Re: Some other things about contrib/bloom and generic_xlog.c

Hi, Tom!

On Sun, Apr 10, 2016 at 7:49 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> 1. It doesn't seem like generic_xlog.c has thought very carefully about
> the semantics of the "hole" between pd_lower and pd_upper. The mainline
> XLOG code goes to some lengths to ensure that the hole stays all-zeroes;
> for example RestoreBlockImage() explicitly zeroes the hole when restoring
> from a full-page image that has a hole. But generic_xlog.c's redo routine
> does not do anything comparable, nor does GenericXLogFinish make any
> effort to ensure that the "hole" is all-zeroes after normal application of
> a generic update. The reason this is of interest is that it means the
> contents of the "hole" could diverge between master and slave, or differ
> between the original state of a database and what it is after a crash and
> recovery. That would at least complicate forensic comparisons of pages,
> and I think it might also break checksumming. We thought that this was
> important enough to take the trouble of explicitly zeroing holes during
> mainline XLOG replay. Shouldn't generic_xlog.c take the same trouble?
>
> 2. Unless I'm missing something, contrib/bloom is making no effort
> to ensure that bloom index pages can be distinguished from other pages
> by pg_filedump. That's fine if your expectation is that bloom will always
> be a toy with no use in production; but otherwise, not so much. You need
> to make sure that the last two bytes of the page special space contain a
> uniquely identifiable code; compare spgist_page_id in SPGiST indexes.
>

Thank you for spotting these issues.
I'm going to prepare patches for fixing both of them.

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