Clustered and Secondary Indexes
secondary index
A type of InnoDB index that represents a subset of table columns. An InnoDB table can have zero, one, or many
secondary indexes. (Contrast with the clustered index, which is required for each InnoDB table, and stores the data for
all the table columns.)
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 firstUNIQUE
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 suitableUNIQUE
index,InnoDB
internally generates a hidden clustered 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.
How the Clustered Index Speeds Up Queries
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 example, MyISAM
uses one file for data rows and another
for index records.)
How Secondary Indexes Relate to the Clustered Index
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.
Clustered and Secondary Indexes