SELECT 
  tag_id 
FROM 
  tmall_tags 
WHERE 
  tag = 'Books'

Query time 0.00778

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.002024411,
    "nested_loop": [
      {
        "table": {
          "table_name": "tmall_tags",
          "access_type": "ref",
          "possible_keys": ["tag"],
          "key": "tag",
          "key_length": "767",
          "used_key_parts": ["tag"],
          "ref": ["const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.002024411,
          "filtered": 100,
          "index_condition": "tmall_tags.tag = 'Books'"
        }
      }
    ]
  }
}