Indexing Best Practices
Tags: sql, mysql, postgres
Published: 2024-03-09 04:32:31

## Determine Our Needs

Before creating any indexes, we need to have a clear understanding of the application's requirements. This involves a thorough analysis of the application’s workload. Identify the most common queries and understand how frequently they are used. In addition, determine the application's read-to-write ratio. Indexing improves read performance but can slow down write performance. If the application performs more writes than reads, too many indexes could adversely affect overall performance. Similarly, if the application is read-heavy, appropriate indexing can significantly enhance efficiency.

## Choose the Right Columns for Indexing

The decision of which columns to index should be based on their usage in queries. Indexes should ideally be created on columns that are frequently used in WHERE clauses, ORDER BY clauses, JOIN conditions, or used for sorting and grouping data.

Columns with a high degree of uniqueness are ideal candidates for indexing. Indexes on such columns allow the database engine to quickly filter out a majority of the data. It leads to faster query results. Avoid indexing columns with many null values or those that have a lot of similar values.

## Weigh the Cost of Updates to Indexed Columns

While indexes speed up data retrieval, they slow down data modification. This is because each time data is added, deleted, or modified, the corresponding indexes need to be updated as well. If a column is frequently updated, the overhead of updating the index might negate the performance benefits gained during data retrieval. If the index update time outweighs the time saved during data retrieval, it might not be worth it to maintain the index.

## Limit the Number of Indexes

While indexes are beneficial for query performance, having too many can negatively impact the performance of write operations and consume more disk space. Each time data is inserted or updated, every index on the table must be updated. The cost of maintaining the index might outweigh the performance benefits it provides. It is important to maintain a healthy balance and limit the number of indexes based on the nature of the workload. Utilize monitoring tools provided by the database system to identify if the update time on the indexes is increasing disproportionately.

## Use Composite Indexes Effectively

Composite indexes, which are made up of two or more columns, can be very beneficial for complex queries that involve multiple columns in the WHERE clause. The order of the columns in the composite index is critical. As a general rule of thumb, it should be based on the cardinality of the columns, with the column having the highest number of distinct values appearing first in the index. This order allows the database engine to efficiently filter out unneeded data.  For example, if we are creating a composite index on "CustomerName" and "Country" columns in a 'Orders' table, and there are fewer distinct countries than customer names, the index should be (Country, CustomerName).

It is important to note that this guideline is a general rule of thumb, and it is not always correct. Verify with the optimizer that it indeed uses the composite index as intended.

## Leverage Covering Indexes

A covering index includes all the columns that a query needs, both in the WHERE clause and the SELECT list. This means that the database engine can locate all the required data within the index itself, without having to perform additional lookups in the main table. This results in a significant performance boost because accessing an index is typically faster than accessing the table data. Consider using covering indexes for frequently used, read-intensive queries.

## Regularly Monitor and Optimize the Indexes

Indexes are not a set-it-and-forget-it part of the database. As the data grows and changes, the indexes need to be monitored and optimized. Over time, as data is added, updated, and deleted, indexes can become fragmented, which can negatively impact their performance. Regularly performing index maintenance tasks, such as rebuilding or reorganizing fragmented indexes, can help ensure that they continue to provide optimal performance. Database tools such as SQL Server's Database Engine Tuning Advisor or MySQL's OPTIMIZE TABLE command are some examples of tools to use. Monitoring logs like MySQL’s slow query log is also important in detecting issues early.

## Drop Unused Indexes

Not all indexes end up being used as intended. Some may be rarely used, or not at all. Such indexes impose unnecessary overhead on write operations and waste storage space. Use the database's built-in tools to monitor index usage, and do not hesitate to drop indexes that are no longer serving their purpose. In PostgreSQL, for instance, we can use the pg_stat_user_indexes view to track index usage.

## Conclusion

Database indexing is a critical part in optimizing database efficiency. It's a key component in the balancing act between speed of data retrieval and the performance of write operations.

However, indexing is not a one-size-fits-all solution. Careful design, regular monitoring and maintenance are vital components in maximizing the benefits of the indexing strategies. Recognizing the indexing techniques specific to different databases can significantly improve the data operations.

A well-implemented indexing strategy is fundamental to a high-performing database. Mastering the art of database indexing is an indispensable skill for anyone building large-scale data-driven applications.