Zoltan Boszormenyi <zboszor@dunaweb.hu> writes:
> We have some large views, consisting of 6+ UNION members.
> Recently, I rewrote them to use UNION ALL, since the members
> give distinct rows across the whole view.
> The performance difference between the two is enormous, e.g. using UNION,
> the rows starts flowing only after 30-32 seconds but when using UNION ALL,
> it starts instantly.
Well, UNION requires fetching all the source rows and then doing a
duplicate-elimination step (sort+uniq basically). Why does the
performance change surprise you?
regards, tom lane
