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
This adds a new executor node named TID Range Scan. The query planner
will generate paths for TID Range scans when quals are discovered on base
relations which search for ranges on the table's ctid column. These
ranges may be open at either end. For example, WHERE ctid >= '(10,0)';
will return all tuples on page 10 and over.
To support this, two new optional callback functions have been added to
table AM. scan_set_tidrange is used to set the scan range to just the
given range of TIDs. scan_getnextslot_tidrange fetches the next tuple
in the given range.
For AMs were scanning ranges of TIDs would not make sense, these functions
can be set to NULL in the TableAmRoutine. The query planner won't
generate TID Range Scan Paths in that case.
Author: Edmund Horner, David Rowley
Reviewed-by: David Rowley, Tomas Vondra, Tom Lane, Andres Freund, Zhihong Yu
Discussion: https://postgr.es/m/CAMyN-kB-nFTkF=VA_JPwFNo08S0d-Yk0F741S2B7LDmYAi8eyA@mail.gmail.com
