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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
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. Copyright and license information can be found in the file COPYRIGHT. General documentation about this version of PostgreSQL can be found at: https://www.postgresql.org/docs/devel/ In particular, information about building PostgreSQL from the source code can be found at: https://www.postgresql.org/docs/devel/installation.html The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.