DROP TABLE IF EXISTS behavior difference

[morgo]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

tidb> drop table if exists idontexistatall;
Query OK, 0 rows affected (0.00 sec)

2. What did you expect to see?

mysql57> drop table if exists idontexistatall;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql57> show warnings;
+-------+------+--------------------------------------+
| Level | Code | Message                              |
+-------+------+--------------------------------------+
| Note  | 1051 | Unknown table 'test.idontexistatall' |
+-------+------+--------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?

TiDB does not return a warning in the scenario the table didn't exist.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

MySQL [test]> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v2.1.0-rc.3-25-g3104c8723
Git Commit Hash: 3104c87230600e7514b3a7190689ebea30a1d8ee
Git Branch: newmaster
UTC Build Time: 2018-10-10 02:41:43
GoVersion: go version go1.11 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false
1 row in set (0.00 sec)

This bug does not affect any programs to my knowledge, but is a nice to have compatibility issue.

[shenli]

@winkyao PTAL

[zhexuany]

@ciscoxll Please update a timeline to fix this issue.