postgres indexes in join tables #319

Hi,

I found major issue with postgres dialect. It seems to create multiple indexes for many-to-many relations.

There has been same kind issue 24, but this is only for relations.

Table "public.company_files"
Column | Type | Modifiers
------------+---------+-----------
company_id | integer |
files_id | integer |
Indexes:
"company_files_company_id_files_id_idx" btree (company_id, files_id)
"company_files_company_id_files_id_idx1" btree (company_id, files_id)
"company_files_company_id_files_id_idx2" btree (company_id, files_id)
"company_files_files_id_company_id_idx" btree (files_id, company_id)
"company_files_files_id_company_id_idx1" btree (files_id, company_id)
"company_files_files_id_company_id_idx2" btree (files_id, company_id)
....

currently i seems to have ~2500 index per join table in my DB. My ORM version is 2.1.0