performance: use pread instead of lseek+read

Hi all,

postgresql tries very hard to avoid calling lseek if not needed,
probably to avoid doing unnecessary syscalls.
What about removing lseek entirely and using the p{read,write}?

pread is identical to the normal read syscall, except that it has one
additional parameter: the position from which the data should be read.
All recent unices support that, it's part of POSIX.1c.

Attached is a patch vs the cvs tree.
It seems to work - 7.3.2 with the patch applied passes the regression
test suite on RH Linux.
Untested with cvs-head, preproc.y causes a parser overflow.

What do you think?
- configure: I test for existance of pread, and assume that pwrite will
exist, too. Acceptable?
- Are you interested in further patches? xlog.c contains a few lseeks,
but I doubt that they are in the critical path.

--
Manfred