Quoting Tom Lane <[EMAIL PROTECTED]>: > Kim Bisgaard <[EMAIL PROTECTED]> writes: > > SELECT station_id, timeobs,temp_grass, temp_dry_at_2m > > FROM temp_dry_at_2m a > > FULL OUTER JOIN temp_grass b > > USING (station_id, timeobs) > > WHERE station_id = 52981 > > AND timeobs = '2004-1-1 0:0:0' > > > explain analyse SELECT b.station_id, b.timeobs,temp_grass, temp_dry_at_2m > > FROM temp_dry_at_2m a > > FULL OUTER JOIN temp_grass b > > USING (station_id, timeobs) > > WHERE b.station_id = 52981 > > AND b.timeobs = '2004-1-1 0:0:0' > > > Why will PostgreSQL not use the same plan for both these queries - they > > are virtually identical?? > > Because they're semantically completely different. The second query is > effectively a RIGHT JOIN, because join rows in which b is all-null will > be thrown away by the WHERE. The optimizer sees this (note your second > plan doesn't use a Full Join step anywhere) and is able to produce a > much better plan. Full outer join is difficult to optimize, in part > because we have no choice but to use a merge join for it --- the other > join types don't support full join. > > regards, tom lane >
Yes I am aware that they are not "identical", they also give different results, but the data nessesary to compute the results is (0-2 rows, 0-1 row from each table), and thus ideally have the potential to have similar performance - to my head anyway, but I may not have grasped the complete picture yet :-) Regards, Kim. ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings
