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. ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/users-lounge/docs/faq.html
