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 optimise the most common lookup and DML operations for each table.
- 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 firstUNIQUEindex where all the key columns areNOT NULLandInnoDBuses it as the clustered index. - If the table has no
PRIMARY KEYor suitableUNIQUEindex,InnoDBinternally generates a hidden clustered index on a synthetic column containing row ID values. The rows are ordered by the ID thatInnoDBassigns 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 organisations 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. InnoDBÂ 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.