plan: improve row count estimation using column order correlation #9839
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eurekaka
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winoros
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## master #9839 +/- ##
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| @@ -44,18 +44,3 @@ IndexLookUp_11 0.00 root | |||
| ├─IndexScan_8 1.00 cop table:outdated_statistics, index:a, b, range:[1 1,1 1], keep order:false | |||
| └─Selection_10 0.00 cop eq(test.outdated_statistics.c, 1) | |||
| └─TableScan_9 1.00 cop table:outdated_statistics, keep order:false | |||
| CREATE TABLE `unknown_correlation` ( | |||
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It's better to keep this test, or move it to another file. In order to discover plan regression.
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This test is moved to TestLimitCrossEstimation.
| for _, col := range cols { | ||
| hist, ok := histColl.Columns[col.ID] | ||
| if !ok { | ||
| return nil, 0 |
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Why breaking here? Should we continue to compare other columns?
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If we don't know correlation of a column, we assume they are independent, so the whole condition should be treated as independent?
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What problem does this PR solve?
Fix #9067
What is changed and how it works?
When estimating row count for TableScan with Limit pushed down:
tidb_opt_correlation_thresholdandtidb_opt_correlation_exp_factor;Check List
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