inconsistent result of greatest("2017-02-30", cast("2017-02-02" as date))

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

2. What did you expect to see?

mysql(127.0.0.1)> select greatest(cast("2017-02-02" as date),"2017-02-30");
Field   1:  `greatest(cast("2017-02-02" as date),"2017-02-30")`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8_general_ci (33)
Length:     30
Max_length: 10
Decimals:   31
Flags:


+---------------------------------------------------+
| greatest(cast("2017-02-02" as date),"2017-02-30") |
+---------------------------------------------------+
| 2017-02-30                                        |                                    <====
+---------------------------------------------------+

mysql(127.0.0.1)> select greatest(cast("2017-02-02" as date),cast("2017-02-03" as date));
Field   1:  `greatest(cast("2017-02-02" as date),cast("2017-02-03" as date))`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       DATE   <====
Collation:  utf8_general_ci (33)
Length:     9
Max_length: 10
Decimals:   0
Flags:


+-----------------------------------------------------------------+
| greatest(cast("2017-02-02" as date),cast("2017-02-03" as date)) |
+-----------------------------------------------------------------+
| 2017-02-03                                                      |
+-----------------------------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?

tidb(127.0.0.1)> select greatest(cast("2017-02-02" as date),"2017-02-30");
Field   1:  `greatest(cast("2017-02-02" as date),"2017-02-30")`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8_general_ci (33)
Length:     0
Max_length: 19
Decimals:   31
Flags:


+---------------------------------------------------+
| greatest(cast("2017-02-02" as date),"2017-02-30") |
+---------------------------------------------------+
| 2017-02-02 00:00:00                               |                                <====
+---------------------------------------------------+
1 row in set, 1 warning (0.00 sec)


tidb(127.0.0.1)> select greatest(cast("2017-02-02" as date),cast("2017-02-03" as date));
Field   1:  `greatest(cast("2017-02-02" as date),cast("2017-02-03" as date))`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING        <====
Collation:  utf8_general_ci (33)
Length:     0
Max_length: 19
Decimals:   31
Flags:


+-----------------------------------------------------------------+
| greatest(cast("2017-02-02" as date),cast("2017-02-03" as date)) |
+-----------------------------------------------------------------+
| 2017-02-03 00:00:00                                             |
+-----------------------------------------------------------------+
1 row in set (0.00 sec)

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

Release Version: v2.0.0-rc.4-273-gf6a3c3f
Git Commit Hash: f6a3c3f0a55abf454d352ffa2089d7983fc18307
Git Branch: master
UTC Build Time: 2018-06-04 03:37:54
GoVersion: go version go1.10.1 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.0.0-rc.4.1