Re: Nested loop join and date range query

On 5/2/06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Ian Burrell" <ianburrell@gmail.com> writes:
> > We recently upgraded to PostgreSQL 8.1 from 7.4 and a few queries are
> > having performance problems and running for very long times.  The
> > commonality seems to be PostgreSQL 8.1 is choosing to use a nested
> > loop join because it estimates there will be only be a single row.
>
> We've already noted that there's a problem with estimating zero-width
> ranges (too lazy to search the archives, but this has come up at least
> twice recently).  Can you modify your app to generate something like
>
>         week >= x and week < x+1
>
> instead of
>
>         week >= x and week <= x
>

I am working on modifying the SQL generation code to replace the
zero-width range with an equals.

Does BETWEEN have the same bug?

> ?  My recollection is that the fix will probably be complicated
> enough to not get back-patched into 8.1.
>
> BTW, AFAIK the same problem exists in 7.4.  What kind of estimates/plans
> were you getting for this case in 7.4?
>

We get similar rows=1 estimates on 7.4.  7.4 doesn't choose to use the
nested loop joins so it performs fine.

We have been getting similar rows=1 estimates and nested loop joins
with some other queries. But I think those are caused by not
frequently analyzing log type tables and then searching for recent
days which it doesn't think exist.

 - Ian