0ca3b16973
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/.