Both Filter and Group By is useful for user to quickly filter data in Odoo. Default group is often used so we can see tally data when a list is first loaded.
Define search view to a model
<record model="ir.actions.act_window" id="action_custom_model">
<field name="name">Custom Model</field>
<field name="res_model">custom.model</field>
<field name="view_type">form</field>
<field name="view_mode">tree,form</field>
<field name="view_id" ref="custom_model_view_tree"/>
<field name="search_view_id" ref="custom_model_search_form_view"/>
<field name="context">{'search_default_group_product': 1}</field>
</record>
{'search_default_group_product': 1} in context means that a filter with name as group_product is set as default.
Search view
<record id="custom_model_search_form_view" model="ir.ui.view">
<field name="name">custom.model.search.stock.form</field>
<field name="model">custom.model</field>
<field name="mode">primary</field>
<field name="arch" type="xml">
<search string="Custom Model">
<field name="name" string="Name"
filter_domain="['|', '|', ('default_code', 'ilike', self), ('name', 'ilike', self), ('barcode', 'ilike', self)]"/>
<filter string="State" name="confirm" domain="[('state','=','confirm')]" help="Confirmed Data"/>
<group expand="0" name="group_by" string="Group By">
<filter name="group_product" string="Product" domain="[]"
context="{'group_by' : 'product_id'}"/>
</group>
</search>
</field>
</record>
