SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6616, 6582, 6835, 7568, 7166, 6660, 6730, 
    6859, 7206, 7579, 6773, 6691, 7588, 
    6806, 7599, 6890, 6917, 6955, 6988, 
    7019, 7064, 7126, 7245, 7277, 7303, 
    7324, 7365, 7611, 7625
  )

Query time 0.00078

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "15.43"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 36,
      "rows_produced_per_join": 36,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "8.24",
        "eval_cost": "7.20",
        "prefix_cost": "15.44",
        "data_read_per_join": "213K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6616,6582,6835,7568,7166,6660,6730,6859,7206,7579,6773,6691,7588,6806,7599,6890,6917,6955,6988,7019,7064,7126,7245,7277,7303,7324,7365,7611,7625))"
    }
  }
}