Detecting a change in the execution plan of a query

My report used to complete in seconds and now is running for hours! Do you know what is going on?

Sounds familiar? It is very common in Data Warehouses to experience a sudden performance degradation in the execution time of a report, or of an individual ETL mapping. The query was running fast for months and then one day everything changed!

Such a behavior is a sign of a change in the execution plan. Of course there are a million other reasons that might have caused the performance degradation but a change in the execution plan is one of the most common. The reasons for such a change are numerous (stale statistics, change triggered by some DDL -e.g. a new index created, a change in the optimizer environment -e.g., by changing the value of some relevant  parameter at the session level etc.)

In this post, we want to show how easy it is to detect a change in the execution plan of a statement (technically represented by a unique sql id) with the help of historic data maintained by Oracle in the AWR. To this end we will present some examples and some simple scripts to do it. However, we will not deal at all with the reasons that might trigger the change of plan. This is a long discussion ...

Find all available Execution Plans for a specific SQL statement

When you have a specific SQL statement that is been running in your database (e.g., it might be part of nightly ETL flow that is used for loading your Data Warehouse), then how can you get a list of all available execution plans that the optimizer has chosen for this specific statement?

Each statement is uniquely identified by an SQL_ID and whenever the statement undergoes a hard parsing, then an execution plan is generated, which is identified uniquely by a hash value, called plan_hash_value.