Sync Scan update

I have updated my Synchronized Scan patch and have had more time for
testing.

Go to http://j-davis.com/postgresql/syncscan-results10.html
where you can download the patch, and see the benchmarks that I've run.

The results are very promising. I did not see any significant slowdown
for non-concurrent scans or for scans that fit into memory, although I
do need more testing in this area.

The benchmarks that I ran tested the concurrent performance, and the
results were excellent.

I also added two new simple features to the patch (they're just
#define'd tunables in heapam.h):
(1) If the table is smaller than
effective_cache_size*SYNC_SCAN_THRESHOLD then the patch doesn't do
anything different from current behavior.
(2) The scans can start earlier than the hint implies by setting
SYNC_SCAN_START_OFFSET between 0 and 1. This is helpful because it makes
the scan start in a place where the cache trail is likely to be
continuous between the starting point and the location of an existing scan.

I'd like any feedback, particularly any results that show a slowdown
from current behavior. I think I fixed Luke's problem (actually, it was
a fluke that it was even working at all), but I haven't heard back. Some
new feedback would be very helpful.

Thanks.

Regards,
Jeff Davis