opt: fix distinct count estimation for outer joins #38229
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Prior to this commit, it was possible for the distinct count of a column in an outer join to be zero when the row count was non-zero. Since we require that the distinct count is always non-zero when the row count is non-zero, this caused an error. This commit fixes the issue by ensuring that there is always at least one distinct value for each column in an outer join when the row count is non-zero. Fixes cockroachdb#38091 Release note: None
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justinj
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Jun 17, 2019
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complete! 1 of 0 LGTMs obtained (waiting on @justinj, @RaduBerinde, and @rytaft)
pkg/sql/opt/memo/testdata/stats/join, line 1125 at r1 (raw file):
│ │ ├── left-join │ │ │ ├── outer: (2) │ │ │ ├── stats: [rows=1000, distinct(2)=1, null(2)=0]
Did this distinct count just happen to get computed during optimization? Is that why it's the only one that is showing up?
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pkg/sql/opt/memo/testdata/stats/join, line 1125 at r1 (raw file): Previously, justinj (Justin Jaffray) wrote…
Yea - it gets computed because column 2 is part of the join filter condition, and we need to know the distinct count to estimate selectivity. |
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TFTR! bors r+ |
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38229: opt: fix distinct count estimation for outer joins r=rytaft a=rytaft Prior to this commit, it was possible for the distinct count of a column in an outer join to be zero when the row count was non-zero. Since we require that the distinct count is always non-zero when the row count is non-zero, this caused an error. This commit fixes the issue by ensuring that there is always at least one distinct value for each column in an outer join when the row count is non-zero. Fixes #38091 Release note: None Co-authored-by: Rebecca Taft <becca@cockroachlabs.com>
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Prior to this commit, it was possible for the distinct count of a
column in an outer join to be zero when the row count was non-zero.
Since we require that the distinct count is always non-zero when the
row count is non-zero, this caused an error. This commit fixes the
issue by ensuring that there is always at least one distinct value
for each column in an outer join when the row count is non-zero.
Fixes #38091
Release note: None