mlw wrote: > Bruce Momjian wrote: > > 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. > > It took a bike ride to think about this one. The supposed advantage of a > sequential read over an random read, in an active multitasking system, is a > myth. If you are executing one query and the system is doing only that query, > you may be right. > > Execute a number of queries at the same time, the expected benefit of a > sequential scan goes out the window. The OS will be fetching blocks, more or > less, at random.
OK, yes, sequential scan _can_ be as slow as index scan, but sometimes it is faster. Can you provide reasoning why index scan should be preferred, other than the admin created it, which I already addressed? -- Bruce Momjian | http://candle.pha.pa.us [EMAIL PROTECTED] | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/users-lounge/docs/faq.html
