Re: Seq scans roadmap

> In reference to the seq scans roadmap, I have just submitted
> a patch that addresses some of the concerns.
>
> The patch does this:
>
> 1. for small relation (smaller than 60% of bufferpool), use
> the current logic 2. for big relation:
> - use a ring buffer in heap scan
> - pin first 12 pages when scan starts
> - on consumption of every 4-page, read and pin the next 4-page
> - invalidate used pages of in the scan so they do not
> force out other useful pages

A few comments regarding the effects:

I do not see how this speedup could be caused by readahead, so what are
the effects ?
(It should make no difference to do the CPU work for count(*) inbetween
reading each block when the pages are not dirtied)
Is the improvement solely reduced CPU because no search for a free
buffer is needed and/or L2 cache locality ?

What effect does the advance pinnig have, avoid vacuum ?

A 16 x 8k page ring is too small to allow the needed IO blocksize of
256k.
The readahead is done 4 x one page at a time (=32k).
What is the reasoning behind 1/4 ring for readahead (why not 1/2), is
3/4 the trail for followers and bgwriter ?

I think in anticipation of doing a single IO call for more that one
page, the KillAndReadBuffer function should be split into two parts. One
that does the killing
for n pages, and one that does the reading for n pages.
Killing n before reading n would also have the positive effect of
grouping perhaps needed writes (not interleaving them with the reads).

I think the 60% Nbuffers is a very good starting point. I would only
introduce a GUC when we see evidence that it is needed (I agree with
Simon's partitioning comments, but I'd still wait and see).

Andreas