8000 Merge Join's row count estimation doesn't consider the case that the join key degenerates to other condition · Issue #57691 · pingcap/tidb · GitHub
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Merge Join's row count estimation doesn't consider the case that the join key degenerates to other condition #57691

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winoros opened this issue Nov 25, 2024 · 0 comments
Open

Merge Join's row count estimation doesn't consider the case that the join key degenerates to other condition #57691

winoros opened this issue Nov 25, 2024 · 0 comments
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sig/planner SIG: Planner type/enhancement The issue or PR belongs to an enhancement.

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winoros commented Nov 25, 2024 •
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Enhancement

For example, if we have table t(a int, b int, key a). And the SQL select ... from t t1 join t t2 on t1.a=t2.a and t1.b=t1.b. To use merge join, we'll only regard a as the join key and the t1.b=t2.b will become a normal filter after we finish the join key matching.
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But, from the picture, you can see that the row count estimation and the cost calculation don't consider such a case.
@winoros winoros added sig/planner SIG: Planner type/enhancement The issue or PR belongs to an enhancement. labels Nov 25, 2024
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