FR: Option to have all queries low priority

[morgo]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Currently, it is possible to explicitly set individual queries to either low or high priority:
https://github.com/pingcap/docs/blob/master/FAQ.md#how-to-control-or-change-the-execution-priority-of-sql-commits

I would like to have an option to be able to set all queries from a TiDB server to Low Priority.

2. What did you expect to see?

An option. i.e. tidb_force_priority = LOW_PRIORITY.

3. What did you see instead?

Individual queries require hints.

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

MySQL [(none)]> SELECT tidb_version()\G                                                                                                                                                                                   *************************** 1. row ***************************
tidb_version(): Release Version: v2.1.0-rc.1-13-g1255973
Git Commit Hash: 1255973764f72575d71920b95fc0861d1677c6c5
Git Branch: master
UTC Build Time: 2018-08-28 07:35:57
GoVersion: go version go1.10.2 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)

[shenli]

@jackysp We need to add an option in the configure file to make the option persistent.

[jackysp]

@shenli , the current implementation could make the effect persistent for the TiDB server. Do we need to add one more option in the configure file?

[shenli]

If the tidb instance is rebooted, does the varible still take effort?

[jackysp]

No, but there is rarely to specify a configuration file for a TiDB server. The deployment tools could not support it. The user needs to change the configuration file manually. If there is such a requirement, we can add it.

[morgo]

In MySQL 8.0, such a feature is called SET PERSIST.

[jackysp]

@morgo , it is really a very useful feature. We could try to support it.

[morgo]

Yes, it has been really popular. There were a couple of additions before 8.0 went GA: SET PERSIST_ONLY and RESET PERSIST. I think in TiDB's case a logical extension would be to set persistent configuration for the whole cluster (using pd to distribute).