[7.0] Sales Orders: group by 'Customers' raises a MemoryError

[Guewen Baconnier @ Camptocamp]

Steps to reproduce:

Database with about 258000 sales orders and 176000 customers.

Go to 'Sales > Sales > Sales Orders'
Group by Customers

Tested on a 64 bits computer with 12GB of RAM and 4GB of swap.

Result:

    2013-11-21 13:39:59,610 30665 ERROR openerp_trunk openerp.netsvc:
    Traceback (most recent call last):
      File "/snip/openerp/trunk/parts/server/openerp/netsvc.py", line 292, in dispatch_rpc
        result = ExportService.getService(service_name).dispatch(method, params)
      File "/snip/openerp/trunk/parts/server/openerp/service/web_services.py", line 626, in dispatch
        res = fn(db, uid, *params)
      File "/snip/openerp/trunk/parts/server/openerp/osv/osv.py", line 188, in execute_kw
        return self.execute(db, uid, obj, method, *args, **kw or {})
      File "/snip/openerp/trunk/parts/server/openerp/osv/osv.py", line 131, in wrapper
        return f(self, dbname, *args, **kwargs)
      File "/snip/openerp/trunk/parts/server/openerp/osv/osv.py", line 197, in execute
        res = self.execute_cr(cr, uid, obj, method, *args, **kw)
      File "/snip/openerp/trunk/parts/server/openerp/osv/osv.py", line 185, in execute_cr
        return getattr(object, method)(cr, uid, *args, **kw)
      File "/snip/openerp/trunk/parts/server/openerp/osv/orm.py", line 2692, in read_group
        data = self.read(cr, uid, data_ids, [groupby], context=context)
      File "/snip/openerp/trunk/parts/server/openerp/osv/orm.py", line 3613, in read
        result = self._read_flat(cr, user, select, fields, context, load)
      File "/snip/openerp/trunk/parts/server/openerp/osv/orm.py", line 3733, in _read_flat
        res2 = self._columns[f].get(cr, self, ids, f, user, context=context, values=res)
      File "/snip/openerp/trunk/parts/server/openerp/osv/fields.py", line 467, in get
        context=context))
      File "/snip/openerp/trunk/parts/server/openerp/addons/base/res/res_partner.py", line 545, in name_get
        name = record.name
      File "/snip/openerp/trunk/parts/server/openerp/osv/orm.py", line 486, in __getattr__
        return self[name]
      File "/snip/openerp/trunk/parts/server/openerp/osv/orm.py", line 474, in __getitem__
        new_data[field_name] = result_line[field_name]
    MemoryError