3f44959f47
Teach nbtree backwards scans to avoid relocking a just-read leaf page to read its current left sibling link when it isn't truly necessary. This happened inside _bt_readnextpage whenever _bt_readpage had already determined that there'll be no further matches to the left (or at least none for the current primitive index scan, for a scan with array keys). A new precheck inside _bt_readnextpage is all that we need to avoid these useless lock acquisitions. Arguably, using a precheck like this was a missed opportunity for commit 2ed5b87f96, which taught nbtree to drop leaf page pins early to avoid blocking cleanup by VACUUM. Forwards scans already managed to avoid relocking the page like this. The optimization added by this commit is particularly helpful with backwards scans that use array keys where the scan must perform multiple primitive index scans. Such backwards scans will now avoid a useless leaf page re-lock at the end of each primitive index scan. Note that this commit does not attempt to avoid needlessly re-locking a leaf page that was just read when the scan must follow the leaf page's left link. That more ambitious optimization could work by stashing the left link when the page is first read by a backwards scan, allowing the subsequent _bt_readnextpage call to optimistically skip re-reading the original page just to get a new copy of its left link. For now we only address cases where we don't care about our original page's left link. Author: Peter Geoghegan <pg@bowt.ie> Reviewed-By: Matthias van de Meent <boekewurm+postgres@gmail.com> Discussion: https://postgr.es/m/CAH2-Wz=xgs7PojG=EUvhgadwENzu_mY_riNh-w9wFPsaS717ew@mail.gmail.com
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.
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