On 14 June 2013 16:09, Hannu Krosing <ha...@2ndquadrant.com> wrote: > What skytools/pgq/londiste uses for comparing tables on master > and slave is query like this > > select sum(hashtext(t.*::text)) from <yourtable> t; > > This is non-modulo sum and does not use md5 but relies on > whatever the hashtext() du jour is :) > > So it is not comparable to anything external (like the md5sum > compatible idea above) but is usually good enough for fast > checks of compatible tables. > > As tables are unordered by definition anyway, this should be > good enough for most SQL. > > The speed comes from both fast(er) hashtext() function and > avoiding the sort. >
That sounds like a pretty good approach. We could do that if we had a version of md5() that returned numeric. My impression is that numeric computations are pretty fast compared to the sorting overhead. On the other hand, if there is a usable index, select md5_agg(..) from (sub-query) will do and index scan rather than a sort, making it much faster than using an ORDER BY in the aggregate. Regards, Dean -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
