SQL Plan Directives

The aim of this post is to point out an issue (bug?) that I recently discovered. But, before talking about it, I need to go through a rather long introduction about the state of SQL plan directive (SPD).

As of version 12.1.0.2, an SPD has two state information. You can see both of them through the CDB/DBA_SQL_PLAN_DIRECTIVES data dictionary views:

The aim of this post is to summarize the knowledge about the 12.1 and 12.2 adaptive query optimizer configuration that, as far as I know, is spread over a number of (too many) different sources.

First of all, let’s shortly review which adaptive query optimization features exist:

In a previous post I described a situation where the creation of an extension, independently of whether it’s carried out manually by a person or automatically by the database engine, can lead to the invalidation of PL/SQL objects. In this second post on that subject, I describe, with the help of an example (I love examples!) based on the extension_invalidate_pkg_remote.sql script, what can happen when the table on which the extension is created is stored in a remote database.

Let’s start by creating the required objects. Notice that all objects are local but, because the package body references the table through a database link that points to the very same schema owning all other objects, the database engine considers it a remote table.

The fact that an extension explicitly created by a user through DBMS_STATS can invalidate objects like packages is not new in 12c. It has been like that since the introduction of extensions in 11g. In my opinion, since such an invalidation takes place only when a developer or DBA triggers it, I do not consider it a major problem.

SQL plan directives are a new concept introduced in version 12.1. Their purpose is to help the query optimizer cope with misestimates. To do so, they store in the data dictionary information about the predicates that cause misestimates. Simply put, the purpose of SQL plan directives is to instruct the database engine to either use dynamic sampling or automatically create extended statistics (specifically, column groups).

Since the database engine automatically maintains (e.g. creates and purges) SQL plan directives, in some situations it is necessary to copy the SQL plan directives created in one database to another one. This can be done with the help of the DBMS_SPD package.

Here are the key steps for such a copy: