Jim C. Nasby wrote:
On Mon, Feb 26, 2007 at 10:18:36PM -0500, Matthew T. O'Connor wrote:
Jim C. Nasby wrote:
Here is a worst case example: A DB with 6 tables all of which are highly active and will need to be vacuumed constantly. While this is totally hypothetical, it is how I envision things working (without the threshold).


I fail to see how a simple 6 table case is 'worst case'. It's common to
see hundreds of tables, and I've run across more than one database with
thousands of tables (think partitioning). In cases like those it's
certainly possible, perhaps even likely that you would get many daemons
running in the database at one time just from different tables suddenly
needing vacuuming and appearing at a higher point in the list than other
tables. With 100 ~1G tables getting updates it certainly wouldn't be
hard to end up with 10 of those being vacuumed all at the same time.

Yes 6 tables is small, the worst-case part of the example was that all the tables would need to be vacuumed constantly. Most databases only have a few hot tables. Most tables only need to vacuumed every once in a while.

I do like the idea since it should be easier to tune, but I think we
still need some limit on it. Perhaps as a first-pass we could just have
a hard limit and log a message and/or set a flag any time we hit it.
That would hopefully allow us to get information about how big a problem
it really is. We could go one step further and say that the last daemon
that can start in a database will only vacuum tables that can be done
quickly; that's essentially what we've been talking about, except the
limit we've been discussing would be hard-coded at 2.

I'm confused, what limit would be set at 2? The number of concurrent workers? I've never said that.

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