Making slow queries fast using composite indexes in MySQL

Let us first see what is Composite Index −

• A composite index is an index that is used on multiple columns.

• It is also known as a multiple-column index.

• MySQL allows the user to create a composite index which can consist of up to 16 columns.

• The query optimizer uses the composite indexes for queries which will test all columns in the index.

• It can also be used for queries which will test the first columns, the first two columns, and so on.

• If the columns are specified in the right order in the index definition, a single composite index can be used that would speed up certain kinds of queries on the same table.

Let us see how a composite index can be created, during the creation of a table. It can be done using the below statement −

Query

CREATE TABLE table_name (
   c1 data_type PRIMARY KEY,
   c2 data_type,
   c3 data_type,
   c4 data_type,
   INDEX index_name (c2,c3,c4)
);

In the above statement, the composite index consists of three columns c2, c3, and c4.

A composite index can also be added into an existing table using the ‘CREATE INDEX’ statement. Let us see how this can be done −

Query

CREATE INDEX index_name
ON table_name(c2,c3,c4);

Let us see how sloq queries can be made fast using Composite Index −

• The speed of the query execution depends on its duration.

• Using index hints will enace the query speed.

• The MySQL optimizer can be used to make the right decision while choosing an index.

• But this should be done only on static queries.

• If the queries are added where the ‘WHERE’ clause change, the performance of the queries will worsen, since it won’t let the optimizer do its job.

• The ‘FORCE INDEX’ statement acts like ‘USE INDEX (index_list), in addition that a table scan is considered as an expensive task.

• A table scan is required only if there is no way to use a named index to find the rows in a table.