On Thu, 25 Mar 2004, Ara Anjargolian wrote:
> I've run into this odd planner choice which I don't quite understand.
>
> I have two tables articles, users and
> articles.article_id and users.user_id are primary keys.
>
> Insides articles there are two optional fields author_id1, author_id2
> which all reference users.user_id.
>
> And now the plans:
> (by the way this is pg 7.4 and I set enable_seqscan to off).
>
> jargol=# explain select user_id, first_names, last_name from articles, users
> where article_id = 5027 and (articles.author_id1 = users.user_id);
> QUERY PLAN
> ----------------------------------------------------------------------------
> ------
> Nested Loop (cost=0.00..4.04 rows=1 width=26)
> -> Index Scan using articles_pk on articles (cost=0.00..2.01 rows=1
> width=4)
> Index Cond: (article_id = 5027)
> -> Index Scan using users_pk on users (cost=0.00..2.01 rows=1 width=26)
> Index Cond: ("outer".author_id1 = users.user_id)
> (5 rows)
>
> jargol=# explain select user_id, first_names, last_name from articles, users
> where article_id = 5027 and (articles.author_id1 = users.user_id or
> articles.author_id2 = users.user_id);
> QUERY PLAN
> ----------------------------------------------------------------------------
> -----------------------
> Nested Loop (cost=100000000.00..100000003.11 rows=2 width=26)
> Join Filter: (("outer".author_id1 = "inner".user_id) OR
> ("outer".author_id2 = "inner".user_id))
> -> Index Scan using articles_pk on articles (cost=0.00..2.01 rows=1
> width=8)
> Index Cond: (article_id = 5027)
> -> Seq Scan on users (cost=100000000.00..100000001.04 rows=4 width=26)
> (5 rows)
>
> Why does it think it MUST do a seq-scan in the second case? users.user_id is
> a primary key,
> so shouldn't it behave exactly as in the first case?
>
> Any enlightenment on this problem will be much appreciated.
Are articles.author_id1 and users.user_id the same type? Have you tried
casting one to the other's type if they're different?
