On 14/07/18 10:26, Andrey Borodin wrote:
This is tradeoff between complex concurrency feature and possibility
of few dead tuples left after VACUUM. I want to understand: is it
something dangerous in this dead tuples?
Yeah, that's bad. When a new heap tuple is inserted in the same
location, the old index tuple will point to the new, unrelated, tuple,
and you will get incorrect query results.
There is one more serious race condition: result of first scan is
just a hint where to look for downlinks to empty pages. If internal
page splits between scan and cleanup, offsets of downlinks will be
changed, cleanup will lock pages, see non-empty pages and will not
delete them (though there are not dead tuples, just not deleted empty
leafs).
That's fine. Leaving behind a few empty pages is harmless, the next
vacuum will pick them up.
- Heikki
