On Mon, Jun 15, 2020 at 10:29:41PM +0900, Michael Paquier wrote: > Attempting to run installcheck with 13~ and a value of work_mem lower > than the default causes two failures, both related to incremental > sorts (here work_mem = 1MB): > 1) Test incremental_sort: > @@ -4,12 +4,13 @@ > select * from (select * from tenk1 order by four) t order by four, ten; > QUERY PLAN > ----------------------------------- > - Sort > + Incremental Sort > Sort Key: tenk1.four, tenk1.ten > + Presorted Key: tenk1.four > -> Sort > Sort Key: tenk1.four > -> Seq Scan on tenk1 > -(5 rows) > +(6 rows)
Looking at this one, it happens that this is the first test in incremental_sort.sql, and we have the following comment: -- When we have to sort the entire table, incremental sort will -- be slower than plain sort, so it should not be used. explain (costs off) select * from (select * from tenk1 order by four) t order by four, ten; When using such a low value of work_mem, why do we switch to an incremental sort if we know that it is going to be slower than a plain sort? Something looks wrong in the planner choice here. -- Michael
signature.asc
Description: PGP signature
