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  3. May 2021

Re: SQL and UNION: why do some UNIONs significantly inflate the processing time, while others don't?

On 5/20/21 10:20 PM, Birgitta Hauser wrote:
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)

I actually tried UNION ALL, and it had little effect.

And as to Visual Explain (it's been so long I'd forgotten what it was called!), well, the last time we were an ACS shop, it was still called "PC Support."

--
JHHL


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