Why using a column from a virtual table makes SQL in Oracle slow

Question:

Good.

Not wanting to make the subject too "gaseous" but I just wanted tips on things to investigate in the environment.

I have a SQL that uses a virtual table, for a number of reasons that don't really matter.

Something like :

select *
from
(select a,b,c,x,virtual.d
 from tabela1,
(select a,b,c,d
 from tabela2
 where ....) virtual
where virtual.a = tabela1.a
and virtual.b = tabela1.b
virtual.c = tabela1.c)

SQL runs fast , but I need to do a type test

 select *
    from
    (select a,b,c,virtual.d
     from tabela1,
    (select a,b,c,d
     from tabela2
     where ....) virtual
    where virtual.a = tabela1.a
    and virtual.b = tabela1.b
    virtual.c = tabela1.c)
where ((c <> d) or (a=1))

By doing this the SQL is extremely slow.

What could be investigated.

I don't know if details like description, indexes etc would help, the basic question is:

Why does using a column from a virtual table make SQL in Oracle slow?

Grateful.

Answer:

If it is of interest to others , I solved it with a HINT

SELECT /*+USE_CONCAT*/ * from
    (select a,b,c,virtual.d
     from tabela1,
    (select a,b,c,d
     from tabela2
     where ....) virtual
    where virtual.a = tabela1.a
    and virtual.b = tabela1.b
    virtual.c = tabela1.c)
where ((c <> d) or (a=1))

Source