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Here we adjust the partial path generation for parallel DISTINCT queries to add Sort nodes on top of any unsorted partial distinct paths. This increases the likelihood of the planner pushing a Sort below a Gather Merge which enables the final phase of the parallel distinct to be implemented using a Unique node in more cases. Sorting the partial distinct paths is particularly useful when the DISTINCT query has an ORDER BY and LIMIT clause as this can allow cheaper plans by having the workers Hash Aggregate then Sort before feeding the results into the Gather Merge. The non-parallel portion of the plan then becomes very cheap as it leaves only Unique and Limit to do in the leader process. Author: Richard Guo Reviewed-by: David Rowley Discussion: https://postgr.es/m/CAMbWs48u9VoVOouJsys1qOaC9WVGVmBa+wT1dx8KvxF5GPzezA@mail.gmail.com |
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