planner: Exists subquery to join variable

[terry1purcell]

What problem does this PR solve?

Issue Number: ref #44850

Problem Summary:

What changed and how does it work?

Create global/session variable tidb_opt_enable_semi_join_rewrite to allow users to control semi join rewrite of EXISTS correlated subqueries without needing the hint.

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.

CREATE TABLE `t1` (
  `a` int,
  `b` int,
  `c` int,
  Primary KEY `ia` (`a`),
  KEY `ib` (`b`)
);

CREATE TABLE `t2` (
  `a` int,
  `b` int,
  `c` int,
  Unique KEY `ia` (`a`),
  KEY `ib` (`b`)
);
set @@cte_max_recursion_depth=10000000;
INSERT INTO t1 (a, b, c)
SELECT a, mod(a, 1000) AS b, mod(a, 10) AS c
FROM (
    WITH RECURSIVE x AS (
        SELECT 1 AS a
        UNION ALL
        SELECT a + 1 AS a
        FROM x
        WHERE a < 1000000
    )
    SELECT a
    FROM x
) AS subquery;
Insert into t2 select * from t1;
Analyze table t1, t2;

To test before change, and with hint:

explain analyze select count(*) from t1 where exists (select * from t2 where t1.b = t2.b and t2.a < 100);
explain analyze select t1.* from t1 where exists (select /*+ SEMI_JOIN_REWRITE() */ * from t2 where t1.b = t2.b and t2.a < 100);

Test with new variable:

set tidb_opt_enable_semi_join_rewrite=ON;
explain analyze select count(*) from t1 where exists (select * from t2 where t1.b = t2.b and t2.a < 100);

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

[tiprow]

Hi @terry1purcell. Thanks for your PR.

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[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.2548%. Comparing base (d022959) to head (45b41a8).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #63416        +/-   ##
================================================
+ Coverage   72.7831%   73.2548%   +0.4717%     
================================================
  Files          1835       1835                
  Lines        496868     497013       +145     
================================================
+ Hits         361636     364086      +2450     
+ Misses       113266     111055      -2211     
+ Partials      21966      21872        -94     

Flag	Coverage Δ
integration	42.0445% <68.4210%> (?)
unit	72.3744% <89.4736%> (+0.0540%)	⬆️

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	46.5294% <ø> (+0.0355%)	⬆️

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[Copilot]

Pull Request Overview

This PR introduces a new global/session variable tidb_opt_enable_semi_join_rewrite to control the automatic rewriting of EXISTS correlated subqueries to inner joins with aggregation, similar to what the SEMI_JOIN_REWRITE() hint already provides.

• Adds session variable tidb_opt_enable_semi_join_rewrite to enable semi-join rewrite optimization without requiring the hint
• Updates semi-join cardinality estimation logic to improve accuracy for semi-join operations
• Modifies plan generation and binding logic to support the new variable

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/sessionctx/vardef/tidb_vars.go	Defines the new variable constant and default value
pkg/sessionctx/variable/sysvar.go	Registers the new system variable with session setter
pkg/sessionctx/variable/session.go	Adds EnableSemiJoinRewrite field to SessionVars struct
pkg/planner/core/rule_semi_join_rewrite.go	Updates rewrite logic to check both hint and session variable
pkg/planner/core/logical_plan_builder.go	Enables semi-join rewrite when session variable is set
pkg/planner/core/operator/logicalop/logical_join.go	Improves semi-join cardinality estimation algorithm
pkg/planner/core/expression_rewriter.go	Adds comment about variable usage in semi-join context
pkg/bindinfo/	Updates binding plan generation to handle new variable
tests/integrationtest/r/planner/core/casetest/physicalplantest/physical_plan.result	Updated test expectations

---

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-09-10 01:11:57.684714119 +0000 UTC m=+410184.244595606: ☑️ agreed by qw4990.
• 2025-09-10 02:48:26.034447438 +0000 UTC m=+415972.594328935: ☑️ agreed by hawkingrei.

[hawkingrei]

/ok-to-test

[terry1purcell]

/retest-required

[hawkingrei]

/retest

[hawkingrei]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, qw4990, yudongusa

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

The pull request process is described here

Needs approval from an approver in each of these files:

• pkg/bindinfo/OWNERS [hawkingrei,qw4990]
• pkg/planner/OWNERS [hawkingrei,qw4990]
• 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

[hawkingrei]

/retest

[hawkingrei]

/retest

[terry1purcell]

/cherry-pick release-8.5

[ti-chi-bot]

@terry1purcell: new pull request created to branch release-8.5: #63540.
But this PR has conflicts, please resolve them!

In response to this:

/cherry-pick release-8.5

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