SELECT 
  cscart_vendor_plans.allowed_vendor_subdomain 
FROM 
  cscart_companies 
  INNER JOIN cscart_vendor_plans ON cscart_vendor_plans.plan_id = cscart_companies.plan_id 
WHERE 
  cscart_companies.company_id = 3364

Query time 0.00349

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_companies",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["company_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_vendor_plans",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["plan_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    }
  }
}