planner: fix agg elimination logic after agg pushed down through a join (#44941)

[ti-chi-bot]

This is an automated cherry-pick of #44941

What problem does this PR solve?

Issue Number: close #44795

Problem Summary:

What is changed and how it works?

code in aggregationPushDownSolver for the switch p.SCtx().GetSessionVars().AllowAggPushDown path is too old to use.

briefly speaking, when we try to push agg down through a join (that's what aggregationPushDownSolver does) and try to combine some aggregation elimination (for example, group item covering unique key, then the aggregation itself can be eliminated):

buildKeyInfo(join)
proj := a.tryToEliminateAggregation(agg, opt). // here use an old pointer, whose child is already changed during the logic above
if proj != nil {
	p = proj
}

the comment place is actually to eliminate the new pushed-down agg since agg's children have changed, maybe some new unique key can be detected to eliminate itself. Or we should say, the old agg elimination logic is quite different from normal agg eliminations (see comments for more detail)

old agg rewriting still uses the ifnull(col#19, 0, 1)'s logic, taking every row as a count 1, while after the agg has been pushed down, the col#19 here is already the final aggregation result, rather than in the process of aggregation. Keep it real is the true path, while for now, we banned this kind of old agg elimination logic.

so the handling logic is a mess.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  scripts:

sql = $below

SELECT c_count, count(*) as custdist
from ( SELECT c_custkey, count(o_orderkey)  as  c_count
       from customer left join orders on c_custkey = o_custkey and o_comment not like '%special%requests%'
       group by c_custkey ) c_orders
group by c_count
order by custdist desc, c_count desc;

execute $sql + "into outfile 'tai1.txt'" under set tidb_opt_agg_push_down=ON;
execute $sql + "into outfile 'tai2txt'" under set tidb_opt_agg_push_down=OFF;

diff tai1.txt tai2.txt to see nothing strange

• 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

Please refer to Release Notes Language Style Guide to write a quality release note.

planner: fix agg elimination logic after agg pushed down to join

[AilinKid]

/retest

[elsa0520]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: elsa0520, time-and-fate

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-14 09:44:49.764816296 +0000 UTC m=+537854.313832278: ☑️ agreed by elsa0520.
• 2023-08-14 10:04:38.815424745 +0000 UTC m=+539043.364440716: ☑️ agreed by time-and-fate.

[codecov]

Codecov Report

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Additional details and impacted files

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