InnoDB table has a special index called
the clustered index
where the data for the rows is stored. Typically, the clustered
index is synonymous with the
primary key. To get the
best performance from queries, inserts, and other database
operations, you must understand how InnoDB uses the clustered
index to optimize the most common lookup and DML operations for
When you define a
PRIMARY KEYon your table,
InnoDBuses it as the clustered index. Define a primary key for each table that you create. If there is no logical unique and non-null column or set of columns, add a new auto-increment column, whose values are filled in automatically.
If you do not define a
PRIMARY KEYfor your table, MySQL locates the first
UNIQUEindex where all the key columns are
InnoDBuses it as the clustered index.
If the table has no
PRIMARY KEYor suitable
InnoDBinternally generates a hidden clustered index on a synthetic column containing row ID values. The rows are ordered by the ID that
InnoDBassigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order.
Accessing a row through the clustered index is fast because the
index search leads directly to the page with all the row data. If
a table is large, the clustered index architecture often saves a
disk I/O operation when compared to storage organizations that
store row data using a different page from the index record. (For
MyISAM uses one file for data rows and
another for index records.)
All indexes other than the clustered index are known as
secondary indexes. In
InnoDB, each record in a secondary index
contains the primary key columns for the row, as well as the
columns specified for the secondary index.
InnoDB uses this primary key value to search
for the row in the clustered index.
If the primary key is long, the secondary indexes use more space, so it is advantageous to have a short primary key.