query planner uses sequencial scan instead of index scan

Hi all, iam having trouble with a query, in this query we have parameters, to indicate the starting month and the
endingmonth (commented line in the query). 

You can see the explain using the parameters of month from 1 to 6

EXPLAIN ANALYZE
select pq.nome, mv.data, mv.valor,
          gprd.tipo,
          tprd.tipo,
          prd.nome,
          qtd.ano, qtd.mes, qtd.sum,
          rct.sum,
          rcb.ano, rcb.sigla, rcb.n_recibo, rcb.data, rcb.ncontrib, rcb.nome_cl, rcb.morada_cl, rcb.codpostal_cl
 from parques pq,
           movimentos mv left join a_mov_rcb amr on mv.idmovimento=amr.idmov left join recibos rcb on
amr.idrecibo=rcb.idrecibo,
           (select idmov,ano,mes,idasso,sum(valor) from receitas group by 1,2,3,4) rct,
           (select idmov,ano,mes,idasso,sum(quantidade) from quantidades group by 1,2,3,4) qtd,
           produtos prd,
           tipoprodutos tprd,
           grp_prod gprd,
           a_prk_prod app
  where pq.idparque=mv.idparque
      and pq.idparque=10
      and rct.ano=2009
      and rct.mes between 1 and 6  /* HERE IS THE STARTING AND ENDING MONTH */
      and mv.idtipo_mv=21
      and mv.vivo
      and mv.idmovimento=rct.idmov
      and rct.idmov=qtd.idmov
      and rct.idasso=qtd.idasso
      and rct.ano=qtd.ano
      and rct.mes=qtd.mes
      and rct.idasso=app.idasso and app.idproduto=prd.idproduto
      and prd.idtipoproduto=tprd.idtipoproduto
      and prd.idgrp_prod=gprd.idgrp_prod
  order by mv.data, prd.idproduto, gprd.idgrp_prod, rcb.sigla, rcb.n_recibo, qtd.ano, qtd.mes




