SELECT 
  company_id 
FROM 
  cscart_companies

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_companies",
      "access_type": "index",
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["company_id"],
      "rows": 1,
      "filtered": 100,
      "using_index": true
    }
  }
}

Result