Re: Materialized views WIP patch

On Thu, Jan 24, 2013 at 03:14:15PM -0500, Kevin Grittner wrote:
> Noah Misch wrote:
> > On Thu, Jan 24, 2013 at 01:29:10PM -0500, Kevin Grittner wrote:
> >> Noah Misch wrote:
> >>> For the benefit of the archives, I note that we almost need not truncate an
> >>> unlogged materialized view during crash recovery. MVs are refreshed in a
> >>> VACUUM FULL-like manner: fill a new relfilenode, fsync it, and point the MV's
> >>> pg_class to that relfilenode. When a crash occurs with no refresh in flight,
> >>> the latest refresh had been safely synced. When a crash cuts short a refresh,
> >>> the pg_class update will not stick, and the durability of the old heap is not
> >>> in doubt. However, non-btree index builds don't have the same property; we
> >>> would need to force an immediate sync of the indexes to be safe here. It
> >>> would remain necessary to truncate unlogged MVs when recovering a base backup,
> >>> which may contain a partially-written refresh that did eventually commit.
> >>> Future MV variants that modify the MV in place would also need the usual
> >>> truncate on crash.
> >> 
> >> Hmm. That's a very good observation. Perhaps the issue can be
> >> punted to a future release where we start adding more incremental
> >> updates to them. I'll think on that, but on the face of it, it
> >> sounds like the best choice.
> > 
> > That situation is challenging for the same reason pg_class.relisvalid was hard
> > to implement for unlogged relations. The startup process doesn't know the
> > relkind of the unlogged-relation relfilenodes it cleans. If you can work
> > through all that, it's certainly a nice endpoint to not lose unlogged snapshot
> > MVs on crash. But I intended the first half of my message as the
> > recommendation and the above as a wish for the future.
> 
> Well, if I just don't create an init fork for MVs, they are left as
> they were on recovery, aren't they? So for 9.3, that solves that
> issue, I think. pg_class.relisvald is a separate issue.

The startup process just looks for init forks, yes.  But it's acceptable to
leave the unlogged MV materials alone during *crash* recovery only.  When
recovering from a base backup, we once again need an init fork to refresh the
unlogged-MV relations.  In turn, we would still need a relisvalid
implementation that copes.  This is all solvable, sure, but it looks like a
trip off into the weeds relative to the core aim of this patch.

> > Why is the dependency problem of ordering MV refreshes and MV index builds so
> > different from existing pg_dump dependency problems?
> 
> If mva has indexes and is referenced by mvb, the CREATE statements
> are all properly ordered, but you want mva populated and indexed
> before you attempt to populate mvb. (Populated to get correct
> results, indexed to get them quickly.) We don't have anything else
> like that.

Is the REFRESH order just a replay of the CREATE order (with index builds
interspersed), or can it differ?

nm