Vitalii Tymchyshyn <tivv00@xxxxxxxxx> writes: > 24.03.11 20:41, Merlin Moncure ÎÁÐÉÓÁ×(ÌÁ): >> ISTM if you add statistics miss and 'risk margin' to the things the >> planner would have to consider while generating a plan, you are >> greatly increasing the number of plan paths that would have to be >> considered for any non trivial query. > Why so? I simply change cost estimation functions. This won't change > number of pathes. If you have multiple figures of merit, that means you have to keep more paths, with consequent slowdown when it comes to choosing which path to use at higher join levels. As an example, we used to keep only the paths with best total cost. When we started to optimize LIMIT, we had to keep around paths with best startup cost too, in case that made for the best combined result at a higher join level. If you're going to consider "risk" while choosing paths, that means you'll start keeping paths you would have discarded before, while not necessarily getting rid of any other paths. The only way to avoid that would be to have a completely brain-dead notion of risk that wasn't affected by how the path is used at a higher join level, and I'm pretty sure that that wouldn't solve anybody's problem. Any significant expansion of the planner's fundamental cost model *will* make it slower. By a lot. Rather than going into this with fantasies of "it won't cost anything", you should be worrying about how to keep the speed penalty to factor-of-two rather than factor-of-ten. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
