Re: vacuum pg_attribute causes high load

After a long battle with technology, kristhegambler(at)yahoo(dot)com (kris pal), an earthling, wrote:
>  
>
>>>>>>>>>>>>>>>>>>>>>>>>>>
>>> 1) Can you tell me if there is any other work around to vacuum the
>>> pg_attribute system table ? Like doing it in parts so that it won't
>> >cause a high load.
>
> Chris Browne <cbbrowne(at)acm(dot)org> wrote:
>>No, you need to let the vacuum finish.
>>If you stop it early, it'll just increase system load and accomplish _nothing_.
>
>>>>>>>>>>>>>>>>>>>>>>>>>>

> But while doing vacuum the load is getting so high that the system
> is almost freezing.

Right. You said that the table was consuming 2GB of space. It has to
read in that 2GB, and discard the bits that aren't necessary.

> Does the performance of 'Vacuum' command depend on other system/
> database parameters?  The 'vacuum pg_attribute" command never
> completes, its causing high load on the server. I had to wait for 15
> mins and kill it.  Because the system load was touching 10, slowing
> down the system therefore not letting anyone access the server.

What's happening, is, at root, that the vacuum process loads all the
data in the table (2GB, you indicated) into memory, looks at it, and
writes it back out.

That presumably leads to about 2GB of reads and 2GB of writes, which
takes a while.

If you want this to go as quickly as possible, shut the database down
and start the postmaster in single user mode so that there's no other
activity competing for the system's resources.

> Can we vacuum the "pg_attribiute" in parts? So that 'vacuum' will
> complete faster and there won't be load issues. Thanks Tom Lane and
> Chris Browne for your help so far.

As I said, no, you need to let the vacuum finish. It's one table, and
must be vacuumed in one piece. If you stop it early, you're just
wasting your time. You cannot do it in pieces, and if you did, that
wouldn't make it complete faster anyways. That would make it take
MORE time.

Do the vacuum, and hold on until it is done.

Once that vacuum is done, later vacuums will take place MUCH quicker,
and in that you are clearly making enormous numbers of modifications
to the table, you need to vacuum the table regularly.
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