Re: Finding overlapping records

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Doh. Just read your whole message, so I see why you were only checking
for the one case. I think the full discussion is still worthwhile though.

Jon Erdman wrote:
>
> Frank,
>
> First of all, you've got a big hole in your overlap check. You're only
> checking for (new span is --- existing is +++):
>
> *+++++++*
> *-------*
>
> when you really need to check for:
>
> *++++++++*
> *---------*
> *-------*
> *--------------*
> *---*
>
> /me pulls out Celko's SQL For Smarties...
>
> So what you would naturally write is perhaps (s1 and e1 are start and
> end of existing span, s2 and e2 are the new span):
>
> WHERE
> s2 between s1 and e1
> OR e2 between s1 and e1
> OR s1 between s2 and e2
> OR e1 between s2 and e2;
>
> which is a bit long and ugly. There's a shortcut you can take, here's
> how you would search for things that *don't* overlap:
>
> *+++++*
> *----*
> *-----*
>
> so you can write it as:
>
> WHERE NOT ((e2 < s1) OR (s2 > e1));
>
> which is *much* cleaner, no? ;)
>
> Credit goes to Joe Celko, SQL for Smarties, Chapter 13: Between and
> Overlaps Predicate, 13.2 Overlaps Predicate, page 279.
>
> Postgres actually has OVERLAPS, so you can just say:
>
> WHERE (s2, e2) OVERLAPS (s1, e1);
>
> however, at least in 8.1, that doesn't use the indexes on the start_date
> and end_date. The shortcut above does use those indexes and is nice and
> fast.
>
> You should test and see if 8.3 or 8.4 will use the indexes for OVERLAPS
> though...

- --

Jon T Erdman

Chief Information Officer voice: (210) 400-5717
Progressive Practice, Inc. jon(at)progressivepractice(dot)com
P.O. Box 17288 www.progressivepractice.com
Rochester, NY 14617

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