Hello Richard, Thank you for the response. I did forget to mention that the columns have the following meanings.
One, if a begin or end date is null, it means that the role is open ended in that direction. For example, if there is no end date, that means currently the role will go on forever beginning with the start date. Your idea of using minimal and maximum dates is an interesting one and not one that I had considered. I will do some testing later today and see if that makes a difference. The other option I am toying with is simply having a status column which is updated nightly via a cron job. This will probably be the most efficient and can be indexed. I also forgot to say that we have seen this behavior on 2 boxes both on Linux (Red Hat ES & Mandrake) and both are running Postgres 8.0 (8.0.1 and 8.0.3). Strangely, after playing with statistics some yesterday (setting from 10 to 100 to 1000 and back to 10 and analyzing), the 8.0.1 machine picks a different plan and runs in a 101.104ms. The larger machine (dual proc Opt, 6 disk raid 10, etc) with 8.0.3 still takes 3-5minutes to run the same query with the same data set even after playing with statistics and repeated analyze on the same table. It just seems odd. It seems it is picking the incorrect plan based off of an overly optimistic estimate of rows returned from the calculation. The other frustration with this is that this sql is machine generated which is why we have some of the awkwardness in the calculation. That calc gets used for a lot of different things including column definitions when people want to see the column on screen. Thanks, -Chris On Wednesday 14 September 2005 05:13 am, Richard Huxton wrote: > Chris Kratz wrote: > > Hello All, > > > > We are struggling with a specific query that is killing us. When doing > > explain analyze on the entire query, we *seem* to be getting killed by > > the estimated number of rows on a case statement calculation. > > > > I've included a snippet from the explain analyze of the much larger > > query. The line in question, (cost=0.00..106.52 rows=1 width=16) (actual > > time=0.048..67.728 rows=4725 loops=1) shows that it returned 4700 rows > > instead of 1 which when coupled with a later join causes the statement to > > run over 3 minutes. > > > > It seems that it thinks that the scan on role_id is going to return 1 > > row, but in reality returns 4725 rows. The case statement causing the > > problem uses todays date to see if a particular row is still active. > > Here is a test case showing how far off the estimate is from the reality. > >  > > > >  A much simpler statement triggers the incorrect row counts here. > > > > explain analyze > > select * > > from roles rol > > where > > > > CASE > > WHEN rol.role_id IS NULL > > THEN NULL > > WHEN rol."begin" IS NOT NULL and rol."end" IS NOT NULL > > THEN > > CASE WHEN TIMESTAMP 'now'>=rol."begin" and TIMESTAMP > > 'now'<=rol."end" > > THEN 'Active' > > ELSE 'Inactive' END > > WHEN rol."begin" IS NOT NULL > > THEN > > CASE WHEN TIMESTAMP 'now'>=rol."begin" > > THEN 'Active' > > ELSE 'Inactive' END > > WHEN rol."end" IS NOT NULL > > THEN > > CASE WHEN TIMESTAMP 'now'<=rol."end" > > THEN 'Active' > > ELSE 'Inactive' END > > ELSE 'Active' > > END = 'Active' > > Aside #1 - I'm not entirely clear how role_id can be null since you > seemed to be joining against it in the real query. > > Aside #2 - You're probably better off with CURRENT_DATE since begin/end > seem to be dates, rather than TIMESTAMP 'now' - and in any case you > wanted "timestamp with time zone" > > OK, I think the root of your problem is your use of null to mean "not > ended" or "not started" (whatever 'not started' means). PostgreSQL has > the handy timestamptz value "infinity", but only for timestamps and not > for dates. I'd probably cheat a little and use an end date of > '9999-12-31' or similar to simulate "infinity". Then your test is simply: > > WHERE > ... > AND (rol.begin <= CURRENT_DATE AND rol.end >= CURRENT_DATE) > > That should estimate simply enough. > > -- > Richard Huxton > Archonet Ltd > > ---------------------------(end of broadcast)--------------------------- > TIP 3: Have you checked our extensive FAQ? > > http://www.postgresql.org/docs/faq -- Chris Kratz ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq