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Re: query performance optimization
Posted by: Øystein Grøvlen
Date: January 29, 2016 04:37AM

Rick James Wrote:
> Øystein -- I used to say " OR clauses tend to
> prevent usage of indexes" for things like
> (SAM.admin_user_id IS NULL OR
> SAM.admin_user_id='24577')
> But then I discovered "ref_or_null". However, it
> does not seem to be used in his case. Can you
> elaborate?

You are right. ref_and_null can be used for conditions like above. However, I am not quite sure what happens in this case where you have an additional join condition on another column. Anyhow, EXPLAIN show that there is only one row in this table. I guess that is why an index is not used.

Øystein Grøvlen,
Senior Principal Software Engineer,
MySQL Group, Oracle,
Trondheim, Norway

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