On Wednesday 07 Nov 2001 18:13, Tom Lane wrote:
> Gordan Bobic <gordan@bobich.net> writes:
> > And is the overhead of running multiple UNION queries greater than the
> > overhead of doing a DISTINCT? I need to sort the records anyway, so the
> > fact that DISTINCT does a SORT is a bonus in this case.
>
> UNION implies DISTINCT, so you're going to get sort and uniq steps in
> either case.
Yes, but I thought that if I have 50 UNION queries, that would do a sort +
uniq for each "append" between them, whereas in the distinc case it only gets
done once, albeit on a bigger data set.
> What this is really going to boil down to is how the
> restriction and join steps are done, and you haven't given enough info
> to speculate about that.
Well, what I said is pretty much it.
It's the case of either doing single FTI term search per query and doing
UNION (for OR search) or INTERSECT (for AND search) of multiple queries.
If the search is executed in this way, and each UNION segment is executed in
sequence, then that means N queries, where N is the number of search terms.
In the SELECT DISTINCT case where multiple terms are ORed in the WHERE
clause, it is vaguely concievable that the entire query (at least in the
UNION case) could be executed in a single pass. Is that the how it works? Or
is each OR term located in a separate pass?
What I'm really trying to figure out is if there is an advantage (in theory
at least) in doing one slightly more complex query, or lots of simpler ones.
> Try some experimentation with EXPLAIN to see
> what kinds of plans you get.
Well, all the fields that are searched on are indexed, and for testing I
usually enable_seqscan=off. What I am going to do is re-write my parser/SQL
generator and give it a go - with a bit of luck, there will be a noticeable
difference in performance.
Thanks.
Gordan