On Sun, Feb 13, 2011 at 3:29 PM, Josh Berkus <josh@xxxxxxxxxxxx> wrote: > I've wordsmithed Chris's changes some, and then spun off a completely > separate page for Hints discussion, since the NotToDo item was becoming > too long. > >> Something like this syntax?: >> >> JOIN WITH (correlation_factor=0.3) > > Please, NO! > > This is exactly the kind of hint that I regard as a last resort if we > run out of implementation alternatives. Any hint which gets coded into > the actual queries becomes a *massive* maintenance and upgrade headache > thereafter. If we're implementing a hint alternative, we should look at > stuff in this priority order: > > 1. Useful tuning of additional cost parameters by GUC (i.e. > cursor_tuple_fraction) > 2. Modifying cost parameters on database *objects* (i.e. "ndistinct=500") > 3. Adding new parameters to modify on database objects (i.e. > "distribution=normal(1.5,17)","new_rows=0.1") > 4. Query hints (if all of the above fails to give fixes for some tested > problem) I fail to see how 1 through 3 can tell the planner the correlation between two fields in two separate tables. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
