*: make tidb_replica_read work with set_var hint

[crazycs520]

cherry-pick #48473

What problem does this PR solve?

Issue Number: close #48472

Problem Summary: make tidb_read_staleness and tidb_replica_read work with set_var hint

What is changed and how it works?

This PR:

mysql> select /*+ SET_VAR(tidb_replica_read = 'follower')*/ @@tidb_replica_read;
+---------------------+
| @@tidb_replica_read |
+---------------------+
| follower            |
+---------------------+
1 row in set (0.00 sec)

mysql> select  @@tidb_replica_read;
+---------------------+
| @@tidb_replica_read |
+---------------------+
| leader              |
+---------------------+
1 row in set (0.00 sec)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

Hi @crazycs520. Thanks for your PR.

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[Tema]

@crazycs520 thanks!

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@crazycs520 thanks!

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[zyguan]

Setting IsHintUpdatable to true may not actually take effect, the value is cached in SessionVars.stmtVars but we still read from the field (eg SessionVars.ReadStaleness).

[cfzjywxk]

Setting IsHintUpdatable to true may not actually take effect, the value is cached in SessionVars.stmtVars but we still read from the field (eg SessionVars.ReadStaleness).

Seems to be a problem, @crazycs520 PTAL

[hawkingrei]

/retest

[crazycs520]

Setting IsHintUpdatable to true may not actually take effect, the value is cached in SessionVars.stmtVars but we still read from the field (eg SessionVars.ReadStaleness).

Nice catch, Currently the fix is not really work. Since SET_VAR feature is GA on v7.4.0, the SET_VAR implementation is a little different between this branch(v6.5.4) and v7.4.0. So I need another work to make tidb_read_staleness and tidb_replica_read work with set_var hint.

cc @Tema

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, Tema, zyguan

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• OWNERS [cfzjywxk,zyguan]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-01 05:54:30.101712743 +0000 UTC m=+3018867.688822888: ☑️ agreed by cfzjywxk.
• 2023-11-06 08:19:15.789639643 +0000 UTC m=+3459553.376749788: ☑️ agreed by zyguan.

[hawkingrei]

/retest