This depend only on reduction scale.If your particullar queries have many
duplicates e.g 90% or more, than separate distinct can be better from resource
POV. But if it is opposite, then eliminate particular distinct operations can
have huge performance boost.Regards,Karol Bieniaszewski
-------- Oryginalna wiadomość --------Od: "Kjell Rilbe
[email protected] [firebird-support]"
<[email protected]> Data: 02.01.2020 21:49 (GMT+01:00) Do:
[email protected] Temat: [firebird-support] Union with or
without distinct in the separate queries?
Hi,
Not important, just got curious: Consider a query like this:
select c1 from t1
union select c1 from t2;
This will eliminate all duplicates, since I didn't specify "union all".
But would this be more or less efficient than (or the same as) this:
select distinct c1 from t1
union select distinct c1 from t2;
I'm thinking that the latter will reduce the number of records to
consider "as early as possible" any may reduce resource usage that way.
On the other hand it would involve three "distinct" operations rather
than a single one... So maybe the answer is that it depends on the data,
which indices are available and applicable, and may differ from case to
case? Or perhaps the query engine recognizes that the two queries are
logically equivalent and executes them identically?
Regards,
Kjell
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