Pg_upgrade speed for many tables

Magnus reported that a customer with a million tables was finding
pg_upgrade slow. I had never considered many table to be a problem, but
decided to test it. I created a database with 2k tables like this:

CREATE TABLE test1990 (x SERIAL);

Running the git version of pg_upgrade on that took 203 seconds. Using
synchronous_commit=off dropped the time to 78 seconds. This was tested
on magnetic disks with a write-through cache. (No change on an SSD with
a super-capacitor.)

I don't see anything unsafe about having pg_upgrade use
synchronous_commit=off. I could set it just for the pg_dump reload, but
it seems safe to just use it always. We don't write to the old cluster,
and if pg_upgrade fails, you have to re-initdb the new cluster anyway.

Patch attached. I think it should be applied to 9.2 as well.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +