How do you use explain plans to identify performance bottlenecks in Teradata SQL queries?

1 What is an explain plan?

An explain plan is a report that shows how Teradata executes your SQL query. It breaks down the query into steps and shows the estimated cost, time, and spool usage of each step. It also shows the join methods, indexes, partitions, and other optimization techniques that Teradata applies to your query. You can generate an explain plan by adding the keyword EXPLAIN before your SQL statement.

2 Why use an explain plan?

An explain plan can help you understand how Teradata processes your query and how efficient it is. You can use an explain plan to compare different versions of your query and see how changing the syntax, filters, joins, or indexes affects the performance. You can also use an explain plan to identify potential problems, such as skewed data distribution, excessive spool usage, or suboptimal join methods.

3 How to read an explain plan?

An explain plan consists of a series of steps that describe the operations that Teradata performs to execute your query. Each step has a number, a description, and a cost indicator. The cost indicator is a percentage that represents the relative cost of the step compared to the total cost of the query. The lower the percentage, the less costly the step. The steps are ordered from the most costly to the least costly, so you can focus on the top steps to find the bottlenecks.

4 How to interpret an explain plan?

Analyzing an explain plan can provide you with valuable information about the performance of your query, but it's important to know how to interpret it correctly. Be on the lookout for high-cost steps that take a long time or use a lot of spool space as they may indicate inefficient join methods, missing indexes, or large intermediate results. Additionally, look out for steps that involve redistribution or duplication of data across the nodes of the Teradata system as this could be an indication of skewed data distribution, poor table statistics, or improper partitioning. Furthermore, steps that involve sorting or aggregation of data may indicate unnecessary or redundant operations, or opportunities for using summary tables or derived tables. Finally, steps that involve full table scans or product joins may suggest a lack of indexes, filters, or join conditions, or poor query design.

5 How to improve an explain plan?

Identifying performance bottlenecks in your query can be addressed by applying various techniques. Consider rewriting the query to simplify it and eliminate unnecessary operations, or using subqueries or common table expressions to create derived tables. Additionally, adding or modifying filters, join conditions, or group by clauses can reduce the amount of data Teradata needs to process. Indexes, partitions, or primary keys are also useful for faster and more efficient data access. Lastly, updating or collecting table statistics and using hints or qualifiers can help Teradata optimize the query plan and choose the best join methods and data distribution strategies.

6 How to use explain plans in practice?

An explain plan is a powerful tool that can help you tune your SQL queries and improve their performance. However, an explain plan is not a definitive answer, but a guide that you need to verify and test. You should always run your queries and measure their actual execution time, resource consumption, and results. You should also compare different versions of your queries and their explain plans to find the best solution. Finally, you should monitor and review your queries regularly, as the performance may change over time due to data growth, system changes, or user behavior.