The solution used in 0ca3b1697 to determine the Parallel TID Range
Scan's start location was to modify the signature of
table_block_parallelscan_startblock_init() to allow the startblock
to be passed in as a parameter. This allows the scan limits to be
adjusted before that function is called so that the limits are picked up
when the parallel scan starts. The commit made it so the call to
table_block_parallelscan_startblock_init uses the HeapScanDesc's
rs_startblock to pass the startblock to the parallel scan. That all
works ok for Parallel TID Range scans as the HeapScanDesc rs_startblock
gets set by heap_setscanlimits(), but for Parallel Seq Scans, initscan()
does not initialize rs_startblock, and that results in passing an
uninitialized value to table_block_parallelscan_startblock_init() as
noted by the buildfarm member skink, running Valgrind.
To fix this issue, make it so initscan() sets the rs_startblock for
parallel scans unless we're doing a rescan. This makes it so
table_block_parallelscan_startblock_init() will be called with the
startblock set to InvalidBlockNumber, and that'll allow the syncscan
code to find the correct start location (when enabled). For Parallel
TID Range Scans, this InvalidBlockNumber value will be overwritten in
the call to heap_setscanlimits().
initscan() is a bit light on documentation on what's meant to get
initialized where for parallel scans. From what I can tell, it looks like
it just didn't matter prior to 0ca3b1697 that rs_startblock was left
uninitialized for parallel scans. To address the light documentation,
I've also added some comments to mention that the syncscan location for
parallel scans is figured out in table_block_parallelscan_startblock_init.
I've also taken the liberty to adjust the if/else if/else code in
initscan() to make it clearer which parts apply to parallel scans and
which parts are for the serial scans.
Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAApHDvqALm+k7FyfdQdCw1yF_8HojvR61YRrNhwRQPE=zSmnQA@mail.gmail.com
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.
David Rowley
d167c19295