util/stmtsummary: discard the plan if it is too long and enlarge the tidb_stmt_summary_max_stmt_count value to 3000 (#25843)

[ti-srebot]

cherry-pick #25843 to release-5.1
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---

Signed-off-by: crazycs [email protected]

What problem does this PR solve?

1. Currently statements_summary doesn't restrict the plan size, in some extreme scenarios, TIDB might OOM. This PR discards the statements_summary plan if it is too long (encode plan size more than 1MB) to avoid occupies too much memory.

2. Enlarge tidb_stmt_summary_max_stmt_count default value from 200 to 3000. Since 200 is too small when troubleshooting.

What is changed and how it works?

About Enlarge tidb_stmt_summary_max_stmt_count default value, this doc record the test process. The final conclusion is:
Enlarge the tidb_stmt_summary_max_stmt_count default value to 3000:

• If the SQL size is 1024, cache 3000 statements only consume 55MB, memory.
• If the SQL size is 10240, cache 3000 statements only consume 200MB memory.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:

Check List

Tests

• Unit test
• Manual test

Side effects

• Performance regression
  • Consumes more MEM
• Breaking backward compatibility

Release note

• Enlarge the variable tidb_stmt_summary_max_stmt_count default value from 200 to 3000

[ti-srebot]

/run-all-tests

[ti-srebot]

@crazycs520 you're already a collaborator in bot's repo.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• breeswish
• djshow832

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[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 31bf315

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