On Sat, Mar 9, 2013 at 4:20 PM, Paul Jungwirth <pj@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:I have a long-running multi-row UPDATE that is deadlocking with a single-row UPDATE:
2013-03-09 11:07:51 CST ERROR: deadlock detected
2013-03-09 11:07:51 CST DETAIL: Process 18851 waits for ShareLock on transaction 10307138; blocked by process 24203.
Process 24203 waits for ShareLock on transaction 10306996; blocked by process 18851.
Process 18851: UPDATE taggings tg
SET score_tier = COALESCE(x.perc, 0)
FROM (SELECT tg2.id,
percent_rank() OVER (PARTITION BY tg2.tag_id ORDER BY tg2.score ASC) AS perc
FROM taggings tg2, tags t
WHERE tg2.score IS NOT NULL
AND tg2.tag_id = t.id
AND t.tier >= 2) AS x
WHERE tg.id = x.id
AND tg.score IS NOT NULL
;
Process 24203: UPDATE "taggings" SET "score" = 2 WHERE "taggings"."id" = 29105523
Note that these two queries are actually updating different columns, albeit apparently in the same row.
Is there anything I can do to avoid a deadlock here? The big query does nothing else in its transaction; the little query's transaction might update several rows from `taggings`, which I guess is the real reason for the deadlock.
I'd be pretty satisfied with approximate values for the big query. As you can see, it is just taking the `score` of each `tagging` and computing the percentage of times it beats other taggings of the same tag. Is there something I can do with transaction isolation levels here? I don't care if the big query operates on slightly-out-of-date values. Since each query updates different columns, I think there should be no issue with them overwriting each other, right?
Thanks,
Paulit *might* help to do the calculation work (all those nested SELECTs) and store the results in a temporary table, then do the update as a second, simpler join to the temp table.
All the suggestions thus far only reduce the window in which a dead lock can occur.
If you really need to prevent that, you can split off the columns for one of the two types of updates into a separate table with a foreign key to the original table.
If you can't see the forest for the trees,
Cut the trees and you'll see there is no forest.
That way your updates happen in different tables and there's no chance on a deadlock between the two types of queries.
-- If you can't see the forest for the trees,
Cut the trees and you'll see there is no forest.
