[GENERAL] select on view shows different plan than select on table

Hi list,

I have a view defined as:
CREATE VIEW kkm_ergebnisse.v_protokoll_details_mit_dauer AS
SELECT
   id,
   schritt,
   objekt_typ,
   objekt_id,
   zeit,
   rechenweg_thema,
   rechenweg_variante,
   rechenweg_stoffgruppe,
   formel_inhalt,
   formel_stoff,
   formel_variablen,
   ergebnis_variable,
   ergebnis_wert,
   status,
   id_rechenlauf,
   formel_id,
   formel_name,
   formel_variante,
   ergebnis_variable_einheit,
   zeitpunkt,
   DATE_PART('seconds', zeitpunkt::time without time zone -
LAG(zeitpunkt::time without time zone) OVER (ORDER BY id)) * 1000 AS
dauer_in_ms
FROM
   kkm_ergebnisse.t_protokoll_details ;

Table kkm_ergebnisse.t_protokoll_details is defined like this:
CREATE TABLE kkm_ergebnisse.t_protokoll_details
(
   id serial NOT NULL,
   schritt integer,
   objekt_typ smallint NOT NULL,
   objekt_id integer,
   zeit integer,
   rechenweg_thema character varying(256),
   rechenweg_variante character varying(256),
   rechenweg_stoffgruppe character varying(256),
   formel_inhalt character varying(4000),
   formel_stoff character varying(256),
   formel_variablen character varying(4000),
   ergebnis_variable character varying(256),
   ergebnis_wert double precision,
   status character varying(4000),
   id_rechenlauf integer NOT NULL,
   formel_id integer,
   formel_name character varying(256),
   formel_variante character varying(256),
   ergebnis_variable_einheit character varying(255),
   zeitpunkt time with time zone DEFAULT clock_timestamp(),
   CONSTRAINT pk PRIMARY KEY (id),
   CONSTRAINT fk_rechenlauf FOREIGN KEY (id_rechenlauf)
       REFERENCES kkm_ergebnisse.t_rechenlaeufe (id) MATCH SIMPLE
       ON UPDATE CASCADE ON DELETE CASCADE,
   CONSTRAINT ck_protokoll_details_objekt_typ CHECK (objekt_typ = ANY
(ARRAY[1, 2]))
);

Why do I get different execution plans when querying the view like this:
SELECT
   *
FROM
   kkm_ergebnisse.v_protokoll_details_mit_dauer
WHERE
   id_rechenlauf = 123
ORDER BY
   schritt ;

opposed to querying against the definition of the view?
SELECT
   id,
   schritt,
   objekt_typ,
   objekt_id,
   zeit,
   rechenweg_thema,
   rechenweg_variante,
   rechenweg_stoffgruppe,
   formel_inhalt,
   formel_stoff,
   formel_variablen,
   ergebnis_variable,
   ergebnis_wert,
   status,
   id_rechenlauf,
   formel_id,
   formel_name,
   formel_variante,
   ergebnis_variable_einheit,
   zeitpunkt,
   DATE_PART('seconds'::text, zeitpunkt::time without time zone -
LAG(zeitpunkt::time without time zone) OVER (ORDER BY id)) *
1000::double precision AS dauer_in_ms
FROM
   kkm_ergebnisse.t_protokoll_details
WHERE
   id_rechenlauf = 123
ORDER BY
   schritt ;

Execution plan query against view (slow):
Sort  (cost=570776.54..570779.14 rows=1037 width=347) (actual
time=4067.919..4068.133 rows=11250 loops=1)
   Output: v_protokoll_details_mit_dauer.id,
v_protokoll_details_mit_dauer.schritt,
v_protokoll_details_mit_dauer.objekt_typ,
v_protokoll_details_mit_dauer.objekt_id,
v_protokoll_details_mit_dauer.zeit,
v_protokoll_details_mit_dauer.rechenweg_thema, v_proto (...)
   Sort Key: v_protokoll_details_mit_dauer.schritt
   Sort Method: quicksort  Memory: 6185kB
   Buffers: shared hit=30925 read=166050
   ->  Subquery Scan on v_protokoll_details_mit_dauer
(cost=0.43..570724.60 rows=1037 width=347) (actual
time=4038.722..4062.481 rows=11250 loops=1)
         Output: v_protokoll_details_mit_dauer.id,
v_protokoll_details_mit_dauer.schritt,
v_protokoll_details_mit_dauer.objekt_typ,
v_protokoll_details_mit_dauer.objekt_id,
v_protokoll_details_mit_dauer.zeit,
v_protokoll_details_mit_dauer.rechenweg_thema, v (...)
         Filter: (v_protokoll_details_mit_dauer.id_rechenlauf = 123)
         Rows Removed by Filter: 3091203
         Buffers: shared hit=30925 read=166050
         ->  WindowAgg  (cost=0.43..531778.35 rows=3115700 width=339)
(actual time=0.127..3839.099 rows=3102453 loops=1)
               Output: pd.id, pd.schritt, pd.objekt_typ, pd.objekt_id,
pd.zeit, pd.rechenweg_thema, pd.rechenweg_variante,
pd.rechenweg_stoffgruppe, pd.formel_inhalt, pd.formel_stoff,
pd.formel_variablen, pd.ergebnis_variable, pd.ergebnis_wert, pd.status,
p (...)
               Buffers: shared hit=30925 read=166050
               ->  Index Scan using pk on
kkm_ergebnisse.t_protokoll_details pd  (cost=0.43..446096.60
rows=3115700 width=339) (actual time=0.109..1682.884 rows=3102453 loops=1)
                     Output: pd.id, pd.schritt, pd.objekt_typ,
pd.objekt_id, pd.zeit, pd.rechenweg_thema, pd.rechenweg_variante,
pd.rechenweg_stoffgruppe, pd.formel_inhalt, pd.formel_stoff,
pd.formel_variablen, pd.ergebnis_variable, pd.ergebnis_wert, pd.sta (...)
                     Buffers: shared hit=30925 read=166050
Planning time: 0.323 ms
Execution time: 4069.073 ms

Execution plan of query against table (fast):
Sort  (cost=305.58..308.17 rows=1037 width=339) (actual
time=51.558..52.140 rows=11250 loops=1)
   Output: id, schritt, objekt_typ, objekt_id, zeit, rechenweg_thema,
rechenweg_variante, rechenweg_stoffgruppe, formel_inhalt, formel_stoff,
formel_variablen, ergebnis_variable, ergebnis_wert, status,
id_rechenlauf, formel_id, formel_name, formel_variante, (...)
   Sort Key: t_protokoll_details.schritt
   Sort Method: quicksort  Memory: 6185kB
   Buffers: shared hit=687
   ->  WindowAgg  (cost=222.52..253.63 rows=1037 width=339) (actual
time=15.925..37.210 rows=11250 loops=1)
         Output: id, schritt, objekt_typ, objekt_id, zeit,
rechenweg_thema, rechenweg_variante, rechenweg_stoffgruppe,
formel_inhalt, formel_stoff, formel_variablen, ergebnis_variable,
ergebnis_wert, status, id_rechenlauf, formel_id, formel_name, formel_var
(...)
         Buffers: shared hit=687
         ->  Sort  (cost=222.52..225.12 rows=1037 width=339) (actual
time=15.905..16.660 rows=11250 loops=1)
               Output: id, schritt, objekt_typ, objekt_id, zeit,
rechenweg_thema, rechenweg_variante, rechenweg_stoffgruppe,
formel_inhalt, formel_stoff, formel_variablen, ergebnis_variable,
ergebnis_wert, status, id_rechenlauf, formel_id, formel_name, form (...)
               Sort Key: t_protokoll_details.id
               Sort Method: quicksort  Memory: 6185kB
               Buffers: shared hit=687
               ->  Index Scan using fki_protokoll_details_id_rechenlauf
on kkm_ergebnisse.t_protokoll_details  (cost=0.43..170.58 rows=1037
width=339) (actual time=0.037..7.281 rows=11250 loops=1)
                     Output: id, schritt, objekt_typ, objekt_id, zeit,
rechenweg_thema, rechenweg_variante, rechenweg_stoffgruppe,
formel_inhalt, formel_stoff, formel_variablen, ergebnis_variable,
ergebnis_wert, status, id_rechenlauf, formel_id, formel_name (...)
                     Index Cond: (t_protokoll_details.id_rechenlauf = 123)
                     Buffers: shared hit=687
Planning time: 0.229 ms
Execution time: 55.245 ms

Thanks for any insight...