On Thursday 2005-12-22 14:28, Lukas Kahwe Smith wrote: > Bruce Momjian wrote: > > Right, if the cardinality changes, you realize this before execution and > > optimize/save the plan again. A further optimization would be to save > > _multiple_ plans for a single prepared plan based on constants and > > choose one of the other, but that is beyond where we are willing to > > consider at this stage, I think. > > ok .. so you store the cardinality that was used when generating the > original plan. on the next execution you look up the cardinality again > and compare it, if its off too much, you replan. however this could in > extreme cases mean that you replan on every execution and thereby > killing off the entire advantage of storing the plan. but thats the > absolute worse case scenario. > > regards, > Lukas > > PS: bruce original email was only send to me directly ..
So you have a parameterized query (one parameter for simplicity of argument), as the parameter changes, cardinality changes dramatically. It seems to me that in this case better than replanning is building a data structure that associates different parameter values with appropriate plans. The plans can be reused until, as would be the case with an no-parameter query, a parameter specific plan should be flushed (or the entire family of plans can be flushed). ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
