value of tidb_sysdate_is_now seems cached in expression in planCache and not respect the current setting

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

TiDB root@127.0.0.1:test> create table t (a int);
Query OK, 0 rows affected
Time: 0.088s
TiDB root@127.0.0.1:test> insert into t values(1);
Query OK, 1 row affected
Time: 0.005s
TiDB root@127.0.0.1:test> set @@tidb_sysdate_is_now=0;
Query OK, 0 rows affected
Time: 0.000s
TiDB root@127.0.0.1:test> prepare s from "select sleep(a), now(6), sysdate(6), sysdate(6)=now(6) from t";
Query OK, 0 rows affected
Time: 0.001s
TiDB root@127.0.0.1:test> execute s;
+----------+----------------------------+----------------------------+-------------------+
| sleep(a) | now(6)                     | sysdate(6)                 | sysdate(6)=now(6) |
+----------+----------------------------+----------------------------+-------------------+
| 0        | 2023-12-08 17:55:36.618077 | 2023-12-08 17:55:36.618089 | 0                 |
+----------+----------------------------+----------------------------+-------------------+
1 row in set
Time: 1.009s
TiDB root@127.0.0.1:test> set @@tidb_sysdate_is_now=1;
Query OK, 0 rows affected
Time: 0.001s
TiDB root@127.0.0.1:test> execute s;
+----------+----------------------------+----------------------------+-------------------+
| sleep(a) | now(6)                     | sysdate(6)                 | sysdate(6)=now(6) |
+----------+----------------------------+----------------------------+-------------------+
| 0        | 2023-12-08 17:55:43.594673 | 2023-12-08 17:55:43.594688 | 0                 |
+----------+----------------------------+----------------------------+-------------------+
1 row in set
Time: 1.013s

2. What did you expect to see? (Required)

It should respect the current value of tidb_sysdate_is_now like no use plan cache:

TiDB root@127.0.0.1:test> set @@tidb_enable_prepared_plan_cache=0;
Query OK, 0 rows affected
Time: 0.001s
TiDB root@127.0.0.1:test> set @@tidb_sysdate_is_now=0;
Query OK, 0 rows affected
Time: 0.001s
TiDB root@127.0.0.1:test> prepare s from "select sleep(a), now(6), sysdate(6), sysdate(6)=now(6) from t";
Query OK, 0 rows affected
Time: 0.001s
TiDB root@127.0.0.1:test> execute s;
+----------+----------------------------+----------------------------+-------------------+
| sleep(a) | now(6)                     | sysdate(6)                 | sysdate(6)=now(6) |
+----------+----------------------------+----------------------------+-------------------+
| 0        | 2023-12-08 17:59:51.246963 | 2023-12-08 17:59:52.248985 | 0                 |
+----------+----------------------------+----------------------------+-------------------+
1 row in set

TiDB root@127.0.0.1:test> set @@tidb_sysdate_is_now=1;
Query OK, 0 rows affected
Time: 0.001s
TiDB root@127.0.0.1:test> execute s;
+----------+----------------------------+----------------------------+-------------------+
| sleep(a) | now(6)                     | sysdate(6)                 | sysdate(6)=now(6) |
+----------+----------------------------+----------------------------+-------------------+
| 0        | 2023-12-08 18:00:28.162819 | 2023-12-08 18:00:28.162819 | 1                 |
+----------+----------------------------+----------------------------+-------------------+
1 row in set

3. What did you see instead (Required)

The sysdate(6) = now(6) is false even we change tidb_sysdate_is_now to 1 when plan cache enabled.

4. What is your TiDB version? (Required)

Tested in nightly build

TiDB root@127.0.0.1:test> select tidb_version();
+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v7.6.0-alpha                             |
| Edition: Community                                        |
| Git Commit Hash: b74d64c7a48189f00fbae0c29ad1435b60b8c51d |
| Git Branch: heads/refs/tags/v7.6.0-alpha                  |
| UTC Build Time: 2023-12-07 14:25:39                       |
| GoVersion: go1.21.4                                       |
| Race Enabled: false                                       |
| Check Table Before Drop: false                            |
| Store: tikv                                               |
+-----------------------------------------------------------+
1 row in set