Re: Deleting older versions in unique indexes to avoid page splits

On Tue, Nov 17, 2020 at 7:05 AM Victor Yegorov <vyegorov@gmail.com> wrote:
> I've looked over the BTP_HAS_GARBAGE modifications, they look sane.
> I've double checked that heapkeyspace indexes don't use this flag (don't rely on it),
> while pre-v4 ones still use it.

Cool.

> I have a question. This flag is raised in the _bt_check_unique() and in _bt_killitems().
> If we're deprecating this flag, perhaps it'd be good to either avoid raising it at least for
> _bt_check_unique(), as it seems to me that conditions are dealing with postings, therefore
> we are speaking of heapkeyspace indexes here.

Well, we still want to mark LP_DEAD bits set in all cases, just as
before. The difference is that heapkeyspace indexes won't rely on the
page-level flag later on.

> If we'll conditionally raise this flag in the functions above, we can get rid of blocks that drop it
> in _bt_delitems_delete(), I think.

I prefer to continue to maintain the flag in the same way, regardless
of which B-Tree version is in use (i.e. if it's heapkeyspace or not).
Maintaining the flag is not expensive, may have some small value for
forensic or debugging purposes, and saves callers the trouble of
telling  _bt_delitems_delete() (and code like it) whether or not this
is a heapkeyspace index.

-- 
Peter Geoghegan