> -----Original Message----- > From: Tom Lane [mailto:[EMAIL PROTECTED] > Sent: Monday, April 25, 2005 10:23 AM > To: Simon Riggs > Cc: josh@agliodbs.com; Greg Stark; Marko Ristola; pgsql-perform; > pgsql-hackers@postgresql.org > Subject: Re: [HACKERS] [PERFORM] Bad n_distinct estimation; hacks > suggested? > > [...] > It's not just the scan --- you also have to sort, or something like > that, if you want to count distinct values. I doubt anyone is really > going to consider this a feasible answer for large tables.
How about an option to create a stat hashmap for the column that maps distinct values to their number of occurrences? Obviously the map would need to be updated on INSERT/DELETE/UPDATE, but if the table is dominated by reads, and an accurate n_distinct is very important, there may be people willing to pay the extra time and space cost. __ David B. Held Software Engineer/Array Services Group 200 14th Ave. East, Sartell, MN 56377 320.534.3637 320.253.7800 800.752.8129 ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings
