RE: Index Skip Scan

>
> * Suspicious performance difference between different type of workload,
>   mentioned by Tomas (unfortunately I had no chance yet to investigate).
>

His benchmark results indeed most likely point to multiple comparisons being done. Since the most likely place where
theseoccur is _bt_readpage, I suspect this is called multiple times. Looking at your patch, I think that's indeed the
case.For example, suppose a page contains [1,2,3,4,5] and the planner makes a complete misestimation and chooses a skip
scanhere. First call to _bt_readpage will compare every tuple on the page already and store everything in the
workspace,which will now contain [1,2,3,4,5]. However, when a skip is done the elements on the page (not the workspace)
arecompared to find the next one. Then, another _bt_readpage is done, starting at the new offnum. So we'll compare
everytuple (except 1) on the page again. Workspace now contains [2,3,4,5]. Next tuple we'll end up with [3,4,5] etc. So
tuple5 actually gets compared 5 times in _bt_readpage alone. 

-Floris