Re: Add SKIP LOCKED to VACUUM and ANALYZE

On Wed, Sep 05, 2018 at 03:24:21PM +0000, Bossart, Nathan wrote:
> And here it is.  Here is a summary of the notable changes:
>
>  1) Patches v8-0003 and v8-0008 have been discarded.  These patches
>     added SKIP_LOCKED behavior when opening a relation's indexes.
>     Instead, I've documented that VACUUM and ANALYZE may still block
>     on indexes in v9-0007.
>  2) Patches v8-0004 and v8-0005 have been discarded, as they have
>     already been committed.
>  3) Patch v8-0011 has been discarded.  As previously noted, VACUUM
>     (SKIP_LOCKED, FULL) is already handled in vacuum_rel(), so no
>     changed are required to cluster_rel().  However, we will need
>     something similar to v8-0011 if we ever add SKIP_LOCKED to
>     CLUSTER.
>  4) The option has been renamed to SKIP_LOCKED (with the underscore)
>     for consistency with the DISABLE_PAGE_SKIPPING option.
>  5) In the documentation, I've listed the caveats for SKIP_LOCKED and
>     partitioned tables.  I tried to make all the new documentation as
>     concise as possible.

Thanks for the new patches.  So I have begun looking at the full set,
beginning by 0001 which introduces a new common routine to get the
elevel associated to a skipped relation.  I have been chewing on this
one, and I think that the patch could do more in terms of refactoring by
introducing one single routine able to open a relation which is going to
be vacuumed or analyzed.  This removes quite a lot of duplication
between analyze_rel() and vacuum_rel() when it comes to using
try_relation_open().  The result is attached, and that makes the code
closer to what the recently-added vacuum_is_relation_owner does.
Nathan, what do you think?

Please note that I am on the edge of discarding the stuff in
find_all_inheritors and such, as well as not worrying about the case of
analyze which could block for some index columns, but I have not spent
much time studying this part yet.  Still the potential issues around
complicating this code, particularly when things come to having a
partial analyze or vacuum done rather scares me.
--
Michael