"Sort  (cost=23852.81..23852.82 rows=1 width=526) (actual time=339.156..339.197 rows=146 loops=1)"
"  Sort Key: mv.data, prd.idproduto, gprd.idgrp_prod, rcb.sigla, rcb.n_recibo, qtd.ano, qtd.mes"
"  ->  Nested Loop  (cost=23682.31..23852.80 rows=1 width=526) (actual time=319.009..338.801 rows=146 loops=1)"
"        ->  Nested Loop  (cost=23682.31..23851.49 rows=1 width=462) (actual time=318.986..337.758 rows=146 loops=1)"
"              ->  Nested Loop  (cost=23682.31..23851.18 rows=1 width=334) (actual time=318.952..337.159 rows=146
loops=1)"
"                    ->  Nested Loop  (cost=23682.31..23850.87 rows=1 width=202) (actual time=318.917..336.602 rows=146
loops=1)"
"                          ->  Nested Loop  (cost=23682.31..23849.43 rows=1 width=126) (actual time=318.880..335.960
rows=146loops=1)" 
"                                ->  Hash Join  (cost=23682.31..23841.15 rows=1 width=130) (actual
time=318.809..335.161rows=146 loops=1)" 
"                                      Hash Cond: ((rct.idmov = mv.idmovimento) AND (rct.idasso = qtd.idasso) AND
(rct.mes= qtd.mes))" 
"                                      ->  HashAggregate  (cost=5143.05..5201.88 rows=4706 width=24) (actual
time=69.150..79.543rows=14972 loops=1)" 
"                                            ->  Seq Scan on receitas  (cost=0.00..5033.23 rows=8786 width=24) (actual
time=0.236..55.824rows=15668 loops=1)" 
"                                                  Filter: ((ano = 2009) AND (mes >= 1) AND (mes <= 6))"
"                                      ->  Hash  (cost=18539.12..18539.12 rows=8 width=126) (actual
time=249.418..249.418rows=146 loops=1)" 
"                                            ->  Hash Join  (cost=18332.76..18539.12 rows=8 width=126) (actual
time=232.701..249.272rows=146 loops=1)" 
"                                                  Hash Cond: (qtd.idmov = mv.idmovimento)"
"                                                  ->  HashAggregate  (cost=3716.55..3810.31 rows=7501 width=24)
(actualtime=61.735..72.593 rows=15497 loops=1)" 
"                                                        ->  Seq Scan on quantidades  (cost=0.00..3526.18 rows=15230
width=24)(actual time=0.223..48.616 rows=15750 loops=1)" 
"                                                              Filter: (ano = 2009)"
"                                                  ->  Hash  (cost=14588.99..14588.99 rows=2178 width=102) (actual
time=170.719..170.719rows=2559 loops=1)" 
"                                                        ->  Hash Left Join  (cost=7052.05..14588.99 rows=2178
width=102)(actual time=166.942..169.261 rows=2559 loops=1)" 
"                                                              Hash Cond: (amr.idrecibo = rcb.idrecibo)"
"                                                              ->  Hash Left Join  (cost=4706.50..11472.92 rows=2178
width=24)(actual time=77.667..93.502 rows=2559 loops=1)" 
"                                                                    Hash Cond: (mv.idmovimento = amr.idmov)"
"                                                                    ->  Bitmap Heap Scan on movimentos mv
(cost=3058.71..9558.85rows=2178 width=20) (actual time=28.338..35.229 rows=2559 loops=1)" 
"                                                                          Recheck Cond: ((idtipo_mv = 21) AND (10 =
idparque))"
"                                                                          Filter: vivo"
"                                                                          ->  BitmapAnd  (cost=3058.71..3058.71
rows=2205width=0) (actual time=28.196..28.196 rows=0 loops=1)" 
"                                                                                ->  Bitmap Index Scan on
idx_03_idtipo_mv (cost=0.00..583.08 rows=33416 width=0) (actual time=7.307..7.307 rows=46019 loops=1)" 
"                                                                                      Index Cond: (idtipo_mv = 21)"
"                                                                                ->  Bitmap Index Scan on
idx_02_idparque (cost=0.00..2474.29 rows=141577 width=0) (actual time=19.948..19.948 rows=136676 loops=1)" 
"                                                                                      Index Cond: (10 = idparque)"
"                                                                    ->  Hash  (cost=812.13..812.13 rows=49413 width=8)
(actualtime=49.178..49.178 rows=49385 loops=1)" 
"                                                                          ->  Seq Scan on a_mov_rcb amr
(cost=0.00..812.13rows=49413 width=8) (actual time=0.069..24.160 rows=49385 loops=1)" 
"                                                              ->  Hash  (cost=1030.13..1030.13 rows=49313 width=86)
(actualtime=69.384..69.384 rows=49348 loops=1)" 
"                                                                    ->  Seq Scan on recibos rcb  (cost=0.00..1030.13
rows=49313width=86) (actual time=0.018..31.965 rows=49348 loops=1)" 
"                                ->  Index Scan using asso_prk_prod_pkey on a_prk_prod app  (cost=0.00..8.27 rows=1
width=8)(actual time=0.003..0.004 rows=1 loops=146)" 
"                                      Index Cond: (rct.idasso = app.idasso)"
"                          ->  Index Scan using produtos_pkey on produtos prd  (cost=0.00..1.43 rows=1 width=80)
(actualtime=0.002..0.003 rows=1 loops=146)" 
"                                Index Cond: (app.idproduto = prd.idproduto)"
"                    ->  Index Scan using grp_prod_pkey on grp_prod gprd  (cost=0.00..0.30 rows=1 width=136) (actual
time=0.002..0.002rows=1 loops=146)" 
"                          Index Cond: (prd.idgrp_prod = gprd.idgrp_prod)"
"              ->  Index Scan using tipoprodutos_pkey on tipoprodutos tprd  (cost=0.00..0.30 rows=1 width=136) (actual
time=0.002..0.002rows=1 loops=146)" 
"                    Index Cond: (prd.idtipoproduto = tprd.idtipoproduto)"
"        ->  Seq Scan on parques pq  (cost=0.00..1.30 rows=1 width=72) (actual time=0.002..0.005 rows=1 loops=146)"
"              Filter: (idparque = 10)"
"Total runtime: 339.973 ms"

now here is the explain using the parameters from 1 to 4.

