MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0
Every 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 each table.
When you define a PRIMARY KEY
on your
table, InnoDB
uses 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 KEY
for your
table, MySQL locates the first UNIQUE
index
where all the key columns are NOT NULL
and
InnoDB
uses it as the clustered index.
If the table has no PRIMARY KEY
or suitable
UNIQUE
index, InnoDB
internally generates a hidden clustered index named
GEN_CLUST_INDEX
on a synthetic column
containing row ID values. The rows are ordered by the ID that
InnoDB
assigns 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.
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.
For guidelines to take advantage of InnoDB
clustered and secondary indexes, see
Section 8.3, “Optimization and Indexes”.