* Noah Misch (n...@leadboat.com) wrote:
> On Wed, Apr 20, 2016 at 10:50:21PM -0400, Stephen Frost wrote:
> > I'm certainly open to improving these issues now if we agree that they
> > should be fixed for 9.6.  If we don't want to include such changes in 9.6
> > then I will propose then for post-9.6.
> Folks run clusters with ~1000 databases; we previously accepted at least one
> complex performance improvement[1] based on that use case.  On the faster of
> the two machines I tested, the present thread's commits slowed "pg_dumpall
> --schema-only --binary-upgrade" by 1-2s per database.  That doubles pg_dump
> runtime against the installcheck regression database.  A run against a cluster
> of one hundred empty databases slowed fifteen-fold, from 8.6s to 131s.
> "pg_upgrade -j50" probably will keep things tolerable for the 1000-database
> case, but the performance regression remains jarring.  I think we should not
> release 9.6 with pg_dump performance as it stands today.

After looking through the code a bit, I realized that there are a lot of
object types which don't have ACLs at all but which exist in pg_catalog
and were being analyzed because the bitmask for pg_catalog included ACLs
and therefore was non-zero.

Clearing that bit for object types which don't have ACLs improved the
performance for empty databases quite a bit (from about 3s to a bit
under 1s on my laptop).  That's a 42-line patch, with comment lines
being half of that, which I'll push once I've looked into the other
concerns which were brought up on this thread.

Much of the remaining inefficiancy is how we query for column
information one table at a time (that appears to be around 300ms of the
900ms or so total time).  I'm certainly interested in improving that but
that would require adding more complex data structures to pg_dump than
what we use currently (we'd want to grab all of the columns we care
about in an entire schema and store it locally and then provide a way to
look up that information, etc), so I'm not sure it'd be appropriate to
do now.

I'll look into it again once I've addressed the rest of the issues and
add the TAP-based tests which I've also been working on to actually get
direct testing of pg_dump in the regression suite.



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