On Sun, Aug 26, 2012 at 11:01:31PM +0200, Thomas Kellerer wrote: > I was inspired by this question on StackOverflow: > http://stackoverflow.com/questions/12128501/fastest-way-to-count-the-rows-in-any-database-table/12128545#12128545 > > Which shows Oracle's behaviour with an index scan for the count(*) operation. Interesting, It shows indeed Oracle uses the index to do the operation. For postgres it's not so simple for a few reasons, I'm not sure how oracle avoids the same issues: - The index has no visibility information, so you can't tell if an index entry refers to a row you can actually see in your session. The visibility map might help here in the future. - Different versions of the same row (after an UPDATE for example) may both be in the index, Now if you're counting a primary key column you can work around that. But frankly, counting all the rows in a table is something I never do. The system tables carry estimates which have proved good enough for statistical purposes when I need them. Have a nice day, -- Martijn van Oosterhout <kleptog@xxxxxxxxx> http://svana.org/kleptog/ > He who writes carelessly confesses thereby at the very outset that he does > not attach much importance to his own thoughts. -- Arthur Schopenhauer
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