Re: An out-of-date comment in nodeIndexonlyscan.c

On Thu, Jun 27, 2019 at 4:33 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:

> Here's a patch I'd like to commit to fix the comment. We could look
> at improving the actual code after
> https://commitfest.postgresql.org/23/2169/ is done.
>

Change looks good to me.

> I wonder if it might be possible to avoid page locks on both the heap
> *and* index in some limited cases, as I mentioned over here (just an
> idea, could be way off base):
>
>
> https://www.postgresql.org/message-id/CA%2BhUKGJGDVfhHmoUGzi-_J%2BN8FmRjmWTY0MCd1ZV5Fj9qdyb1w%40mail.gmail.com

I am in same boat as you. One can get serializable conflict error today
based on tuple gets HOT updated or not. HOT is logically internal code
optimization and not so much user functionality, so ideally feels shouldn't
affect serializable behavior. But it does currently, again, due to index
locking. Disable HOT update and 4 isolation tests fail due to "could not
serialize access due to read/write dependencies among transactions"
otherwise not. If the tuple happens to fit on same page user will not get
the error, if the tuple gets inserted to different page the error can
happen, due to index page locking. I had discussed this with Heikki and
thinking is we shouldn't need to take the lock on the index page, if the
index key was not changed, even if it was a non-HOT update. Not sure of
complications and implications, but just a thought.