avoid complaining about the binary collation in prepare

Signed-off-by: Yang Keao <[email protected]>

Author: YangKeao

pkg/types/field_type.go

@@ -179,7 +179,7 @@ func SetTypeFlag(flag *uint, flagItem uint, on bool) {
 // InferParamTypeFromDatum is used for plan cache to infer the type of a parameter from its datum.
 func InferParamTypeFromDatum(d *Datum, tp *FieldType) {
 	InferParamTypeFromUnderlyingValue(d.GetValue(), tp)
-	if IsStringKind(d.k) {
+	if IsStringKind(d.k) || d.GetValue() == nil {
 		// consider charset and collation here
 		c, err := collate.GetCollationByName(d.collation)
 		if err != nil || c == nil {
@@ -197,6 +197,10 @@ func InferParamTypeFromUnderlyingValue(value any, tp *FieldType) {
 		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`.
+		tp.SetCharset(mysql.DefaultCharset)
+		tp.SetCollate(mysql.DefaultCollationName)
 	default:
 		DefaultTypeForValue(value, tp, mysql.DefaultCharset, mysql.DefaultCollationName)
 		if hasVariantFieldLength(tp) {

tests/integrationtest/r/expression/json.result

@@ -806,3 +806,8 @@ SELECT JSON_KEYS(b2) FROM t1;
 Error 3144 (HY000): Cannot create a JSON value from a string with CHARACTER SET 'binary'.
 SELECT JSON_SCHEMA_VALID(b2, '{}') FROM t1;
 Error 3144 (HY000): Cannot create a JSON value from a string with CHARACTER SET 'binary'.
+prepare stmt from 'select json_object(?, ?)';
+set @a=1;
+execute stmt using @a, @a;
+json_object(?, ?)
+{"1": 1}

tests/integrationtest/t/expression/json.test

@@ -527,3 +527,8 @@ SELECT JSON_SEARCH(b2, 'one', '1') FROM t1;
 SELECT JSON_KEYS(b2) FROM t1;
 -- error 3144
 SELECT JSON_SCHEMA_VALID(b2, '{}') FROM t1;
+
+# TestIssue54044
+prepare stmt from 'select json_object(?, ?)';
+set @a=1;
+execute stmt using @a, @a;