release-19.1: opt: improve per-ON condition cost adjustment for lookup join #39028
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justinj
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complete! 1 of 0 LGTMs obtained (waiting on @justinj, @RaduBerinde, and @rytaft)
pkg/sql/opt/xform/testdata/coster/join, line 395 at r1 (raw file):
opt SELECT w, x, y, z FROM wxyzijklmn
how come this is different from the original commit?
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pkg/sql/opt/xform/testdata/coster/join, line 395 at r1 (raw file): Previously, justinj (Justin Jaffray) wrote…
How is it different? |
justinj
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pkg/sql/opt/xform/testdata/coster/join, line 395 at r1 (raw file):
Previously, RaduBerinde wrote…
How is it different?
oops, I was looking at the first query in the original commit and this one in this commit, ignore me!
Issue cockroachdb#34810 tracks taking into account the internal row count of lookup joins. We currently have a hack in place to always prefer looking indexes that constrain more columns. We encountered a case where this adjustment doesn't work: when the estimated row count is very very small (which happens when there are a lot of conditions), the per-row cost adjustment ends up not making a difference (this is because of limited floating point precision, and the "tolerance" built into `Cost.Less()`). To address this, we also add a constant per-ON condition cost which isn't scaled by the row count. Release note (bug fix): Fixed bug in the optimizer causing a bad index for lookup join in some cases.
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Backport 1/1 commits from #39016.
/cc @cockroachdb/release
Issue #34810 tracks taking into account the internal row count of
lookup joins. We currently have a hack in place to always prefer
looking indexes that constrain more columns (see #35587). We encountered a case
where this adjustment doesn't work: when the estimated row count is
very very small (which happens when there are a lot of conditions),
the per-row cost adjustment ends up not making a difference (this is
because of limited floating point precision, and the "tolerance" built
into
Cost.Less()). To address this, we also add a constant per-ONcondition cost which isn't scaled by the row count.
Release note (bug fix): Fixed bug in the optimizer causing a bad index
for lookup join in some cases.