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...