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Save a few cycles during nbtree VACUUM.

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Avoid calling RelationGetNumberOfBlocks() unnecessarily in the common
case where there are no deleted but not yet recycled pages to recycle
during a cleanup-only nbtree VACUUM operation.

Follow-up to commit e5d8a999, which (among other things) taught the
"skip full scan" nbtree VACUUM mechanism to only trigger a full index
scan when the absolute number of deleted pages in the index is
considered excessive.
This commit is contained in:
Peter Geoghegan 2021-03-11 14:18:23 -08:00
parent effdd3f3b6
commit 7bb97211a5
1 changed files with 5 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
src/backend/access/nbtree/nbtree.c
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@ -825,9 +825,10 @@ _bt_vacuum_needs_cleanup(IndexVacuumInfo *info)
* calls. That is, we can end up scanning the entire index without ever
* placing even 1 of the prev_num_delpages pages in the free space map, at
* least in certain narrow cases (see nbtree/README section on recycling
* deleted pages for details). This rarely matters in practice.
* deleted pages for details). This rarely comes up in practice.
*/
if (prev_num_delpages > RelationGetNumberOfBlocks(info->index) / 20)
if (prev_num_delpages > 0 &&
prev_num_delpages > RelationGetNumberOfBlocks(info->index) / 20)
return true;
return false;
@ -916,17 +917,12 @@ btvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
}
/*
* By here, we know for sure that this VACUUM operation won't be skipping
* its btvacuumscan() call. Maintain num_delpages value in metapage.
* This information will be used by _bt_vacuum_needs_cleanup() during
* future VACUUM operations that don't need to call btbulkdelete().
* Maintain num_delpages value in metapage for _bt_vacuum_needs_cleanup().
*
* num_delpages is the number of deleted pages now in the index that were
* not safe to place in the FSM to be recycled just yet. We expect that
* it will almost certainly be possible to place all of these pages in the
* FSM during the next VACUUM operation. _bt_vacuum_needs_cleanup() will
* force the next VACUUM to consider this before allowing btvacuumscan()
* to be skipped entirely.
* FSM during the next VACUUM operation.
*/
Assert(stats->pages_deleted >= stats->pages_free);
num_delpages = stats->pages_deleted - stats->pages_free;