Subject: More performance improvements for pg_dump in binary upgrade mode

From: Daniel Gustafsson <[email-masked]>
Date: Wed, 15 May 2024 22:15:13 +0200

Prompted by an off-list bugreport of pg_upgrade hanging (which turned out to be
slow enough to be perceived to hang) for large schemas I had a look at pg_dump
performance during --binary-upgrade mode today.  My initial take was to write
more or less exactly what Nathan did in [0], only to realize that it was a)
already proposed and b) I had even reviewed it.  Doh.

The next attempt was to reduce more per-object queries from binary upgrade, and
the typarray lookup binary_upgrade_set_type_oids_by_type_oid seemed like a good
candidate for a cache lookup.  Since already cache type TypeInfo objects, if we
add typarray to TypeInfo we can use the existing lookup code.

As a baseline, pg_dump dumps a synthetic workload of 10,000 (empty) relations
with a width of 1-10 columns:

$ time ./bin/pg_dump --schema-only --quote-all-identifiers --format=custom \
  --file a postgres > /dev/null

real	0m1.256s
user	0m0.273s
sys	0m0.059s

The same dump in binary upgrade mode runs significantly slower:

$ time ./bin/pg_dump --schema-only --quote-all-identifiers --binary-upgrade \
  --format=custom --file a postgres > /dev/null

real	1m9.921s
user	0m0.782s
sys	0m0.436s

With the typarray caching from the patch attached here added:

$ time ./bin/pg_dump --schema-only --quote-all-identifiers --binary-upgrade \
  --format=custom --file b postgres > /dev/null

real	0m45.210s
user	0m0.655s
sys	0m0.299s

With the typarray caching from the patch attached here added *and* Nathan's
patch from [0] added:

$ time ./bin/pg_dump --schema-only --quote-all-identifiers --binary-upgrade \
  --format=custom --file a postgres > /dev/null

real	0m1.566s
user	0m0.309s
sys	0m0.080s

The combination of these patches thus puts binary uphrade mode almost on par
with a plain dump, which has the potential to make upgrades of large schemas
faster.  Parallel-parking this patch with Nathan's in the July CF, just wanted
to type it up while it was fresh in my mind.

--
Daniel Gustafsson

[0] https://commitfest.postgresql.org/48/4936/