For database tables that have a small number of rows, the performance of scans versus index searches is not that different, and depending on each particular case one may perform slightly better than the other. There are many different primitive operations, but if we only look at those that retrieve data, they can be divided into two main groups: operations that look for the required data by reading sequentially (sometimes called 'sequential scans' or 'table scans'), and operations that look for data by searching an index (often called 'index searches', or 'index scans').Īre sequential scans slower than index scans? In general yes, but there is no absolute answer. Complex queries that involve joins, grouping or similar constructs require several operations, each building on the results of the previous one. For a simple query, this may be a single operation. Once the SQL code for the query is parsed, a query optimizer will look at it and come up with a sequence of operations that need to be performed to produce the results that are requested. To be able to optimize database queries, it is useful to first review how databases implement the resolution of a query. Would you like to support my work, and as a thank you be able to vote on my future articles and have access to a chat room where I hang out and answer questions? Become a Patron! How the Database Resolves a Query This article was voted by my supporters on Patreon. Examples for Postgres, MySQL and SQLite are included! How do you debug and identify this type of problems? In this article I'm going to show you how to fix the most common database performance problems, which are those that are caused by improper indexing. It is when the application has been on production for some time that database performance issues start to appear, causing parts of the application to become slower and slower as the database continues to grow.

Most database problems go unnoticed during development, because we tend to code our applications using small datasets.

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