Finding out what makes SQL slow
An SQL statement can be slow for a lot of reasons. Here, we will give a short list of these, with at least one way of recognizing each reason.
Getting ready
If the SQL statement is still running, look at Chapter 8, Monitoring and Diagnosis.
How to do it…
The core issues are likely to be one of these:
- You're asking it to do too much work
- Something is stopping it from doing the work
This might not sound that helpful at first, but it's good to know that there's nothing really magical going on that you can't understand if you look.
In more detail, the main reasons are:
- Returning too much data
- Processing too much data index needed
- Wrong plan for other reasons
- Cache or I/O problems
- Locking problems
The first reason can be handled as described in the Reducing the number of rows returned recipe.Reasons 2-4can be investigated from two perspectives:TheSQLitselfand the objects that the SQL touches. Let's start by looking at the SQLitselfby running the query with EXPLAINANALYZE. We...
