I've just spent a few hours searching and reading about the postgres way of selecting distinct records. I understand the points made about the ORDER BY limitation of DISTINCT ON, and the abiguity of GROUP BY, but I think there's a (simple, common) case that have been missed in the discussion. Here is my sitation: table "projects": id title more stuff (pretend there's 20 more columns.) ----------------------------------------------------------- 1 buildrome moredata inothercolumns 2 housework evenmoredata letssay20columns table "todos": id projectid name duedate ----------------------------------------- 1 1 conquer 1pm 2 1 laybricks 10pm 3 2 dolaundry 5pm In english, I want to "select projects and order them by the ones that have todos due the soonest." Does that sound like a reasonable request? This won't work in postgresql 8.2.4: select distinct on(a.id) a.* from projects a inner join todos b on b.projectid = a.id order by b.duedate offset 10 limit 20; ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions What to do? I could: Option A. select distinct on(b.duedate, a.id) a.* from projects a inner join todos b on b.projectid = a.id order by b.duedate, a.id offset 10 limit 20; But that's not equivalent because the duedates will break uniqueness. The results will have two records where the a.id is 1. So I guess that's a non-option. Option B. I could try group by: select a.* from projects a inner join todos b on b.projectid = a.id group by a.id order by b.duedate offset 10 limit 20; Query failed: ERROR: column "a.title" must appear in the GROUP BY clause or be used in an aggregate function (And presumably every other column of projects would too). Option C. I could list out each and every one of the 20+ columns of the project table in MAX() functions so that they'd be "in an aggregate function": select MAX(a.title) as title, MAX(a.column2) as column2, MAX(a.column3) as column3, MAX(a.column4) as column4, MAX(a.column5) as column5, MAX(a.column6) as column6, MAX(a.column7) as column7, MAX(a.column8) as column8, MAX(a.column9) as column9, MAX(a.column8) as ihatepostgresql, MAX(a.column9) as imgoingbacktomysql, from projects a inner join todos b on b.projectid = a.id group by a.id order by b.duedate offset 10 limit 20; http://groups.google.com/group/comp.databases.postgresql.general/msg/923aed5b8ea5faa1 Option D: select distinct a.* from projects a inner join todos b on b.projectid = a.id order by b.duedate offset 10 limit 20; Query failed: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list But I don't want b.duedate in my results. And if I add it, it will, again, break the desired uniqueness (see option A). (And, actually even if that one worked, it would suck. Instead of making postgres compare only one column to determine uniqueness, it now has to compare all of them. (In other words, the whole advantage of DISTINCT ON is out the window).) Option E: I could use a subselect. But notice my offset, limit. If I use a subselect, then postgresql would have to build ALL of the results in memory (to create the subselect virtual table), before I apply the offset and limit on the subselect. Any suggestion would be appreciated. BTW for those of you who are curious, in mysql (that other db), this would be: select a.* from projects a inner join todos b on b.projectid = a.id group by a.id order by b.duedate limit 10,20; ---------------------------(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
