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stat4 gathers sample values and sample cardianlities for each index. This helps the sqlite planner choose better plans. Problems can still arise though. One such case exists for terabugs: the issueLabel -> issue join. `SELECT * FROM issueLabel WHERE label_id = xx` If `xx` is not present in stat4, sqlite falls back to an average. The average is roughly the number of unsampled rows divided by the number of rows in the table. If there is a heavy skew, this average can be way off. In terabugs we see it as being 10x off in cases. This PR gathers more stats to get better averages for the case a sample is missing.
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stat4 gathers sample values and sample cardianlities for each index.
This helps the sqlite planner choose better plans.
Problems can still arise though. One such case exists for terabugs:
the issueLabel -> issue join.
SELECT * FROM issueLabel WHERE label_id = xxIf
xxis not present in stat4, sqlite falls back to an average. The average is roughly the number of unsampled rows divided by the number of rows in the table.If there is a heavy skew, this average can be way off. In terabugs we see it as 10x off in cases.
This PR gathers more stats to get better averages for the case a sample is missing.