fix unicode support related bugs in s3select (#7877)

pkg/s3select/sql/stringfuncs_test.go

@@ -105,3 +105,27 @@ func TestEvalSQLLike(t *testing.T) {
 		}
 	}
 }
+
+func TestEvalSQLSubstring(t *testing.T) {
+	evalCases := []struct {
+		s           string
+		startIdx    int
+		length      int
+		resExpected string
+		errExpected error
+	}{
+		{"abcd", 1, 1, "a", nil},
+		{"abcd", -1, 1, "a", nil},
+		{"abcd", 999, 999, "", nil},
+		{"", 999, 999, "", nil},
+		{"测试abc", 1, 1, "测", nil},
+		{"测试abc", 5, 5, "c", nil},
+	}
+
+	for i, tc := range evalCases {
+		res, err := evalSQLSubstring(tc.s, tc.startIdx, tc.length)
+		if res != tc.resExpected || err != tc.errExpected {
+			t.Errorf("Eval Case %d failed: %v %v", i, res, err)
+		}
+	}
+}