SELECT 
  warehouse_id 
FROM 
  cscart_erp_warehouse 
WHERE 
  company_id = 1 
  AND is_main = 'Y'

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_erp_warehouse",
      "access_type": "ALL",
      "rows": 9,
      "filtered": 100,
      "attached_condition": "cscart_erp_warehouse.company_id = 1 and cscart_erp_warehouse.is_main = 'Y'"
    }
  }
}