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

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James, in ACS you can run things through Visual Explain - you'll get all kinds of information doing that.

If you are using Access for i, there's a place to run statements there, as well, and Visual Explain is there - it's been around for a couple decades, almost.

Cheers
Vern

On 5/20/2021 7:20 PM, James H. H. Lampert wrote:

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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