*: 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
• 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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.7359%. Comparing base (4f42486) to head (5f05ef6).
⚠️ 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%)	⬇️
unit	66.0045% <51.5789%> (+0.3730%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

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[lcwangchao]

/retest

[lcwangchao]

/retest

[lcwangchao]

/retest

[lcwangchao]

/unhold

[bb7133]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

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

[lcwangchao]

/retest

[yudongusa]

Please open a document PR

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

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
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[lcwangchao]

Please open a document PR

It's here pingcap/docs-cn#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!