planner: fix begin statement consumed read_ts wrongly (#25492)

[ti-srebot]

cherry-pick #25492 to release-5.1
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git pr https://github.com/pingcap/tidb/pull/25520

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---

Signed-off-by: yisaer disxiaofei@163.com

What problem does this PR solve?

The following behavior is not expected.

mysql> set transaction read only as of timestamp '2021-06-16 11:20:00';
Query OK, 0 rows affected (0.05 sec)

mysql> delete from t1 where id=3;
ERROR 1105 (HY000): only support read-only statement during read-only staleness transactions
mysql> commit;
Query OK, 0 rows affected (0.05 sec)

mysql> start transaction read only as of timestamp now()-interval 1 minute;
ERROR 1105 (HY000): start transaction read only as of is forbidden after set transaction read only as of
mysql> start transaction read only as of timestamp now()-interval 1 minute;
Query OK, 0 rows affected (0.05 sec)

What is changed and how it works?

Fix that the read_ts would be consumed wrongly.

Check List

Tests

• Unit test

Release note

• No release note

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[ti-chi-bot]

@xhebox: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

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[Yisaer]

/merge

[ti-chi-bot]

@Yisaer: /merge in this pull request requires 2 approval(s).

In response to this:

/merge

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[Yisaer]

lgtm

[Yisaer]

/sig sql-infra

[ti-chi-bot]

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• Yisaer
• xhebox

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[xhebox]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: f015587