Re: Sequential Scan Read-Ahead

Curt Sampson wrote:
> At 12:41 PM 4/23/02 -0400, Bruce Momjian wrote:
>
> >This is an interesting point, that an index scan may fit in the cache
> >while a sequential scan may not.
>
> If so, I would expect that the number of pages read is significantly
> smaller than it was with a sequential scan. If that's the case,
> doesn't that mean that the optimizer made the wrong choice anyway?
>
> BTW, I just did a quick walk down this chain of code to see what happens
> during a sequential scan:
>
> access/heap/heapam.c
> storage/buffer/bufmgr.c
> storage/smgr/smgr.c
> storage/smgr/md.c
>
> and it looks to me like individual reads are being done in BLKSIZE
> chunks, whether we're scanning or not.
>
> During a sequential scan, I've heard that it's more efficient to
> read in multiples of your blocksize, say, 64K chunks rather than
> 8K chunks, for each read operation you pass to the OS. Does anybody
> have any experience to know if this is indeed the case? Has anybody
> ever added this to postgresql and benchmarked it?
>
> Certainly if there's a transaction based limit on disk I/O, as well
> as a throughput limit, it would be better to read in larger chunks.

We expect the file system to do re-aheads during a sequential scan.
This will not happen if someone else is also reading buffers from that
table in another place.

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