Re: Why could different data in a table be processed with different performance?

> Since you have a very big toast table, given you are using spinning
disks, I think that increasing the block size will bring benefits.
But will it worsen caching? I will have lesser slots in cache. Also will it
affect required storage space?

>> consecutive runs with SAME parameters do NOT hit the disk, only the
first one does, consequent ones read only from buffer cache.
> I m a bit confused.. every query you pasted contains 'read':
> Buffers: shared hit=50 read=2378
> and 'read' means you are reading from disk (or OS cache). Or not?
Yes, sorry, it was just my misunderstanding of what is "consecutive". To
make it clear: I iterate over all data in table with one request and
different parameters on each iteration (e.g. + 5000 both borders), in this
case I get disk reads on each query run (much more reads on "slow" range).
But if I request data from an area queried previously, it reads from cache
and does not hit disk (both ranges). E.g. iterating over 1M of records with
empty cache takes ~11 minutes in "fast" range and ~1 hour in "slow" range,
while on second time it takes only ~2 minutes for both ranges (if I don't
do drop_caches).

Regards,
Vlad