On Thu, Sep 6, 2012 at 5:12 PM, Alan Hodgson <ahodgson@xxxxxxxxx> wrote: > On Thursday, September 06, 2012 05:06:27 PM Jeff Janes wrote: >> For updating 20 million out of 500 million rows, wouldn't a full table >> scan generally be preferable to an index scan anyway? >> > > Not one table scan for each row updated ... My understanding is that he was only trying to update one row on the parent table anyway, which then cascaded to 20,000,000 rows on the child/fact table. If you mean one table scan for each of the 20,000,000 rows *of the child* being updated, that isn't what it does now, index or not. Even if he were updating 10 rows of the parent table, I think it would still be the case that if one sequential scan of the child/fact was faster than one (large, low-cardinality, unclustered) index scan, then 10 sequential scans would be faster than 10 index scans. Cheers, Jeff -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
