planner: support subquery in SHOW statement

[eurekaka]

What problem does this PR solve?

Before this PR:

mysql> create table tbl(a int, b int);
Query OK, 0 rows affected (0.03 sec)

mysql> show columns from tbl;
+-------+---------+------+------+---------+-------+
| Field | Type    | Null | Key  | Default | Extra |
+-------+---------+------+------+---------+-------+
| a     | int(11) | YES  |      | NULL    |       |
| b     | int(11) | YES  |      | NULL    |       |
+-------+---------+------+------+---------+-------+
2 rows in set (0.01 sec)

mysql> show columns from tbl where field in (select 'a');
ERROR 2013 (HY000): Lost connection to MySQL server during query

What is changed and how it works?

The panic is caused by the fact that we didn't consider the possible subquery in the filter of show statement, if that happens, the rewritten expression would be nil, and hence panic would be raised in ResolveIndices.

This PR solves this panic by:

• change Show from Plan implementation to PhysicalPlan implementation;
• treat Show as a kind of data source, and build physical operators such as aggregation / join on top of it;
• add an additional projection on top of the built physical plan, to keep the output fields consistent with normal show statement;

After this PR:

mysql> show columns from t where field in (select 'b');
+-------+---------+------+------+---------+-------+
| Field | Type    | Null | Key  | Default | Extra |
+-------+---------+------+------+---------+-------+
| b     | int(11) | YES  |      | NULL    |       |
+-------+---------+------+------+---------+-------+

Check List

Tests

• Unit test

Code changes

N/A

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

[codecov]

Codecov Report

Merging #10942 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #10942   +/-   ##
===========================================
  Coverage   81.6603%   81.6603%           
===========================================
  Files           423        423           
  Lines         91152      91152           
===========================================
  Hits          74435      74435           
  Misses        11486      11486           
  Partials       5231       5231

[eurekaka]

/run-all-tests

[eurekaka]

/run-unit-test

[eurekaka]

/run-all-tests

[eurekaka]

/run-unit-test

[alivxxx]

LGTM

[winoros]

lgtm

[zz-jason]

LGTM