Tobias,
Interesting example:

The 'desc' seems to be the guy triggering the sort, e.g:

explain select c.id from c join b on c_id=c.id group by c.id order by
c.id limit 5;
QUERY PLAN

-----------------------------------------------------------------------------------------
Limit (cost=0.00..0.28 rows=5 width=4)
-> Group (cost=0.00..4476.00 rows=80000 width=4)
-> Merge Join (cost=0.00..4276.00 rows=80000 width=4)
Merge Cond: ("outer".id = "inner".c_id)
-> Index Scan using c_pkey on c (cost=0.00..1518.00
rows=80000 width=4)
-> Index Scan using b_on_c on b (cost=0.00..1558.00
rows=80000 width=4)
(6 rows)

Whereas with it back in again:

explain select c.id from c join b on c_id=c.id group by c.id order by
c.id desc limit 5;
QUERY PLAN

--------------------------------------------------------------------------------------
Limit (cost=10741.08..10741.11 rows=5 width=4)
-> Group (cost=10741.08..11141.08 rows=80000 width=4)
-> Sort (cost=10741.08..10941.08 rows=80000 width=4)
Sort Key: c.id
-> Hash Join (cost=1393.00..4226.00 rows=80000 width=4)
Hash Cond: ("outer".c_id = "inner".id)
-> Seq Scan on b (cost=0.00..1233.00 rows=80000
width=4)
-> Hash (cost=1193.00..1193.00 rows=80000 width=4)
-> Seq Scan on c (cost=0.00..1193.00
rows=80000 width=4)
(9 rows)


However being a bit brutal:

set enable_mergejoin=false;
set enable_hashjoin=false;

explain select c.id from c join b on c_id=c.id group by c.id order by
c.id desc limit 5;
QUERY PLAN

--------------------------------------------------------------------------------------------------
Limit (cost=0.00..15.24 rows=5 width=4)
-> Group (cost=0.00..243798.00 rows=80000 width=4)
-> Nested Loop (cost=0.00..243598.00 rows=80000 width=4)
-> Index Scan Backward using c_pkey on c
(cost=0.00..1518.00 rows=80000 width=4)
-> Index Scan using b_on_c on b (cost=0.00..3.01
rows=1 width=4)
Index Cond: (b.c_id = "outer".id)
(6 rows)

What is interesting is why this plan is being rejected...

Cheers

Mark

Tobias Brox wrote:
> Consider this setup - which is a gross simplification of parts of our
> production system ;-)
>
> create table c (id integer primary key);
> create table b (id integer primary key, c_id integer);
> create index b_on_c on b(c_id)
>
> insert into c (select ... lots of IDs ...);
> insert into b (select id, id from c); /* keep it simple :-) */
>
> Now, I'm just interessted in some few rows.
>
> All those gives good plans:
>
> explain select c.id from c order by c.id limit 1;
> explain select c.id from c group by c.id order by c.id limit 1;
> explain select c.id from c join b on c_id=c.id order by c.id limit 1;
>
> ... BUT ... combining join, group and limit makes havoc:
>
> explain select c.id from c join b on c_id=c.id group by c.id order by c.id
> desc limit 5;
>

---------------------------(end of broadcast)---------------------------
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