Hi, Merlin, On Tue, 27 Jul 2004 09:07:02 -0400 "Merlin Moncure" <[EMAIL PROTECTED]> wrote:
> So, for a table t with a three part key over columns a,b,c, the query > to read the next value from t for given values a1, b1, c1 is > > select * from t where > a >= a1 and > (a > a1 or b >= b1) and > (a > a1 or b > b1 or c > c1) You mut not rely on such trickery to get any ordering, as the SQL data model contains no ordering, and a query optimizer is free to deliver you the tuples in any order it feels like. Why don't you add a 'ORDER BY a,b,c ASC' to your query? > Interestingly, it is possible to rewrite the above query by switching > and with or and >= with >. However when written that way, the planner > almost never gets it right. That's the reason why you cannot rely on any implicit ordering, the planner is free to rewrite a query as it likes as long as it delivers the same tuples, but in any order it wants. > My problem is deceptively simple: how you read the next record from a > table based on a given set of values? In practice, this is difficult > to implement. If anybody can suggest a alternative/better way to > this, I'm all ears. So you really want something like 'SELECT * FROM t WHERE a>=a1 AND b>=b1 AND c>=c1 ORDER BY a,b,c ASC LIMIT 1' HTH, Markus -- markus schaber | dipl. informatiker logi-track ag | rennweg 14-16 | ch 8001 zürich phone +41-43-888 62 52 | fax +41-43-888 62 53 mailto:[EMAIL PROTECTED] | www.logi-track.com ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faqs/FAQ.html
