Postgresql – Why does PostgreSQL aggregate result in the Index or Seq scan outputting all columns of the table

Doing a query like this:

SELECT fruit, avg(cost) FROM fruit_table GROUP BY fruit

results in a query plan like this:

Sort  (cost=3713873.41..3713874.46 rows=420 width=16)
  Output: fruit, (avg(cost))
  Sort Key: fruittable.fruit
  ->  HashAggregate  (cost=3713849.87..3713855.12 rows=420 width=16)
      Output: fruit, (avg(cost))
      ->  Seq Scan on public.fruit  (cost=0.00..3677849.91 rows=7199991 width=16)
          Output: fruit, weight, cost, ...

The result is correct, but ideally, for performance reasons I would like to have weight omitted in the output of index/seq scan – what am I doing wrong?

This is the output of EXPLAIN ANALYZE VERBOSE:

Sort  (cost=3713873.41..3713874.46 rows=420 width=16) (actual time=31156.940..31156.958 rows=612 loops=1)
  Output: fruit, (avg(cost))
  Sort Key: fruittable.fruit
  Sort Method: quicksort  Memory: 53kB
  ->  HashAggregate  (cost=3713849.87..3713855.12 rows=420 width=16) (actual time=31156.272..31156.729 rows=612 loops=1)
    Output: fruit, avg(cost)
    ->  Seq Scan on public.fruittable  (cost=0.00..3677849.91 rows=7199991 width=16) (actual time=0.004..15050.636 rows=7199991 loops=1)
      Output: fruit, weight, cost, ...
Total runtime: 31157.097 ms

1. EDIT

I run the following queries based on Craig's comment:

SELECT
pg_stat_get_live_tuples('fruittable'::regclass), 
pg_stat_get_dead_tuples('fruittable'::regclass), 
pg_relation_size('fruittable'::regclass), 
pg_total_relation_size('fruittable'::regclass); 

-- OUTPUT:
pg_stat_get_live_tuples: 0 
pg_stat_get_dead_tuples: 0 
pg_relation_size: 29539123200 
pg_total_relation_size: 32118743040 

and

SELECT avg(pg_column_size(x)) FROM (SELECT * FROM fruittable LIMIT 1000) AS x;

-- OUTPUT
avg: 1354.68

2. EDIT

This is the table definition

CREATE TABLE x_2009
(
  c_a character varying(70),
  c_b character varying(70),
  c_c character varying(70),
  c_d character varying(70),
  c_e character varying(70),
  c_f double precision,
  v_g character varying(70),
  v_h character varying(70),
  c_i double precision,
  c_j double precision,
  d_k character varying(70),
  d_l character varying(70),
  c_m character varying(70),
  c_n character varying(70),
  d_o character varying(70),
  d_p character varying(70),
  d_q character varying(70),
  v_r character varying(70),
  c_s character varying(70),
  v_t double precision,
  v_u double precision,
  c_v character varying(70),
  v_w double precision,
  v_x double precision,
  v_y double precision,
  v_z double precision,
  c_aa double precision,
  v_ab double precision,
  v_ac double precision,
  v_ad double precision,
  v_ae double precision,
  v_af double precision,
  v_ag double precision,
  v_ah double precision,
  v_ai double precision,
  v_aj double precision,
  v_ak double precision,
  v_al double precision,
  v_am double precision,
  v_an double precision,
  v_ao double precision,
  v_ap double precision,
  v_aq double precision,
  v_ar double precision,
  v_as double precision,
  v_at double precision,
  v_au double precision,
  v_av double precision,
  v_aw double precision,
  v_ax double precision,
  v_ay double precision,
  v_az_cost double precision,
  v_ba double precision,
  v_bb double precision,
  v_bc double precision,
  v_bd double precision,
  v_be double precision,
  v_bf double precision,
  v_bg double precision,
  v_bh double precision,
  v_bi double precision,
  v_bj double precision,
  v_bk double precision,
  v_bl double precision,
  v_bm double precision,
  v_bn double precision,
  v_bo double precision,
  v_bp double precision,
  v_bq double precision,
  v_br double precision,
  v_bs double precision,
  v_bt double precision,
  v_bu double precision,
  v_bv double precision,
  v_bw double precision,
  v_bx double precision,
  v_by double precision,
  v_bz double precision,
  v_ca double precision,
  v_cb double precision,
  v_cc double precision,
  v_cd double precision,
  v_ce double precision,
  v_cf double precision,
  v_cg double precision,
  v_ch double precision,
  v_ci double precision,
  v_cj double precision,
  v_ck double precision,
  v_cl double precision,
  v_cm double precision,
  v_cn double precision,
  v_co double precision,
  v_cp double precision,
  v_cq double precision,
  v_cr double precision,
  v_cs double precision,
  v_ct double precision,
  v_cu double precision,
  v_cv double precision,
  v_cw double precision,
  v_cx double precision,
  v_cy double precision,
  v_cz double precision,
  v_da double precision,
  v_db double precision,
  v_dc double precision,
  v_dd double precision,
  v_de double precision,
  v_df double precision,
  v_dg double precision,
  v_dh double precision,
  v_di double precision,
  v_dj double precision,
  v_dk double precision,
  v_dl double precision,
  c_dm character varying(70),
  c_dn double precision,
  v_do double precision,
  v_dp double precision,
  v_dq double precision,
  v_dr double precision,
  v_ds double precision,
  v_dt double precision,
  v_du double precision,
  v_dv double precision,
  v_dw double precision,
  v_dx double precision,
  v_dy double precision,
  v_dz double precision,
  v_ea double precision,
  v_eb double precision,
  v_ec double precision,
  v_ed double precision,
  v_ee double precision,
  v_ef double precision,
  v_eg double precision,
  v_eh double precision,
  v_ei double precision,
  v_ej double precision,
  v_ek double precision,
  v_el double precision,
  v_em double precision,
  v_en double precision,
  v_eo double precision,
  v_ep double precision,
  v_eq double precision,
  v_er double precision,
  v_es double precision,
  v_et double precision,
  v_eu double precision,
  v_ev double precision,
  v_ew double precision,
  v_ex character varying(70),
  c_ey character varying(70),
  c_ez character varying(70),
  c_fa character varying(70),
  c_fb character varying(70),
  d_fc character varying(70),
  v_fd character varying(70),
  v_fe double precision,
  v_ff double precision,
  v_fg double precision,
  v_fh double precision,
  v_fi double precision,
  v_fj double precision,
  c_fk double precision,
  c_fl character varying(70),
  v_fm character varying(70)
)
WITH (
  OIDS=FALSE
);
ALTER TABLE x_2009
  OWNER TO xxx;
GRANT ALL ON TABLE x_2009 TO xxx;

-- Index: x_2009_idx_c_a

CREATE INDEX x_2009_idx_c_a
  ON x_2009
  USING btree
  (c_a);

-- Index: x_2009_idx_c_a_and_v_g

CREATE INDEX x_2009_idx_c_a_and_v_g
  ON x_2009
  USING btree
  (v_g, c_a);

-- Index: x_2009_idx_v_g

CREATE INDEX x_2009_idx_v_g
  ON x_2009
  USING btree
  (v_g);