I agree about the performance; but it won't be that bad if PG can unnest these subqueries and convert them into join views!!! In that case, these views would return just one row (LIMIT 1), and that is the best a developer can do to help the optimizer make the decision. If the optimizer knows that a relation in the join is going to return just one row, it would try to evaluate that relation first, yeilding better performance in the subsequent join operations. But I dont think we have a choice; unless, of course, if someday PG starts supporting Oracle-like 'lateral views', where we can write have predicates in the where clause of a view which refer the columns of another relation in the join which this view is a part of!!! Do let us all know if you find a better solution. Regards, Gurjeet. On 6/20/06, Sim Zacks <sim@xxxxxxxxxxxxxx> wrote:
Thank you for responding. I was thinking along those lines as well, though that would be an absolute performance killer. Gurjeet Singh wrote: > It would have been quite easy if done in Oracle's 'lateral view' > feature. But I think it is achievable in standard SQL too; using > subqueries in the select-clause. > > Try something like this: > > select > Employee, EventDate, > EventTime as e1_time, > EventType as e1_type, > ( select > EventTime > from > Events > where Employee = O.Employee > and EventDate = O.EventDate > and EventTime > O.EventTime > limit 1 > )as e_time_1, > ( select > EventType > from > Events > where Employee = O.Employee > and EventDate = O.EventDate > and EventTime > O.EventTime > limit 1 > ) > from > Events > > Hope it helps... > > Regards, > Gurjeet. > > On 6/20/06, Sim Zacks <sim@xxxxxxxxxxxxxx> wrote: >> I am having brain freeze right now and was hoping someone could help me >> with a (fairly) simple query. >> >> I need to join on the next row in a similar table with specific criteria. >> >> I have a table with events per employee. >> I need to have a query that gives per employee each event and the event >> after it if it happened on the same day. >> >> The Events table structure is: >> >> EventID >> Employee >> EventDate >> EventTime >> EventType >> >> I want my query resultset to be >> Employee,EventDate(1),EventTime(1),EventType(1),EventTime(2),EventType(2) >> Where Event(2) is the first event of the employee that took place after >> the other event. >> >> Example >> EventID Employee EventDate EventTime EventType >> 1 John 6/15/2006 7:00 A >> 2 Frank 6/15/2006 7:15 B >> 3 Frank 6/15/2006 7:17 C >> 4 John 6/15/2006 7:20 C >> 5 Frank 6/15/2006 7:25 D >> 6 John 6/16/2006 7:00 A >> 7 John 6/16/2006 8:30 R >> >> Expected Results >> John, 6/15/2006, 7:00, A, 7:20, C >> Frank, 6/15/2006, 7:15, B, 7:17, C >> Frank, 6/15/2006, 7:17, C, 7:25, D >> John, 6/16/2006, 7:00, A, 8:30, R >> >> To get this result set it would have to be an inner join on employee and >> date where the second event time is greater then the first. But I don't >> want the all of the records with a greater time, just the first event >> after. >> >> Thank You >> Sim >> >> ---------------------------(end of broadcast)--------------------------- >> TIP 1: if posting/reading through Usenet, please send an appropriate >> subscribe-nomail command to majordomo@xxxxxxxxxxxxxx so that your >> message can get through to the mailing list cleanly >> > > ---------------------------(end of broadcast)--------------------------- > TIP 3: Have you checked our extensive FAQ? > > http://www.postgresql.org/docs/faq > ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
