Commit b5a9b18c introduced block streaming infrastructure with a special
fast path for all-cached scans, and commit b7b0f3f2 connected the
infrastructure up to sequential scans. One of the fast path
micro-optimizations had an unintended consequence: it interfered with
parallel sequential scan's block range allocator (from commit 56788d21),
which has its own ramp-up and ramp-down algorithm when handing out
groups of pages to workers. A scan of an all-cached table could give
extra blocks to one worker, when others had finished. In some plans
(probably already very bad plans, such as the one reported by
Alexander), the unfairness could be magnified.
An internal buffer of 16 block numbers is removed, keeping just a single
block buffer for technical reasons.
Back-patch to 17.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/63a63690-dd92-c809-0b47-af05459e95d1%40gmail.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.
Thomas Munro
4effd0844d