Re: Wrong stats for empty tables

From: Tom Lane [tgl(at)sss(dot)pgh(dot)pa(dot)us]
Subject: Re: [HACKERS] Wrong stats for empty tables

"Emmanuel Cecchet" <Emmanuel(dot)Cecchet(at)asterdata(dot)com> writes:
> Is this a bug?

No, it's intentional.

So what is the rationale behind not being able to use indexes and optimizing empty tables as in the following example:

manu=# create table father (id int, val int, tex varchar(100), primary key(id));
manu=# create table other (id1 int, id2 int, data varchar(10), primary key(id1,id2));
insert some data
manu=# explain select father.*,id2 from father left join other on father.id=other.id1 where id2=2 order by id;
QUERY PLAN
------------------------------------------------------------------------
Sort (cost=37.81..37.82 rows=5 width=230)
Sort Key: father.id
-> Hash Join (cost=23.44..37.75 rows=5 width=230)
Hash Cond: (father.id = other.id1)
-> Seq Scan on father (cost=0.00..13.10 rows=310 width=226)
-> Hash (cost=23.38..23.38 rows=5 width=8)
-> Seq Scan on other (cost=0.00..23.38 rows=5 width=8)
Filter: (id2 = 2)
(8 rows)

manu=# create table child1() inherits(father);
manu=# create table child2() inherits(father);
manu=# create table child3() inherits(father);
manu=# create table child4() inherits(father);
manu=# create table child5() inherits(father);
manu=# create table child6() inherits(father);
manu=# create table child7() inherits(father);
manu=# create index i1 on child1(id);
manu=# create index i2 on child2(id);
manu=# create index i3 on child3(id);
manu=# create index i4 on child4(id);
manu=# create index i5 on child5(id);
manu=# create index i6 on child6(id);
manu=# create index i7 on child7(id);
manu=# explain select father.*,id2 from father left join other on father.id=other.id1 where id2=2 order by id;
QUERY PLAN
------------------------------------------------------------------------------------
Sort (cost=140.00..140.16 rows=62 width=230)
Sort Key: public.father.id
-> Hash Join (cost=23.44..138.16 rows=62 width=230)
Hash Cond: (public.father.id = other.id1)
-> Append (cost=0.00..104.80 rows=2480 width=226)
-> Seq Scan on father (cost=0.00..13.10 rows=310 width=226)
-> Seq Scan on child1 father (cost=0.00..13.10 rows=310 width=226)
-> Seq Scan on child2 father (cost=0.00..13.10 rows=310 width=226)
-> Seq Scan on child3 father (cost=0.00..13.10 rows=310 width=226)
-> Seq Scan on child4 father (cost=0.00..13.10 rows=310 width=226)
-> Seq Scan on child5 father (cost=0.00..13.10 rows=310 width=226)
-> Seq Scan on child6 father (cost=0.00..13.10 rows=310 width=226)
-> Seq Scan on child7 father (cost=0.00..13.10 rows=310 width=226)
-> Hash (cost=23.38..23.38 rows=5 width=8)
-> Seq Scan on other (cost=0.00..23.38 rows=5 width=8)
Filter: (id2 = 2)
(16 rows)

I must admit that I did not see what the original intention was to get this behavior.
Emmanuel