Index usage on a temporary table

I have two rather simple queries. The first query UPDATE mp_physical SET periodic_number = '' WHERE periodic_number is NULL; and it's plan duration: 0.125 ms plan: Query Text: UPDATE mp_physical SET periodic_number = '' WHERE periodic_number is NULL; Update on mp_physical (cost=0.42..7.34 rows=1 width=801) -> Index Scan using "_I_periodic_number" on mp_physical (cost=0.42..7.34 rows=1 width=801) Index Cond: (periodic_number IS NULL) And the second one: UPDATE observations_optical_temp SET designation = '' WHERE periodic_number is NULL; and it's plan: duration: 2817.375 ms plan: Query Text: UPDATE observations_optical_temp SET periodic_number = '' WHERE periodic_number is NULL; Update on observations_optical_temp (cost=103.55..9223.01 rows=5049 width=212) -> Bitmap Heap Scan on observations_optical_temp (cost=103.55..9223.01 rows=5049 width=212) Recheck Cond: (periodic_number IS NULL) -> Bitmap Index Scan on "_I_per_num_temp" (cost=0.00..102.29 rows=5049 width=0) Index Cond: (periodic_number IS NULL) I expect the second plan shold be the same the first one. But it's not. Why? Here are dumps of the tables. CREATE TABLE public.mp_physical( id_mpp serial NOT NULL, id_comet_parts integer, "SPK_id" public.nonnegative_int, designation varchar(30), name varchar(100), prefix varchar, "is_NEO" bool, "H" double precision, "G" double precision, diameter public.nonnegative_double, extent varchar(30), extent_error public.nonnegative_double, geometric_albedo public.nonnegative_double, rot_per public.nonnegative_double, "GM" public.nonnegative_double, "BV" public.nonnegative_double, "UB" public.nonnegative_double, "spec_B" varchar(30), "spec_T" varchar(30), lca double precision, multiplicity public.nonnegative_int, polar_ang double precision, polar_slope_ang double precision, a double precision, b double precision, mass public.nonnegative_double, mp_type public.mp_type NOT NULL, periodic_number varchar(5), diameter_method_def varchar(200), discovery_info text, "H_sigma" public.nonnegative_double, "G_sigma" public.nonnegative_double, diameter_sigma public.nonnegative_double, geometric_albedo_sigma public.nonnegative_double, rot_per_sigma public.nonnegative_double, "GM_sigma" public.nonnegative_double, "BV_sigma" public.nonnegative_double, "UB_sigma" public.nonnegative_double, lca_sigma public.nonnegative_double, a_sigma public.nonnegative_double, b_sigma public.nonnegative_double, polar_ang_sigma public.nonnegative_double, mass_sigma public.nonnegative_double, CONSTRAINT "_C_id_ap" PRIMARY KEY (id_mpp) ); CREATE INDEX "_I_name" ON mp_physical USING btree (name); CREATE INDEX "_I_designation" ON mp_physical USING btree(mpp_designation); CREATE INDEX "_I_periodic_number" ON mp_physical USING btree(periodic_number); CREATE INDEX "_I_mp_type" ON mp_physical USING btree(mp_type); And CREATE TEMPORARY TABLE "observations_optical_temp"(note_1,date,"RA","Dec",magnitude,band,id_observatory,id_mpp,"Dec_degree",observatory_code,periodic_number,mpp_designation,mp_type) AS SELECT note_1,date,"RA","Dec",magnitude,band,id_observatory,id_mpp,"Dec_degree",'1'::varchar(3),'1'::varchar(8),'1'::varchar(30),'A'::public.mp_type FROM observations_optical; CREATE TABLE observations_optical( id_obs_o bigint note_1 varchar, date timestamp NOT NULL, "RA" time NOT NULL, "Dec_degree" integer NOT NULL, "Dec" time NOT NULL, magnitude double precision, band varchar, id_observatory integer, id_mpp integer, CONSTRAINT "_PK_id_obs_o" PRIMARY KEY (id_obs_o) ); CREATE INDEX "_I_temp_1" ON observations_optical_temp USING btree(mpp_designation); CREATE INDEX "_I_temp_2" ON observations_optical_temp USING btree(periodic_number); CREATE INDEX "_I_temp_3" ON observations_optical_temp USING btree(mp_type);