select slows from 3 seconds to 30 seconds

Hi folks.

I have the query below which when run takes approx 3 seconds.  However when I 
add the condition 'and w_ws_id = 10' onto the end changes to 30+ seconds. 

Can anyone see why? I've included the explain for the long select.

select w_vin as key, w_vin, v_o_number as vista, v_status,       s_stock_no, s_customer_order,
coalesce(d.r_registration,p.r_registration, w_reg_no) as 
 
reg_no,w_arrival,       w_updated::date,CURRENT_DATE-w_created::date as age, w_model,       w_radio_code, w_key_no,
w_inspected,w_damage,       w_walon_repair,w_collect_date, w_despatch_date, w_sheet,       w_plates, w_accessories,
coalesce(p.d_des, d.d_des) as d_des,      coalesce(p.de_des, d.de_des) as de_des,      p.r_id as pdi,       d.r_id as
delivery,     o.o_id, o.state from walon  left outer join request_details p on p.t_id = 'P' and
substring(p.r_chassisfrom '(.{11}$)') = w_vin left outer join request_details d on d.t_id = 'D' and
substring(d.r_chassisfrom '(.{11}$)') = w_vin left outer join order_details o on        substring(o.o_vin from
'(.{11}$)')= w_vin left outer join stock s on substring(s.s_vin from '(.{11}$)') = w_vin left outer join vista v on
v.v_vin= w_vin where  w_hide = 0.
 


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Gary Stainburn
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