> -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Tom Lane > Sent: Wednesday, May 19, 2004 11:56 AM
> Not unless you add yet another sort step after the fetch step, that is > the idea becomes > 1. read index to get candidate TIDs > 2. sort TIDs into physical order > 3. read heap in physical order, check row visibility > 4. sort selected rows into index order > > That would start to look like an unpleasant amount of overhead, since > the sort would have to be over the entire output; you couldn't divide > the scan into reasonable-size batches, whereas steps 1-3 can easily > be run in batched style. Or: 2. Sort AND COPY TID's into physical order. 3. Read heap in phy. order, match results to un-sorted TID list. That sounds quite cheap. Then you get to drop step 4 (which would *usually* be quite a bit more expensive than a TID sort and copy?). The cost of the proposed implementation would be higher *when everything is in the cache*, granted. As a user, I will take that cost 10 times over in return for such a large improvement in the no-cache situation. -Glen ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match