planner: improve row count estimation of IndexJoin's inner scan

[eurekaka]

What problem does this PR solve?

Currently, the row count estimation of IndexJoin's inner child is not that accurate because:

• we are assuming each outer row would find matches in inner child, in fact this may not hold normally;
• when estimating row count of inner child, we are using Count / NDV, if the index used is composite index, and the join key only covers the prefix of the index, the row count would be smaller than the real count

What is changed and how it works?

Initially, I considered comparing histograms of both child plans to compute an overlapping ratio and using it for row count estimation, but found that it was trivial to handle the mismatch between the data types, and it was hard to compute overlap for composite index cases.

Alternatively, I choose to reuse the estimated row count of join result after evaluating join equal conditions, because leftCnt * rightCnt / max(leftNDV, rightNDV) has already taken the overlap into account more or less, and this approach can give more consistent row count estimations for join operator and its children.

Check List

Tests

• Unit test

Code changes

N/A

Side effects

• Possible performance regression

Related changes

N/A

Release note

• Write release note for bug-fix or new feature.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@440bb74). Click here to learn what that means.
The diff coverage is 100%.

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[eurekaka]

/run-all-tests

[AilinKid]

/run-all-tests

[zyxbest]

/run-unit-test

[winoros]

lgtm

[winoros]

lgtm

[eurekaka]

/bench

[sre-bot]

@@                               Benchmark Diff                               @@
================================================================================
--- tidb: 5c18c5df97d935398ea1b44098a0db3171999466
+++ tidb: 7278ab07104a307f04e72e21d02f6528f8591013
tikv: ff82aa9eba331585aec1c6cdf9e1584512bccb34
pd: ce060a9aeb66d6bbb39159243b879740dffae041
================================================================================
test-1: < oltp_insert >
    * QPS : 21141.44 ± 1.5888% (std=240.80) delta: -0.08%
    * AvgMs : 12.10 ± 1.6193% (std=0.14) delta: 0.08%
    * PercentileMs99 : 42.92 ± 1.0903% (std=0.38) delta: 1.46%
            
test-2: < oltp_update_non_index >
    * QPS : 29436.80 ± 0.2322% (std=47.10) delta: -0.10%
    * AvgMs : 8.69 ± 0.2071% (std=0.01) delta: 0.09%
    * PercentileMs99 : 30.59 ± 1.0788% (std=0.27) delta: 1.09%
            
test-3: < oltp_read_write >
    * QPS : 37051.41 ± 0.3770% (std=80.95) delta: 0.32%
    * AvgMs : 138.73 ± 0.3792% (std=0.31) delta: -0.32%
    * PercentileMs99 : 257.95 ± 0.0000% (std=0.00) delta: 0.00%
            
test-4: < oltp_point_select >
    * QPS : 74734.42 ± 3.2705% (std=1572.71) delta: -0.30%
    * AvgMs : 3.43 ± 3.0940% (std=0.07) delta: 0.40%
    * PercentileMs99 : 7.43 ± 0.0000% (std=0.00) delta: 0.00%
            
test-5: < oltp_update_index >
    * QPS : 16880.32 ± 0.5051% (std=62.94) delta: 0.29%
    * AvgMs : 15.16 ± 0.5012% (std=0.06) delta: -0.29%
    * PercentileMs99 : 48.34 ± 0.0000% (std=0.00) delta: 0.00%
            

https://perf.pingcap.com

[alivxxx]

LGTM
Please resolve the conflicts.

[sre-bot]

Your auto merge job has been accepted, waiting for 12009

[sre-bot]

/run-all-tests