> > * 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 these occur 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 scan here. 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) are compared to find the next one. Then, another _bt_readpage is done, starting at the new offnum. So we'll compare every tuple (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 tuple 5 actually gets compared 5 times in _bt_readpage alone. -Floris
