Query out of memory

I am running the below query. Table has 21 million records. I get an Out Of Memory error after a while.(from both pgadmin and psql). Can someone review DB parameters given below.

select t.*,g.column,a.column from

gk_staging g, transaction t,account a

where

g.accountcodeis not null AND

g.accountcode::text <> '' AND

length(g.accountcode)=13 AND

g.closeid::text=t.transactionid::text AND

subsrting(g.accountcode::text,8)=a.mask_code::text

Below are system parameters.

shared_buffers=3GB

work_mem=2GB

effective_cache_size=10GB

maintenance_work_mem=1GB

max_connections=250

I am unable to paste explain plan here due to security concerns.

Regards,

Aditya.