Reminiscent opened a new pull request, #20004:
URL: https://github.com/apache/doris/pull/20004
# Proposed changes
Before the change:
```
mysql> SET enable_nereids_planner=true;
Query OK, 0 rows affected (0.01 sec)
mysql> explain select /*+ SET_var(enable_nereids_planner = false) */
year_floor(cast('2023-04-28' as date));
-- omit the result here
10 rows in set (0.01 sec)
mysql> select @@enable_nereids_planner;
+--------------------------+
| @@enable_nereids_planner |
+--------------------------+
| 0 |
+--------------------------+
1 row in set (0.00 sec)
```
After the change:
```
mysql> SET enable_nereids_planner=true;
Query OK, 0 rows affected (0.01 sec)
mysql> explain select /*+ SET_var(enable_nereids_planner = false) */
year_floor(cast('2023-04-28' as date));
-- omit the result here
10 rows in set (0.14 sec)
mysql> select @@enable_nereids_planner;
+------+
| TRUE |
+------+
| 1 |
+------+
1 row in set (0.25 sec)
```
## Problem summary
We have already recorded the old session vars when we use the `Nereids` to
handle the `set_var` hint.
But after we change the optimizer to the old one, it will handle the
`set_var` hint again. But it has already taken effect before. So the old value
has already changed. But we will use the changed value to overwrite again.
Describe your changes.
We will check the old session var value when we want to record it first. If
there exists the value, just skip it.
## Checklist(Required)
* [ ] Does it affect the original behavior
* [ ] Has unit tests been added
* [ ] Has document been added or modified
* [ ] Does it need to update dependencies
* [ ] Is this PR support rollback (If NO, please explain WHY)
## Further comments
If this is a relatively large or complex change, kick off the discussion at
[d...@doris.apache.org](mailto:d...@doris.apache.org) by explaining why you
chose the solution you did and what alternatives you considered, etc...
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