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planner: add maybe good heuristics for index selection #26850

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wants to merge 26 commits into from
Closed

planner: add maybe good heuristics for index selection #26850

wants to merge 26 commits into from

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xuyifangreeneyes
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What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What is changed and how it works?

When SQL has order by pk limit n pattern, and one index path only has point ranges and its CountAfterAccess is small enough, we directly choose the index rather than pk. Since the heuristic rule is not always good, we introduce a global/session variable tidb_enable_maybe_good_heuristics and only apply the rule when the switch turns on. When executing EXPLAIN, TiDB will output a NOTE level warning if some path matches the maybe good heuristic rule.

Check List

Tests

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

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 

introduce maybe good heuristics for index selection

xuyifangreeneyes and others added 23 commits July 14, 2021 16:32
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@ti-chi-bot ti-chi-bot added release-note Denotes a PR that will be considered when it comes time to generate release notes. size/XL Denotes a PR that changes 500-999 lines, ignoring generated files. labels Aug 3, 2021
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/sig planner

@ti-chi-bot ti-chi-bot added the sig/planner SIG: Planner label Aug 3, 2021
@github-actions github-actions bot added the sig/execution SIG execution label Aug 3, 2021
@ti-chi-bot ti-chi-bot added the needs-rebase Indicates a PR cannot be merged because it has merge conflicts with HEAD. label Aug 8, 2021
@ti-chi-bot ti-chi-bot added size/L Denotes a PR that changes 100-499 lines, ignoring generated files. and removed needs-rebase Indicates a PR cannot be merged because it has merge conflicts with HEAD. size/XL Denotes a PR that changes 500-999 lines, ignoring generated files. labels Aug 9, 2021
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/cc @winoros

@ti-chi-bot ti-chi-bot requested a review from winoros August 9, 2021 02:44
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Since #27123 can also make the optimizer choose index rather than pk for the situations like where index_col = ... order by pk limit n, the pr can be closed.

@xuyifangreeneyes xuyifangreeneyes deleted the maybe-good-heuristics branch October 30, 2021 05:07
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