SELECT 
  field_id 
FROM 
  cscart_profile_fields 
WHERE 
  profile_type = 'S' 
  AND profile_show = 'Y' 
  AND ec_multiple = 'Y'

Query time 0.00054

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "11.40"
    },
    "table": {
      "table_name": "cscart_profile_fields",
      "access_type": "ALL",
      "possible_keys": [
        "profile_show"
      ],
      "rows_examined_per_scan": 52,
      "rows_produced_per_join": 0,
      "filtered": "1.92",
      "cost_info": {
        "read_cost": "11.20",
        "eval_cost": "0.20",
        "prefix_cost": "11.40",
        "data_read_per_join": "1K"
      },
      "used_columns": [
        "field_id",
        "profile_show",
        "profile_type",
        "ec_multiple"
      ],
      "attached_condition": "((`webmarco`.`cscart_profile_fields`.`profile_type` = 'S') and (`webmarco`.`cscart_profile_fields`.`profile_show` = 'Y') and (`webmarco`.`cscart_profile_fields`.`ec_multiple` = 'Y'))"
    }
  }
}