I\'m dealing with a Postgres table (called \"lives\") that contains records with columns for time_stamp, usr_id, transaction_id, and lives_remaining. I need a query that wil
Actaully there's a hacky solution for this problem. Let's say you want to select the biggest tree of each forest in a region.
SELECT (array_agg(tree.id ORDER BY tree_size.size)))[1]
FROM tree JOIN forest ON (tree.forest = forest.id)
GROUP BY forest.id
When you group trees by forests there will be an unsorted list of trees and you need to find the biggest one. First thing you should do is to sort the rows by their sizes and select the first one of your list. It may seems inefficient but if you have millions of rows it will be quite faster than the solutions that includes JOIN
's and WHERE
conditions.
BTW, note that ORDER_BY
for array_agg
is introduced in Postgresql 9.0
Here's another method, which happens to use no correlated subqueries or GROUP BY. I'm not expert in PostgreSQL performance tuning, so I suggest you try both this and the solutions given by other folks to see which works better for you.
SELECT l1.*
FROM lives l1 LEFT OUTER JOIN lives l2
ON (l1.usr_id = l2.usr_id AND (l1.time_stamp < l2.time_stamp
OR (l1.time_stamp = l2.time_stamp AND l1.trans_id < l2.trans_id)))
WHERE l2.usr_id IS NULL
ORDER BY l1.usr_id;
I am assuming that trans_id
is unique at least over any given value of time_stamp
.
I like the style of Mike Woodhouse's answer on the other page you mentioned. It's especially concise when the thing being maximised over is just a single column, in which case the subquery can just use MAX(some_col)
and GROUP BY
the other columns, but in your case you have a 2-part quantity to be maximised, you can still do so by using ORDER BY
plus LIMIT 1
instead (as done by Quassnoi):
SELECT *
FROM lives outer
WHERE (usr_id, time_stamp, trans_id) IN (
SELECT usr_id, time_stamp, trans_id
FROM lives sq
WHERE sq.usr_id = outer.usr_id
ORDER BY trans_id, time_stamp
LIMIT 1
)
I find using the row-constructor syntax WHERE (a, b, c) IN (subquery)
nice because it cuts down on the amount of verbiage needed.