SELECT 
  DISTINCT object_id 
FROM 
  cscart_bm_blocks_content 
WHERE 
  object_type = 'categories' 
  AND cr_content_type = 'newest' 
  AND object_id = 366

Query time 0.00099

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "51.66"
    },
    "duplicates_removal": {
      "using_filesort": false,
      "table": {
        "table_name": "cscart_bm_blocks_content",
        "access_type": "ALL",
        "possible_keys": [
          "PRIMARY"
        ],
        "rows_examined_per_scan": 470,
        "rows_produced_per_join": 0,
        "filtered": "0.21",
        "cost_info": {
          "read_cost": "51.56",
          "eval_cost": "0.10",
          "prefix_cost": "51.66",
          "data_read_per_join": "415"
        },
        "used_columns": [
          "object_id",
          "object_type",
          "cr_content_type"
        ],
        "attached_condition": "((`CS_Cart_Dev`.`cscart_bm_blocks_content`.`object_id` = 366) and (`CS_Cart_Dev`.`cscart_bm_blocks_content`.`object_type` = 'categories') and (`CS_Cart_Dev`.`cscart_bm_blocks_content`.`cr_content_type` = 'newest'))"
      }
    }
  }
}