Обсуждение: Re: Stopgap solution for table-size-estimate updatingproblem

>> This is not true in my case, since I only "update statistics"/analyze
>> when the tables have representative content (i.e. not empty).
>
> I'm unsure why you feel you need a knob to defeat this.  The only time
> when the plan would change from what you think of as the hand-tuned
> case is when the physical table size is greatly different from what it
> was when you analyzed.

Ok, understood. I just need to make sure I don't "vacuum full" in that case,
which is good anyway if I expect the table to soon grow to this size again.
I think that is good.

I think I recall that lseek may have a negative effect on some OS's readahead
calculations (probably only systems that cannot handle an lseek to the next page
eighter) ? Do you think we should cache the last value to avoid the syscall ?

Andreas

"Zeugswetter Andreas DAZ SD" <ZeugswetterA@spardat.at> writes:
> I think I recall that lseek may have a negative effect on some OS's
> readahead calculations (probably only systems that cannot handle an
> lseek to the next page eighter) ? Do you think we should cache the
> last value to avoid the syscall ?

We really can't, since the point of doing it is to find out whether any
other backends have extended the file since we last looked.  Also, IIRC
seqscan startup does a similar lseek() anyhow, so having the planner do
one will make no difference to the readahead or lack of it in a
subsequent seqscan.
        regards, tom lane