On 5/2/06, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
"Ian Burrell" <ianburrell@xxxxxxxxx> 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
