Lukas Smith wrote: > Jim C. Nasby wrote: > > > Now, if both of these are done using a prepared statement, it's going to > > look like: > > > > SELECT * FROM queue WHERE status='?'; > > > > If the first one to run is the queue processing one, the planner will > > probably choose the index. This means that when we're searching on 'N', > > there will be a fairly small number of tuples read to execute the query, > > but when searching for 'D' a very large number of tuples will be read. > > I do not know how exactly how pg handles this internally, however while > skimming the oracle tuning pocket guide I picked up for 2 euros I > noticed that it mentioned that since oracle 9i bound parameter values > are evaluated before the execution plan is determined. > > Maybe I am mixing up separate concepts (are bound variables and prepared > statements different concepts?) here. I also do not really understand if > that means that oracle does not store a query plan for a prepared query > or if it just does some special handling in case it knows that a > prepared statement column is known to have a highly varying selectivity > per value.
What the Oralce manual means I think is that the plan of the query is delayed until the _first_ EXECUTE, so it has some values to use in the optimizer. The problem is that later queries might use constants of greatly different cardinality. -- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
