SHOW INDEX

Syntax:
SHOW [EXTENDED] {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.

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: 4188
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
Visible: YES
Expression: NULL
*************************** 2. row ***************************
Table: city
Non_unique: 1
Key_name: CountryCode
Seq_in_index: 1
Column_name: CountryCode
Collation: A
Cardinality: 232
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
Visible: YES
Expression: NULL

An alternative to tbl_name FROM db_name syntax is db_name.tbl_name.
These two statements are equivalent:

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

The optional EXTENDED keyword causes the output to include information
about hidden indexes that MySQL uses internally and are not accessible
by users.

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. See also the description for the Expression column.

o Collation

How the column is sorted in the index. This can have values A
(ascending), D (descending), 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. However, prefix lengths for
index specifications in CREATE TABLE, ALTER TABLE, and CREATE INDEX
statements are 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
https://dev.mysql.com/doc/refman/8.0/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.

o Visible

o Expression

MySQL 8.0.13 and higher supports functional key parts (see
https://dev.mysql.com/doc/refman/8.0/en/create-index.html#create-inde
x-functional-key-parts), which affects both the Column_name and
Expression columns:

o For a nonfunctional key part, Column_name indicates the column
indexed by the key part and Expression is NULL.

o For a functional key part, Column_name column is NULL and
Expression indicates the expression for the key part.

Information about table indexes is also available from the
INFORMATION_SCHEMA STATISTICS table. See
https://dev.mysql.com/doc/refman/8.0/en/information-schema-statistics-t
able.html. The extended information about hidden indexes is available
only using SHOW EXTENDED INDEX; it cannot be obtained from the
STATISTICS table.

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