Re: Bug in gin insert redo code path during re-compression of empty gin data leaf pages

Hi!

On Wed, Jul 18, 2018 at 1:40 AM R, Siva <sivasubr@amazon.com> wrote:

> We came across an issue during replay of a gin insert record on a pre-9.4
> uncompressed data leaf page that does not have any items in it. The engine
> version where the replay is done is 9.6.3. The redo logic attempts to
> compress the existing page items before trying to insert  new items from
> the WAL record[1]. In this particular case, we noticed that if the data
> leaf page does not have any items in it (see below on how such a page can
> be present), the compression should not be attempted as the algorithm
> expects the page to have at least one item [2]. This will lead to incorrect
> data size set on the page and also makes the assertion that expects npacked
> and nuncompressed to be equal, false [3].
>
> In Postgres 9.3, when the gin posting tree is vacuumed, the pages that are
> in the leftmost and rightmost branches are not deleted [4]. These empty
> pages can be part of the database after upgrading to 9.6.3. We verified
> this by doing the following test:
>

Thank you for reporting this bug.  At the first glance it seems that your
proposed fix is correct.  I'll review it in more details and commit.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company