ast: reset JoinLevel at the beginning of TableRefsClause.Restore

[Hexilee]

Signed-off-by: hexilee i@hexilee.me

What problem does this PR solve?

Currently, format.RestoreCtx.JoinLevel does not work well in the multi-level subquery.
If you parse the following SQL and restore it, you will get unnecessary pairs of parentheses in the inner subquery.

-- origin
SELECT * FROM (SELECT * FROM (SELECT * FROM `t1`) AS `tmp`) AS `tmp`

-- restored
SELECT * FROM (SELECT * FROM ((SELECT * FROM (`t1`)) AS `tmp`)) AS `tmp`

Unnecessary parentheses can be ignored by tidb. However, in mysql, it will cause a syntax error.

mysql 8.0.21

mysql root@127.0.0.1:test> SELECT * FROM (SELECT * FROM ((SELECT * FROM (`t1`)) AS `tmp`)) AS `tmp`;
(1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')) AS `tmp`' at line 1")

What is changed and how it works?

This PR resets JoinLevel at the beginning of TableRefsClause.Restore, then JoinLevel will never affect subqueries.

Check List

Tests

• Unit test

Code changes

• Has interface methods change

[kennytm]

The culprit is actually the Join structure being used to represent non-JOINs. When the Join does not represent a real JOIN the JoinLevel should not be changed. Please change the patch to

diff --git a/ast/dml.go b/ast/dml.go
index f6e4a1d..ac33cf8 100644
--- a/ast/dml.go
+++ b/ast/dml.go
@@ -91,12 +91,15 @@ func (n *Join) Restore(ctx *format.RestoreCtx) error {
                ctx.WritePlain("(")
                defer ctx.WritePlain(")")
        }
-       ctx.JoinLevel++
+       if n.Right != nil {
+               ctx.JoinLevel++
+       }
        if err := n.Left.Restore(ctx); err != nil {
                return errors.Annotate(err, "An error occurred while restore Join.Left")
        }
-       ctx.JoinLevel--
-       if n.Right == nil {
+       if n.Right != nil {
+               ctx.JoinLevel--
+       } else {
                return nil
        }
        if n.NaturalJoin {

also move the test case to parser_test.go or somewhere that tests a complete statement. TestTableRefsClauseRestore is not the correct function to insert this test.

[Hexilee]

I don't think so. The following example also causes a syntax error in mysql.

-- origin
SELECT * FROM (SELECT * FROM (SELECT * FROM `t1`) AS `tmp`) AS `tmp`, t

-- restored
SELECT * FROM (SELECT * FROM ((SELECT * FROM (`t1`)) AS `tmp`)) AS `tmp`, t

The culprit is the JoinLevel affects subqueries.