Syntax:
SHOW {INDEX | INDEXES | KEYS}
    {FROM | IN} tbl_name
    [{FROM | IN} db_name]
    [WHERE expr]

SHOW INDEX returns table index information. The format resembles that
of the SQLStatistics call in ODBC. This statement requires some
privilege for any column in the table.

You can use db_name.tbl_name as an alternative to the tbl_name FROM
db_name syntax. These two statements are equivalent:

SHOW INDEX FROM mytable FROM mydb;
SHOW INDEX FROM mydb.mytable;

The WHERE clause can be given to select rows using more general
conditions, as discussed in
http://dev.mysql.com/doc/refman/5.7/en/extended-show.html.

mysql> SHOW INDEX FROM City\G
*************************** 1. row ***************************
        Table: city
   Non_unique: 0
     Key_name: PRIMARY
 Seq_in_index: 1
  Column_name: ID
    Collation: A
  Cardinality: 4321
     Sub_part: NULL
       Packed: NULL
         Null: 
   Index_type: BTREE
      Comment: 
Index_comment: 
*************************** 2. row ***************************
        Table: city
   Non_unique: 1
     Key_name: CountryCode
 Seq_in_index: 1
  Column_name: CountryCode
    Collation: A
  Cardinality: 4321
     Sub_part: NULL
       Packed: NULL
         Null: 
   Index_type: BTREE
      Comment: 
Index_comment: 

SHOW INDEX returns the following fields:

o Table

  The name of the table.

o Non_unique

  0 if the index cannot contain duplicates, 1 if it can.

o Key_name

  The name of the index. If the index is the primary key, the name is
  always PRIMARY.

o Seq_in_index

  The column sequence number in the index, starting with 1.

o Column_name

  The column name.

o Collation

  How the column is sorted in the index. This can have values A
  (ascending) or NULL (not sorted).

o Cardinality

  An estimate of the number of unique values in the index. To update
  this number, run ANALYZE TABLE or (for MyISAM tables) myisamchk -a.

  Cardinality is counted based on statistics stored as integers, so the
  value is not necessarily exact even for small tables. The higher the
  cardinality, the greater the chance that MySQL uses the index when
  doing joins.

o Sub_part

  The index prefix. That is, the number of indexed characters if the
  column is only partly indexed, NULL if the entire column is indexed.

  *Note*:

  Prefix limits are measured in bytes, whereas the prefix length in
  CREATE TABLE, ALTER TABLE, and CREATE INDEX statements is interpreted
  as number of characters for nonbinary string types (CHAR, VARCHAR,
  TEXT) and number of bytes for binary string types (BINARY, VARBINARY,
  BLOB). Take this into account when specifying a prefix length for a
  nonbinary string column that uses a multibyte character set.

  For additional information about index prefixes, see
  http://dev.mysql.com/doc/refman/5.7/en/column-indexes.html, and [HELP
  CREATE INDEX].

o Packed

  Indicates how the key is packed. NULL if it is not.

o Null

  Contains YES if the column may contain NULL values and '' if not.

o Index_type

  The index method used (BTREE, FULLTEXT, HASH, RTREE).

o Comment

  Information about the index not described in its own column, such as
  disabled if the index is disabled.

o Index_comment

  Any comment provided for the index with a COMMENT attribute when the
  index was created.

You can also obtain information about table indexes from
INFORMATION_SCHEMA, which contains a STATISTICS table. See
http://dev.mysql.com/doc/refman/5.7/en/statistics-table.html.

You can list a table's indexes with the mysqlshow -k db_name tbl_name
command.

URL: http://dev.mysql.com/doc/refman/5.7/en/show-index.html

