*: support system variable `tidb_index_lookup_pushdown_policy` and hint `NO_INDEX_LOOKUP_PUSHDOWN`

[lcwangchao]

What problem does this PR solve?

Issue Number: ref #62575

What changed and how does it work?

• Introduce a new system variable tidb_index_lookup_pushdown_policy which has 3 options:

  • hint-only: The default value. Only push down the index lookup with hint INDEX_LOOKUP_PUSHDOWN
  • affinity-force: Force to push down the index lookup for tables with AFFINITY set.
  • force: Force to push down the index lookup for all tables.

• Introduce a new hint NO_INDEX_LOOKUP_PUSHDOWN to disable index lookup for the current statement.

Check List

Tests

• [ ] Unit test
• [x] 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

[lcwangchao]

/hold before table affinity implemented

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 68.7359%. Comparing base (4f42486) to head (5f05ef6). :warning: Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #64932        +/-   ##
================================================
- Coverage   70.8840%   68.7359%   -2.1481%     
================================================
  Files          1889       1867        -22     
  Lines        516054     508009      -8045     
================================================
- Hits         365800     349185     -16615     
- Misses       125811     136536     +10725     
+ Partials      24443      22288      -2155     

Flag	Coverage Δ
integration	41.6476% <100.0000%> (-6.4992%)	:arrow_down:
unit	66.0045% <51.5789%> (+0.3730%)	:arrow_up:

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Components	Coverage Δ
dumpling	52.8700% <ø> (ø)
parser	∅ <ø> (∅)
br	39.1464% <ø> (-20.2237%)	:arrow_down:

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[lcwangchao]

/retest

[lcwangchao]

/retest

[lcwangchao]

/retest

[lcwangchao]

/unhold

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-12-10 14:39:36.657444038 +0000 UTC m=+1051921.471221610: :ballot_box_with_check: agreed by winoros.
• 2025-12-15 03:43:38.222131491 +0000 UTC m=+1444563.035909053: :ballot_box_with_check: agreed by bb7133.

[lcwangchao]

/retest

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bb7133, cfzjywxk, winoros, yudongusa

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The pull request process is described here

Needs approval from an approver in each of these files:

• ~~OWNERS~~ [bb7133,cfzjywxk,winoros]
• ~~pkg/parser/OWNERS~~ [bb7133,yudongusa]
• ~~pkg/planner/OWNERS~~ [winoros]
• ~~pkg/sessionctx/vardef/OWNERS~~ [yudongusa]
• ~~pkg/sessionctx/variable/OWNERS~~ [yudongusa]

Approvers can indicate their approval by writing /approve in a comment Approvers can cancel approval by writing /approve cancel in a comment

[lcwangchao]

Please open a document PR

It's here https://github.com/pingcap/docs-cn/pull/20950, I'll complete it for the new variable and hint

[lcwangchao]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #65035. But this PR has conflicts, please resolve them!