Re: adding support for posix_fadvise()

On Mon, 2003-11-03 at 11:11, Tom Lane wrote:
> Why not? The advice says that you're going to access the data
> sequentially in the forward direction. If you're not going to back up,
> there is no point in keeping pages in cache after they've been read.

The advice says: "I'm going to read this data sequentially, going
forward." It doesn't say: "I'm only going to read the data once, and
then not access it again" (ISTM that's what FADV_NOREUSE is for). For
example, the following is a perfectly reasonable sequential access
pattern:

a,b,c,a,b,c,a,b,c,a,b,c

(i.e. repeatedly scanning through a large file, say for a data-analysis
app that does multiple passes over the input data). It might not be a
particularly common database reference pattern, but just because an app
is doing a sequential read says little about the temporal locality of
references to the pages in question.

-Neil