Re: how to efficiently update tuple in many-to-many relationship?

Drew Wilson <drewmwilson(at)gmail(dot)com> writes:
> If I understand the EXPLAIN ANALYZE results below, it looks like the
> time spent applying the "set is_public = true" is much much more than
> the fetch. I don't see any triggers firing.

Nope, there aren't any. 8.2 is smart enough to bypass firing FK
triggers on UPDATE if the key columns didn't change. Of course that
check takes a certain amount of time, but I don't think it's a large
amount. So basically we're looking at index update and WAL logging
as the major costs here, I think.

[ thinks for a bit... ] Part of the problem might be that the working
set for updating all the indexes is too large. What do you have
shared_buffers set to, and can you increase it?

Oh, and checkpoint_segments at 8 is probably still not nearly enough;
if you have disk space to spare, try something like 30 (which'd eat
about 1Gb of disk space). It might be educational to set
checkpoint_warning to 300 or so and watch the log to see how often
checkpoints happen during the update --- you want them at least a couple
minutes apart even during max load.

Also, bumping up wal_buffers a little might help some, for large updates
like this. I've heard people claim that values as high as 64 are helpful.

regards, tom lane