"Sort  (cost=23944.24..23944.24 rows=1 width=526) (actual time=1887457.197..1887457.241 rows=124 loops=1)"
"  Sort Key: mv.data, prd.idproduto, gprd.idgrp_prod, rcb.sigla, rcb.n_recibo, qtd.ano, qtd.mes"
"  ->  Nested Loop  (cost=16068.57..23944.23 rows=1 width=526) (actual time=34392.436..1887456.339 rows=124 loops=1)"
"        Join Filter: (qtd.idmov = mv.idmovimento)"
"        ->  Nested Loop  (cost=9016.52..9328.02 rows=1 width=444) (actual time=156.601..834.424 rows=12586 loops=1)"
"              ->  Nested Loop  (cost=9016.52..9327.70 rows=1 width=316) (actual time=156.572..678.675 rows=12586
loops=1)"
"                    ->  Nested Loop  (cost=9016.52..9327.39 rows=1 width=184) (actual time=156.544..579.804 rows=12586
loops=1)"
"                          ->  Hash Join  (cost=9016.52..9325.95 rows=1 width=108) (actual time=156.501..304.851
rows=12586loops=1)" 
"                                Hash Cond: ((qtd.idasso = app.idasso) AND (qtd.idmov = rct.idmov) AND (qtd.mes =
rct.mes))"
"                                ->  HashAggregate  (cost=3716.55..3810.31 rows=7501 width=24) (actual
time=59.596..106.009rows=15507 loops=1)" 
"                                      ->  Seq Scan on quantidades  (cost=0.00..3526.18 rows=15230 width=24) (actual
time=0.203..46.900rows=15760 loops=1)" 
"                                            Filter: (ano = 2009)"
"                                ->  Hash  (cost=5250.56..5250.56 rows=2824 width=104) (actual time=96.788..96.788
rows=12597loops=1)" 
"                                      ->  Hash Join  (cost=5148.19..5250.56 rows=2824 width=104) (actual
time=64.588..85.241rows=12597 loops=1)" 
"                                            Hash Cond: (rct.idasso = app.idasso)"
"                                            ->  HashAggregate  (cost=5099.11..5134.41 rows=2824 width=24) (actual
time=62.779..71.578rows=12597 loops=1)" 
"                                                  ->  Seq Scan on receitas  (cost=0.00..5033.23 rows=5271 width=24)
(actualtime=11.277..51.444 rows=13173 loops=1)" 
"                                                        Filter: ((ano = 2009) AND (mes >= 1) AND (mes <= 4))"
"                                            ->  Hash  (cost=34.16..34.16 rows=1193 width=80) (actual time=1.790..1.790
rows=1193loops=1)" 
"                                                  ->  Nested Loop  (cost=0.00..34.16 rows=1193 width=80) (actual
time=0.025..1.186rows=1193 loops=1)" 
"                                                        ->  Seq Scan on parques pq  (cost=0.00..1.30 rows=1 width=72)
(actualtime=0.012..0.016 rows=1 loops=1)" 
"                                                              Filter: (idparque = 10)"
"                                                        ->  Seq Scan on a_prk_prod app  (cost=0.00..20.93 rows=1193
width=8)(actual time=0.006..0.402 rows=1193 loops=1)" 
"                          ->  Index Scan using produtos_pkey on produtos prd  (cost=0.00..1.43 rows=1 width=80)
(actualtime=0.015..0.016 rows=1 loops=12586)" 
"                                Index Cond: (app.idproduto = prd.idproduto)"
"                    ->  Index Scan using grp_prod_pkey on grp_prod gprd  (cost=0.00..0.30 rows=1 width=136) (actual
time=0.003..0.005rows=1 loops=12586)" 
"                          Index Cond: (prd.idgrp_prod = gprd.idgrp_prod)"
"              ->  Index Scan using tipoprodutos_pkey on tipoprodutos tprd  (cost=0.00..0.30 rows=1 width=136) (actual
time=0.003..0.009rows=1 loops=12586)" 
"                    Index Cond: (prd.idtipoproduto = tprd.idtipoproduto)"
"        ->  Hash Left Join  (cost=7052.05..14588.99 rows=2178 width=102) (actual time=146.667..148.973 rows=2559
loops=12586)"
"              Hash Cond: (amr.idrecibo = rcb.idrecibo)"
"              ->  Hash Left Join  (cost=4706.50..11472.92 rows=2178 width=24) (actual time=68.974..79.270 rows=2559
loops=12586)"
"                    Hash Cond: (mv.idmovimento = amr.idmov)"
"                    ->  Bitmap Heap Scan on movimentos mv  (cost=3058.71..9558.85 rows=2178 width=20) (actual
time=23.592..25.603rows=2559 loops=12586)" 
"                          Recheck Cond: ((idtipo_mv = 21) AND (10 = idparque))"
"                          Filter: vivo"
"                          ->  BitmapAnd  (cost=3058.71..3058.71 rows=2205 width=0) (actual time=23.474..23.474 rows=0
loops=12586)"
"                                ->  Bitmap Index Scan on idx_03_idtipo_mv  (cost=0.00..583.08 rows=33416 width=0)
(actualtime=6.003..6.003 rows=46024 loops=12586)" 
"                                      Index Cond: (idtipo_mv = 21)"
"                                ->  Bitmap Index Scan on idx_02_idparque  (cost=0.00..2474.29 rows=141577 width=0)
(actualtime=16.570..16.570 rows=136676 loops=12586)" 
"                                      Index Cond: (10 = idparque)"
"                    ->  Hash  (cost=812.13..812.13 rows=49413 width=8) (actual time=45.280..45.280 rows=49387
loops=12586)"
"                          ->  Seq Scan on a_mov_rcb amr  (cost=0.00..812.13 rows=49413 width=8) (actual
time=0.010..21.042rows=49387 loops=12586)" 
"              ->  Hash  (cost=1030.13..1030.13 rows=49313 width=86) (actual time=63.760..63.760 rows=49350
loops=12586)"
"                    ->  Seq Scan on recibos rcb  (cost=0.00..1030.13 rows=49313 width=86) (actual time=0.006..26.959
rows=49350loops=12586)" 
"Total runtime: 1887457.849 ms"

has we can see the query planner, decided to do sequencial scan in "a_mov_rcb" table and "recibos", when i set the flag
"enable_seqscan"to false all goes well. 

Anyways i could probably set the "enable_seqscan" always of but i dont know if thats a good idea, because if it was
thatwould be set as off by default. 

Is there anything i could do to go around this?
Or can anyone give me a hint why query planner goes sequencial scan when i change the parameters.


Thanks in advanced