Re: Improving N-Distinct estimation by ANALYZE

Greg Stark <gsstark@MIT.EDU> writes:

> > > These numbers don't make much sense to me. It seems like 5% is about as slow
> > > as reading the whole file which is even worse than I expected. I thought I was
> > > being a bit pessimistic to think reading 5% would be as slow as reading 20% of
> > > the table.
> 
> I have a theory. My test program, like Postgres, is reading in 8k chunks.
> Perhaps that's fooling Linux into thinking it's a sequential read and reading
> in 32k chunks internally. That would effectively make a 25% scan a full table
> scan. And a 5% scan would be a 20% scan which is about where I would have
> expected the breakeven point to be.

Well my theory was sort of half right. It has nothing to do with fooling Linux
into thinking it's a sequential read. Apparently this filesystem was created
with 32k blocks. I don't remember if that was intentional or if ext2/3 did it
automatically based on the size of the filesystem.

So it doesn't have wide-ranging implications for Postgres's default 8k block
size. But it is a good lesson about the importance of not using a larger
filesystem block than Postgres's block size. The net effect is that if the
filesystem block is N*8k then your random_page_cost goes up by a factor of N.
That could be devastating for OLTP performance.

-- 
greg