if I run this:

Select mp.*, phc.*
From Maestro_Pacientes mp
Left Outer join Paciente_hc phc On mp.Codigo = phc.Paciente
Where mp.Codigo >= 1

the execution plan is this:
Plan
PLAN JOIN (MP INDEX (RDB$PRIMARY58),PHC INDEX (PK_PACIENTE_HC))
Adapted Plan
PLAN JOIN (MP INDEX (INTEG_526),PHC INDEX (PK_PACIENTE_HC))

------ Performance info ------
Prepare time = 0ms
Execute time = 32ms
Avg fetch time = 2.67 ms
Current memory = 1,879,652
Max memory = 2,110,108
Memory buffers = 2,048
Reads from disk to cache = 586
Writes from cache to disk = 0
Fetches from cache = 712

It runs ver fast....
But I need the recordset order By "Codigo"
like thist:

Select mp.*, phc.*
From Maestro_Pacientes mp
Left Outer join Paciente_hc phc On mp.Codigo = phc.Paciente
Where mp.Codigo >= 1 order By mp.Codigo

and the execution plan is this:
Plan
PLAN SORT (JOIN (MP INDEX (RDB$PRIMARY58),PHC INDEX
(PK_PACIENTE_HC)))

Adapted Plan
PLAN SORT (JOIN (MP INDEX (INTEG_526),PHC INDEX (PK_PACIENTE_HC)))

------ Performance info ------
Prepare time = 0ms
Execute time = 8s 266ms
Avg fetch time = 688.83 ms
Current memory = 1,883,264
Max memory = 2,110,108
Memory buffers = 2,048
Reads from disk to cache = 15,661
Writes from cache to disk = 0
Fetches from cache = 1,956,941

It takes 8 seconds. too much compare to the other plan.
I see that Firebird make a sort of all the tables involved in the
plan.
In table (Maestros_Pacientes) i have 335.000 records. And in table
(Paciente_Hc) I have 140.000 records.
The performance Analisis in the second plan says that the indexed
reads in
each table are equal to the number of record in each one.
Wath can i do to improve that ?

Sorry for my english :) ! I speak in spanish.

Thanks

Diego