When using UNION a UNION DISTINCT is performed, i.e. all column and all row
values of all results are compared and duplicates removed. ... which is time
consuming.
If you specify UNION ALL instead, the results of all queries are just merged
and not compared. So an UNION ALL can be (much) faster than and UNION
(DISTINCT)
Mit freundlichen Grüßen / Best regards
Birgitta Hauser
"Shoot for the moon, even if you miss, you'll land among the stars." (Les
Brown)
"If you think education is expensive, try ignorance." (Derek Bok)
"What is worse than training your staff and losing them? Not training them
and keeping them!"
?Train people well enough so they can leave, treat them well enough so they
don't want to.? (Richard Branson)
-----Original Message-----
From: MIDRANGE-L <midrange-l-bounces@xxxxxxxxxxxxxxxxxx> On Behalf Of James
H. H. Lampert
Sent: Freitag, 21. Mai 2021 02:20
To: Midrange Systems Technical Discussion <midrange-l@xxxxxxxxxxxxxxxxxx>
Subject: SQL and UNION: why do some UNIONs significantly inflate the
processing time, while others don't?
Lately, the question has arisen of why, in some SQL queries using UNION, the
UNION, by itself, adds more processing time than the combined total of the
component queries (i.e., adding 10 seconds to UNION together 4-5 queries
that individually give sub-second responses), while in other cases, UNIONing
together a bunch of component queries has no noticeable effect on response
time.
Can anybody shed any light on this?
I have a vague recollection of an Access tool for examining and optimizing
SQL queries (running them, as I recall, in the debugger, and then digesting
the messages into a graphic representation and suggestions for
optimization), but I haven't used it (or even seen it in
use) in a decade and a half.
--
JHHL
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