SQL > Data Definition Language (DDL) > Index

Indexes help us retrieve data from tables quicker. Let's use an example to illustrate this point: Say we are interested in reading about how to grow peppers in a gardening book. Instead of reading the book from the beginning until we find a section on peppers, it is much quicker for us to go to the index section at the end of the book, locate which pages contain information on peppers, and then go to these pages directly. Going to the index first saves us time and is by far a more efficient method for locating the information we need.

The same principle applies for retrieving data from a database table. Without an index, the database system reads through the entire table (this process is called a 'table scan') to locate the desired information. With the proper index in place, the database system can then first go through the index to find out where to retrieve the data, and then go to these locations directly to get the needed data. This is much faster.

Therefore, it is often desirable to create indexes on tables. An index can cover one or more columns. The syntax for creating a table index is shown in the CREATE INDEX section. Below we discuss some general strategies when building and using an index:

1. Build index on columns of integer type

Integers take less space to store, which means the query will be faster. If the column you want to build an index for is not of type integer, consider creating a surrogate integer key (or simply a surrogate column of type integer) which maps one-to-one to the column you want to build the index for.

2. Keep index as narrow as possible

Narrower indexes take less space, require less time to process, which in turn means the query will run faster.

3. Column order is important

For indexes covering multiple columns, the order of the columns in the index is important. The best practice is to use the column with the lowest cardinality first, and the column with the highest cardinality last. Recall cardinality means the number of distinct values for that column. So, if SELECT DISTINCT (COLUMN1) FROM TABLE_NAME; returns 5, that means the cardinality for COLUMN1 is 5.

4. Make sure the column you are building an index for is declared NOT NULL

This can decrease the size of the index, which in turn will speed up the query.

5. Build an index only when necessary

The advantages of using an index do not come without a cost. Indexes take up disk space, and they will cause INSERT, UPDATE, and DELETE operations to go slower, since each time one of these operations is carried out, not only does the database system need to update the values in the table, but it also needs to update the indexes. There are no hard rules on how to build indexes. Deciding what indexes to build is often this is an iterative process over time.

Next: SQL CREATE INDEX




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