SELECT 
  abt__yt_banners_use 
FROM 
  cscart_bm_blocks 
WHERE 
  block_id = 879

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_bm_blocks",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["block_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    }
  }
}