Results of `unix_timestamp()-unix_timestamp(now())` is wrong and not stable

[qw4990]

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.

select unix_timestamp()-unix_timestamp(now());
select unix_timestamp()-unix_timestamp(now());
select unix_timestamp()-unix_timestamp(now());
select unix_timestamp()-unix_timestamp(now());

2. What did you expect to see?
   In Mysql

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                      0 |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                      0 |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                      0 |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                      0 |
+----------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?
   In TiDB

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                      0 |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                     -1 |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                      0 |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select unix_timestamp()-unix_timestamp(now());
+----------------------------------------+
| unix_timestamp()-unix_timestamp(now()) |
+----------------------------------------+
|                                     -1 |
+----------------------------------------+
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)?

+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                                                     |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v3.0.0-beta-212-g0681b8125
Git Commit Hash: 0681b8125080045ee0d4747ce61923164c6f2dd1
Git Branch: master
UTC Build Time: 2019-03-14 04:39:19
GoVersion: go version go1.12 darwin/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)

[qw4990]

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