Query optimization: join prunning and "uniqeness"
维护者通常 5 天内回复
还没有人认领这个 Issue。
评估
调研方向
Start by reading the query transformation and relation metadata code that tracks join conditions and uniqueness; the issue does not name specific files or tests. The proposed optimization also needs a way to determine whether right-side columns are used in the output. Done means safely removing eligible left joins without changing query results; investigate existing tests for join transformations and add coverage for the stated conditions.
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描述
Under certain conditions, join transforms could be removed from the query without changing its result. This would have significantly improve query performance.
Example:
from e = employees
join side:left addresses (e.id == that.employee_id)
select {e.name}
... could be simplified into:
from e = employees
select {e.name}
Conditions:
- when there is a join of left relation with right relation,
- and it is a left join (so no left rows are dropped),
- and right relation is unique on the join condition (so no left rows are duplicated),
- and none of the columns from the right relation are used in the output.
At the moment, PRQL does contain knowledge to detect the conditions 1.-3., but not 4., so this is not possible to implement.
It would be possible to have this "uniqueness" information either:
- declared at relation definition site (in SQL terms, this would be a UNIQUE constraint / PRIMARY KEY),
- inferred from the query (if there is a
group x (take 1), we can infer uniqueness onx).
Then, it would be possible to detect condition 4.
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