parser: fix Text() in SubSelect parse (pingcap#5718)

ex. "SELECT 1 > (select 1)“ , ast.SubqueryExpr#Query's Text() will return "(select ". by mistake.

parser/parser.y

@@ -4366,15 +4366,15 @@ SubSelect:
 		parser.setLastSelectFieldText(s, endOffset)
 		src := parser.src
 		// See the implementation of yyParse function
-		s.SetText(src[yyS[yypt-1].offset-1:yyS[yypt].offset-1])
+		s.SetText(src[yyS[yypt-1].offset:yyS[yypt].offset])
 		$$ = &ast.SubqueryExpr{Query: s}
 	}
 |	'(' UnionStmt ')'
 	{
 		s := $2.(*ast.UnionStmt)
 		src := parser.src
 		// See the implementation of yyParse function
-		s.SetText(src[yyS[yypt-1].offset-1:yyS[yypt].offset-1])
+		s.SetText(src[yyS[yypt-1].offset:yyS[yypt].offset])
 		$$ = &ast.SubqueryExpr{Query: s}
 	}
 

parser/parser_test.go

@@ -1754,6 +1754,25 @@ func (s *testParserSuite) TestCommentErrMsg(c *C) {
 	s.RunErrMsgTest(c, table)
 }
 
+type subqueryChecker struct {
+	text string
+	c    *C
+}
+
+// Enter implements ast.Visitor interface.
+func (sc *subqueryChecker) Enter(inNode ast.Node) (outNode ast.Node, skipChildren bool) {
+	if expr, ok := inNode.(*ast.SubqueryExpr); ok {
+		sc.c.Assert(expr.Query.Text(), Equals, sc.text)
+		return inNode, true
+	}
+	return inNode, false
+}
+
+// Leave implements ast.Visitor interface.
+func (sc *subqueryChecker) Leave(inNode ast.Node) (node ast.Node, ok bool) {
+	return inNode, true
+}
+
 func (s *testParserSuite) TestSubquery(c *C) {
 	defer testleak.AfterTest(c)()
 	table := []testCase{
@@ -1773,6 +1792,23 @@ func (s *testParserSuite) TestSubquery(c *C) {
 		{"SELECT - NOT EXISTS (select 1)", false},
 	}
 	s.RunTest(c, table)
+
+	tests := []struct {
+		input string
+		text  string
+	}{
+		{"SELECT 1 > (select 1)", "select 1"},
+		{"SELECT 1 > (select 1 union select 2)", "select 1 union select 2"},
+	}
+	parser := New()
+	for _, t := range tests {
+		stmt, err := parser.ParseOneStmt(t.input, "", "")
+		c.Assert(err, IsNil)
+		stmt.Accept(&subqueryChecker{
+			text: t.text,
+			c:    c,
+		})
+	}
 }
 func (s *testParserSuite) TestUnion(c *C) {
 	defer testleak.AfterTest(c)()