cmd, expression: fix convert binary string to gbk

cmd/explaintest/t/new_character_set_builtin.test

@@ -57,6 +57,13 @@ select hex(convert('ㅂ' using gbk)), convert('ㅂ' using gbk);
 select convert(a using binary) from t;
 select convert(convert('中文' using gbk) using binary), convert('中文' using binary);
 select convert(convert('ㅂ' using gbk) using binary), convert('ㅂ' using binary);
+drop table if exists t;
+create table t(a binary(10));
+insert into t values (0xe240), (0x01e240);
+set @@tidb_enable_vectorized_expression = true;
+select hex(convert(a using gbk)), convert(a using gbk) from t;
+set @@tidb_enable_vectorized_expression = false;
+select hex(convert(a using gbk)), convert(a using gbk) from t;
 
 -- test for builtin function md5()
 drop table if exists t;

expression/builtin_string.go

@@ -1152,14 +1152,8 @@ func (b *builtinConvertSig) evalString(row chunk.Row) (string, bool, error) {
 	}
 	// if expr is binary string and convert meet error, we should return NULL.
 	if types.IsBinaryStr(b.args[0].GetType()) {
-		target, _, err := transform.String(encoding.NewEncoder(), expr)
-		if err != nil {
-			return "", true, err
-		}
-
-		// we should convert target into utf8 internal.
-		exprInternal, _, _ := transform.String(encoding.NewDecoder(), target)
-		return exprInternal, false, nil
+		exprInternal, _, err := transform.String(encoding.NewDecoder(), expr)
+		return exprInternal, err != nil, nil
 	}
 	if types.IsBinaryStr(b.tp) {
 		enc := charset.NewEncoding(b.args[0].GetType().Charset)

expression/builtin_string_vec.go

@@ -685,7 +685,6 @@ func (b *builtinConvertSig) vecEvalString(input *chunk.Chunk, result *chunk.Colu
 	if encoding == nil {
 		return errUnknownCharacterSet.GenWithStackByArgs(b.tp.Charset)
 	}
-	encoder := encoding.NewEncoder()
 	decoder := encoding.NewDecoder()
 	isBinaryStr := types.IsBinaryStr(b.args[0].GetType())
 	isRetBinary := types.IsBinaryStr(b.tp)
@@ -702,13 +701,12 @@ func (b *builtinConvertSig) vecEvalString(input *chunk.Chunk, result *chunk.Colu
 		}
 		exprI := expr.GetString(i)
 		if isBinaryStr {
-			target, _, err := transform.String(encoder, exprI)
+			target, _, err := transform.String(decoder, exprI)
 			if err != nil {
-				return err
+				result.AppendNull()
+				continue
 			}
-			// we should convert target into utf8 internal.
-			exprInternal, _, _ := transform.String(decoder, target)
-			result.AppendString(exprInternal)
+			result.AppendString(target)
 		} else {
 			if isRetBinary {
 				str, err := enc.EncodeString(exprI)