DuplicateWeedout strategy
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DuplicateWeedout is an execution strategy for Semi-join subqueries.
The idea
The idea is to run semi-join as if it was a regular join, and then eliminate the duplicate record combinations using a temporary table.
DuplicateWeedout in action
TODO
Factsheet
- DuplicateWeedout is shown as "Start temporary/End temporary" in EXPLAIN
- The strategy can handle correlated subqueries, but cannot be applied if the subquery has grouping or aggregate functions.
- DuplicateWeedout allows the optimizer to freely mix subquery's tables and parent select's tables.
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