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