL END ) AS DATE);
+c0
+0
+SET @b = NULL;
+PREPARE prepare_query FROM 'SELECT * FROM test_62564 WHERE CAST((CASE test_62564.c0 WHEN test_62564.c0 THEN CAST(test_62564.c0 AS TIME) ELSE ? END ) AS DATE)';
+EXECUTE prepare_query USING @b;
+c0
+0

tests/integrationtest/t/planner/core/casetest/integration.test

@@ -732,3 +732,11 @@ use test;
 drop table if exists tt;
 create table tt(a bigint, b bigint, c bigint, d bigint, e bigint, primary key(c,d));
 explain format = brief update tt, (select 1 as c1 ,2 as c2 ,3 as c3, 4 as c4 union all select 2,3,4,5 union all select 3,4,5,6) tmp set tt.a=tmp.c1, tt.b=tmp.c2 where tt.c=tmp.c3 and tt.d=tmp.c4 and (tt.c,tt.d) in ((11,111),(22,222),(33,333),(44,444));
+
+# TestIssue62564
+CREATE TABLE test_62564(c0 DECIMAL);
+INSERT INTO test_62564 VALUES (0);
+SELECT * FROM test_62564 WHERE CAST((CASE test_62564.c0 WHEN test_62564.c0 THEN CAST(test_62564.c0 AS TIME) ELSE NULL END ) AS DATE);
+SET @b = NULL;
+PREPARE prepare_query FROM 'SELECT * FROM test_62564 WHERE CAST((CASE test_62564.c0 WHEN test_62564.c0 THEN CAST(test_62564.c0 AS TIME) ELSE ? END ) AS DATE)';
+EXECUTE prepare_query USING @b;