PostgreSQL does not choose my indexes well

[Arcadio Ortega Reinoso <arcadio.ortega@xxxxxxxxx>]

Hi,

I don't understand why postgresql doesn't use clearly the most optimal 
index for a query in PLAN.

Can you help me?


create table public.tabla
(
    cod_tabla bigint not null,
    tabla varchar(31) not null,
    constraint pk_tabla primary key (cod_tabla)
);


create table public.entidad
(
    cod_entidad bigint not null,
    cod_tabla bigint not null,
    cod_entidad_tabla bigint not null,
    constraint pk_entidad primary key (cod_entidad),
    constraint fk_tabla_entidad foreign key (cod_tabla)
        references public.tabla (cod_tabla) match simple
        on update cascade
        on delete cascade
);


CREATE INDEX idx_tabla_entidad
    ON public.entidad USING btree
    (cod_tabla ASC NULLS LAST);

CREATE INDEX idx_entidad_tabla_4
    ON public.entidad USING btree
    (cod_entidad_tabla ASC NULLS LAST)
    INCLUDE(cod_entidad, cod_tabla, cod_entidad_tabla)
    WHERE cod_tabla::bigint = 4;




SELECT count(*) from entidad;
34.413.354

SELECT count(*) from entidad where cod_tabla = 4;
1.409.985



explain (analyze, buffers, format text) select * from entidad where 
cod_tabla = 4


Index Scan using idx_tabla_entidad on entidad (cost=0.56..51121.41 
rows=1405216 width=20) (actual time=0.037..242.609 rows=1409985 loops=1)
  Index Cond: ((cod_tabla)::bigint = 4)
  Buffers: shared hit=12839
Planning Time: 0.158 ms
Execution Time: 311.828 ms



Why postgresql doesnt use the index idx_entidad_tabla_4?????

Thanks in advance