Re: Linux/PostgreSQL scalability issue - problem with 8 cores

 > Okay, for a table of just a few entries I agree that DELETE is
 > probably better.  But don't forget you're going to need to have those
 > tables vacuumed fairly regularly now, else they'll start to bloat.

I think we'll go with DELETE also for another reason:

Just after we figured out the cause of the spikes we started to
investigate a long-term issue we had with PostgreSQL: pg_dump of big
database was blocking some of our applications. And yes, we replaced
TRUNCATE with DELETE and everything is running as expected.

Looking at the docs now I see there is a new paragraph in 8.3 docs
mentioning that TRUNCATE is not MVCC-safe and also the blocking issue.
It's a pity that the warning wasn't there in 7.1 times :-)

Thanks,

Kuba

Tom Lane napsal(a):
> Jakub Ouhrabka <kuba@comgate.cz> writes:
>>>> Huh.  One transaction truncating a dozen tables?  That would match the
>>>> sinval trace all right ...
>
>> It should be 4 tables - the shown log looks like there were more truncates?
>
> Actually, counting up the entries, there are close to 2 dozen relations
> apparently being truncated in the trace you showed.  But that might be
> only four tables at the user level, since each index on these tables
> would appear separately, and you might have a toast table plus index
> for each one too.  If you want to dig down, the table OIDs are visible
> in the trace, in the messages with type -1:
>
>>> LOG:  sending inval msg -1 0 30036 0 30700 3218341912
>                                 ^^^^^   ^^^^^
>                                 DBOID   RELOID
>
> so you could look into pg_class to confirm what's what.
>
>> Yes, performance was the initial reason to use truncate instead of
>> delete many years ago. But today the truncated tables usualy contain
>> exactly one row - quick measurements now show that it's faster to issue
>> delete instead of truncate in this case.
>
> Okay, for a table of just a few entries I agree that DELETE is probably
> better.  But don't forget you're going to need to have those tables
> vacuumed fairly regularly now, else they'll start to bloat.
>
>             regards, tom lane