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) {