session,executor: fix point get under @@tidb_snapshot (#22460)

[ti-srebot]

cherry-pick #22460 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/22527

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/22527:release-4.0-8d6ddedfb101

---

What problem does this PR solve?

Issue Number: close #22436

Problem Summary:

When the @@tidb_snapshot variable is set, a point get request still gets the latest data rather than a snapshot.

What is changed and how it works?

What's Changed:

• In PrepareTSFuture, if sessionVars.SnapshotTS is not 0, do nothing
• In buildExecutor, check sessionVars.SnapshotTS before useMaxTS

How it Works:

In buildExecutor, the useMaxTS logic have a higher priority than sessionVars.SnapshotTS,
so even the @@tidb_snapshot variable is set, the MaxTS is used by point get query.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Release note

• Fix a bug that point get query does not get the snapshot data when the @@tidb_snapshot variable is set.

[ti-srebot]

/run-all-tests

[ti-srebot]

@tiancaiamao you're already a collaborator in bot's repo.

[tiancaiamao]

The original test case is OK on 4.0, but we still need to cherry-pick the PR for this bug:

mysql> create table t (id int key);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into t values (3);
Query OK, 1 row affected (0.00 sec)

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2021-01-26 11:51:06 |
+---------------------+
1 row in set (0.00 sec)

mysql> insert into t values (7);
Query OK, 1 row affected (0.00 sec)

mysql> set @@tidb_snapshot = '2021-01-26 11:51:06';
Query OK, 0 rows affected (0.01 sec)

// This one is OK
mysql> select * from t where id = 7;
Empty set (0.00 sec)

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

// But this one is not correct.
mysql> select * from t where id = 7;
+----+
| id |
+----+
|  7 |
+----+
1 row in set (0.00 sec)

PTAL @lysu @AilinKid

[AilinKid]

LGTM

[lysu]

LGTM

[lysu]

/approve

[qw4990]

LGTM

[qw4990]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[qw4990]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

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[ti-srebot]

@ti-srebot merge failed.

[qw4990]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

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[qw4990]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 21945
• 22136
• 22119
• 21853
• 22174
• 21881

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[ti-srebot]

/run-all-tests

[qw4990]

/run-all-tests