planner: make sure mpp join task's hashCols are all contained of its plan's schema

[yibin87]

What problem does this PR solve?

Issue Number: close #52828

Problem Summary:

1. In planner: Column prune improvement for MPP Join and TableScan+Filter operators #52143, new projections might be added above mpp join operators to prune useless columns. However, each mpp task has an attribute named "hashCols" which are designed to eliminate useless exchange nodes, and these columns may not be contained in pruned columns.

tidb/pkg/planner/core/task.go

Lines 523 to 546 in 3cfea6a

	// outer task is the task that will pass its MPPPartitionType to the join result
	// for broadcast inner join, it should be the non-broadcast side, since broadcast side is always the build side, so
	// just use the probe side is ok.
	// for hash inner join, both side is ok, by default, we use the probe side
	// for outer join, it should always be the outer side of the join
	// for semi join, it should be the left side(the same as left out join)
	outerTaskIndex := 1 - p.InnerChildIdx
	if p.JoinType != InnerJoin {
	if p.JoinType == RightOuterJoin {
	outerTaskIndex = 1
	} else {
	outerTaskIndex = 0
	}
	}
	// can not use the task from tasks because it maybe updated.
	outerTask := lTask
	if outerTaskIndex == 1 {
	outerTask = rTask
	}
	task := &mppTask{
	p: p,
	partTp: outerTask.partTp,
	hashCols: outerTask.hashCols,
	}

For example:
select A.id from A join B on A.id = B.id; Suppose B is probe side, and it's hash inner join.
After logical column pruning, the output schema of A join B will be A.id only; while the task's hashCols will be B.id. To make matters worse, the hashCols may be used to check if extra cast projection needs to be added, then the newly added projection will expect B.id as input schema. So this PR makes sure hashCols are included in task.p's schema.

2. In physical projection elimination, when a projection is above another projection and satisfy certain conditions, we'll remove current projection and use current projection's schema to reset child projection. However,
   empty projection(which schema is totally pruned) with projection child, will set child projection's schema to empty, which doesn't make sense. In such situation, keep child projection's schema seems more reasonable.

What changed and how does it work?

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

[tiprow]

Hi @yibin87. Thanks for your PR.

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[yibin87]

/cc @windtalker @winoros

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.9880%. Comparing base (4354682) to head (111abf3).
Report is 31 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #52836         +/-   ##
=================================================
- Coverage   72.3201%   55.9880%   -16.3322%     
=================================================
  Files          1474       1589        +115     
  Lines        427611     602598     +174987     
=================================================
+ Hits         309249     337383      +28134     
- Misses        99057     242136     +143079     
- Partials      19305      23079       +3774     

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Components	Coverage Δ
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[yibin87]

/test check-dev

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[winoros]

So LGTM currently.
But you could add a TODO for our planner side.
The task.HashCols should be stored in the p.schema. i.e. Planner should maintain it instead of letting you re-add it here.

[yibin87]

/hold

[yibin87]

/test pull-mysql-client-test

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[yibin87]

/unhold

[yibin87]

/cc @SeaRise

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[APPROVALNOTIFIER] This PR is APPROVED

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[yibin87]

/test mysql-test

[yibin87]

/test pull-integration-ddl-test

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[yibin87]

/test check-dev2

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[yibin87]

/hold

[yibin87]

/unhold

[winoros]

/cherrypick release-8.1

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@winoros: new pull request created to branch release-8.1: #55109.

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/cherrypick release-8.1

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[yibin87]

/cherrypick release-7.5

[yibin87]

/cherrypick release-7.1

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@yibin87: new pull request created to branch release-7.5: #55654.

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/cherrypick release-7.5

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[ti-chi-bot]

@yibin87: new pull request created to branch release-7.1: #55655.

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/cherrypick release-7.1

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