executor: support show create sequence

[AilinKid]

What problem does this PR solve?

Support show create sequence syntax.
After this PR, It goes like as follows:

mysql> create sequence seq;
Query OK, 0 rows affected (0.01 sec)

mysql> show create sequence seq;
+-------+----------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                               |
+-------+----------------------------------------------------------------------------------------------------------------------------+
| seq   | CREATE SEQUENCE `seq` start with 1 minvalue 1 maxvalue 9223372036854775806 increment by 1 cache 1000 nocycle ENGINE=InnoDB |
+-------+----------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

What is changed and how it works?

1. add the fetchShowCreateSequence logic in show.go
2. append the ast.ShowCreateSequence case to the ast.ShowCreateTable case in planbuilder.go (in most case, sequence are treated as a table)
3. add some show create sequence test in sequence_test.go

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to update the documentation

Release note

• Support the show create sequence in TiDB.

[AilinKid]

/run-unit-test

[AilinKid]

/build

[AilinKid]

/build

[crazycs520]

BTW

test> show create sequence tt
(1146, u"Table 'test.tt' doesn't exist")  -- shoud be Sequence xxx doesn't exist ?

[AilinKid]

BTW

test> show create sequence tt
(1146, u"Table 'test.tt' doesn't exist")  -- shoud be Sequence xxx doesn't exist ?

It behaviors like MariaDB... I think it's temporary ok here.

[XuHuaiyu]

The rest of the code almost LGTM.

[AilinKid]

/build

[crazycs520]

LGTM

[crazycs520]

/merge

[sre-bot]

/run-all-tests