Clustered and Secondary Indexes

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 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 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.