in mysql:

mysql> show index from users\G;
*************************** 1. row ***************************
       Table: users
  Non_unique: 0
    Key_name: PRIMARY
Seq_in_index: 1
 Column_name: id
   Collation: A
 Cardinality: 3
    Sub_part: NULL
      Packed: NULL
        Null:
  Index_type: BTREE
     Comment:
*************************** 2. row ***************************
       Table: users
  Non_unique: 1
    Key_name: username
Seq_in_index: 1
 Column_name: username
   Collation: A
 Cardinality: 3
    Sub_part: NULL
      Packed: NULL
        Null:
  Index_type: BTREE
     Comment:
2 rows in set (0.01 sec)

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