SELECT 
  privilege 
FROM 
  cscart_usergroup_privileges 
WHERE 
  usergroup_id IN (7)

Query time 0.00037

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_usergroup_privileges",
          "access_type": "ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["usergroup_id"],
          "ref": ["const"],
          "rows": 58,
          "filtered": 100,
          "using_index": true
        }
      }
    ]
  }
}