Bruce Momjian wrote: > > mlw wrote: > > Now, given the choice of the two strategies on a table, both pretty close to > > one another, the risk of poor performance for using the index scan is minimal > > based on the statistics, but the risk of poor performance for using the > > sequential scan is quite high on a large table.
> My second point, that index scan is more risky than sequential scan, is > outlined above. A sequential scan reads each page once, and uses the > file system read-ahead code to prefetch the disk buffers. Index scans > are random, and could easily re-read disk pages to plow through a > significant portion of the table, and because the reads are random, > the file system will not prefetch the rows so the index scan will have > to wait for each non-cache-resident row to come in from disk. That is a very interesting point, but shouldn't that be factored into the cost (random_tuple_cost?) In which case my point still stands. ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org
