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In v14, bb437f995 added support for scanning for ranges of TIDs using a dedicated executor node for the purpose. Here, we allow these scans to be parallelized. The range of blocks to scan is divvied up similarly to how a Parallel Seq Scans does that, where 'chunks' of blocks are allocated to each worker and the size of those chunks is slowly reduced down to 1 block per worker by the time we're nearing the end of the scan. Doing that means workers finish at roughly the same time. Allowing TID Range Scans to be parallelized removes the dilemma from the planner as to whether a Parallel Seq Scan will cost less than a non-parallel TID Range Scan due to the CPU concurrency of the Seq Scan (disk costs are not divided by the number of workers). It was possible the planner could choose the Parallel Seq Scan which would result in reading additional blocks during execution than the TID Scan would have. Allowing Parallel TID Range Scans removes the trade-off the planner makes when choosing between reduced CPU costs due to parallelism vs additional I/O from the Parallel Seq Scan due to it scanning blocks from outside of the required TID range. There is also, of course, the traditional parallelism performance benefits to be gained as well, which likely doesn't need to be explained here. Author: Cary Huang <cary.huang@highgo.ca> Author: David Rowley <dgrowleyml@gmail.com> Reviewed-by: Junwang Zhao <zhjwpku@gmail.com> Reviewed-by: Rafia Sabih <rafia.pghackers@gmail.com> Reviewed-by: Steven Niu <niushiji@gmail.com> Discussion: https://postgr.es/m/18f2c002a24.11bc2ab825151706.3749144144619388582@highgo.ca
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/.