Re: GIN non-intrusive vacuum of posting tree

On Wed, Nov 30, 2016 at 11:38 AM, Andrew Borodin <borodin@octonica.com> wrote:
> This scan acquires cleanup lock on root of scan (not necessarily root
> of posting tree). Cleanup lock ensures no inserts are inside subtree.
> Scan traverses subtree DF taking exclusive locks from left to right.
> For any page being deleted all leftmost pages were locked and unlocked
> before. New reads cannot enter subtree, all old readscans were
> excluded by lock\unlock. Thus there shall not be deadlocks with
> ginStepRight().
>
> We get lock on page being deleted, then on a left page.
> ginStepRight() takes lock on left page, than on right page. But it’s
> presence is excluded by cleanup lock and DFS scan with locks of upper
> and left parts of tree.
>
> Thank you for reading this. Concurrency bothers me a lot. If you see
> that anything is wrong or suspicious, please do not hesitate to post
> your thoughts.

I don't know much about GIN, but this seems like an interesting
improvement.  I hope somebody who knows more about GIN will step up to
review it in depth.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company