Re: AutoVacuum starvation from sinval messages

Tom Lane escribió:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:

> > IIRC the queue has 4K entries, and IIRC a single DDL
> > operation might provoke a couple of sinvals, but I'm thinking that
> > somebody would probably have to be creating >1024 temp tables a minute
> > to overrun the queue, which is very possible but not necessarily
> > common.
>
> Well, one DDL typically generates multiple messages --- one for each
> catalog row added/modified/removed, roughly speaking. When I run the
> constant create/drop example Jeff posted, I see the AV launcher getting
> a catchup signal every few seconds. I didn't try to determine exactly
> how many create/drop cycles that was, but I'm pretty sure it's a lot
> less than 1000.

Just creating the sequence for the serial column means 16 pg_attribute
tuples. There's also two pg_class entries, one more pg_attribute, two
pg_type entries, a bunch of pg_depend entries ... I doubt it's less than
30 catalog tuples, all things considered. Double that for the drop. So
for a 4k entry table that needs to get 50% full, that's only ~35 temp
table creations like that.

I hadn't realized sequences used so many pg_attribute entries.

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Álvaro Herrera http://www.2ndQuadrant.com/
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