| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | ben(at)charterworks(dot)net |
| Cc: | pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Performance of outer joins? |
| Date: | 2006-12-16 22:51:30 |
| Message-ID: | 8617.1166309490@sss.pgh.pa.us |
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| Lists: | pgsql-general |
Benjamin Smith <ben(at)schoolpathways(dot)com> writes:
> FROM customers
> JOIN calendars ON
> (
> -- GIVE A CALENDAR OF POSSIBLE DAYS FOR DELIVERIES
> calendar.day < 20061201
> AND calendar.day >= 20060101
> )
Haven't you written a cross-product join here? Seems like a bad
idea if you'd like it to be fast. Especially since you then left-joined
it to something else, meaning that every row in the cross product
must in fact generate output. Do you really want to be deluged with
a table showing all the customers you DIDN'T deliver to on each day,
for every day in the year?
regards, tom lane
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