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ORA-56955: quarantined plan used

In DBA, New features, Oracle database, Security and auditing, SQL on May 29, 2019 at 13:15

“The way that worms and viruses spread on the Internet is not that different from the way they spread in the real world, and the way you quarantine them is not that different, either” – David Ulevitch

And now, in Oracle 19c, you can do the same with SQL:

 SQL> SELECT client, COUNT(*) OVER (PARTITION BY price) CLIENT_COUNT 
 FROM sales WHERE price IN (2, 91984);
  *
 ERROR at line 1:
 ORA-56955: quarantined plan used
 Elapsed: 00:00:00.00

Error “ORA-56955: quarantined plan used” is new in the Oracle database, it comes when the SQL run fulfills the quarantine conditions.

It is important to differentiate Oracle SQL Quarantines in 19c from Oracle Object Quarantines in 18c. There is also the concept of Offload Quarantines.

1. A good way to start understanding what SQL quarantines are about is to watch the following short video from Rich Niemiec:

2. Check also page 23 of the Optimizer In Oracle Database 19c white paper. “The new Oracle Database 19c feature SQL Quarantine can be used to eliminate the overhead of runaway queries. When DBRM detects a SQL statement is exceeding a resource or run-time limit, the SQL execution plan used by the statement is quarantined. If the SQL statement is executed again and it is using the same SQL execution plan then it will be terminated immediately. This can significantly reduce the amount of system resource that would otherwise be wasted.”

Think of SQL Quarantines as a way to prevent unnecessary SQL being run in the database, of course based on your own definition of unnecessary SQL. You can prevent the use of “bad” execution plans and exhausting the databases from resources.

In the database, there might be SQL statements with high utilization of CPU and IO: you can prevent them from being started so once they are quarantined they no longer consume system resources because they are terminated prior to their execution.

Note that SQL quarantines work only in 19c on Exadata and DBCS/ExaCS. Check out the Database Licensing Information User Manual:

3. You can quarantine a statement based on:

– SQL_ID and one of its execution plan
– SQL_ID and all of its executions plans
– specific SQL_TEXT

You quarantine a statement in 2 steps:

(A) create a SQL Quarantine by using (for example) DBMS_SQLQ.CREATE_QUARANTINE_BY_SQL_ID
(B) add thresholds by using DBMS_SQLQ.ALTER_QUARANTINE

Here are some examples and some more.

4. For some interesting and non-documented stuff check the article by Mahmoud Hatem entitled Oracle 19c : The QUARANTINE hint.

For instance, it shows how you can test by setting “_exadata_feature_on”=true in order to get SQL QUARANTINE feature to work on a non-Exadata box.

The following parameters can affect a quarantine kick off:

CPU_TIME
ELAPSED_TIME
IO_MEGABYTES
IO_REQUESTS
IO_LOGICAL
PHV

There is also the special value called ALWAYS_QUARANTINE.

5. All details can be of course found in the SQL Quarantine documentation.

The following columns of the V$SQL and GV$SQL views show the quarantine information of execution plans of SQL statements:

– SQL_QUARANTINE: This column shows the name of the quarantine configuration for an execution plan of a SQL statement.
– AVOIDED_EXECUTIONS: This column shows the number of times an execution plan of a SQL statement was prevented from running after it was quarantined.

There is a new view in 19c called DBA_SQL_QUARANTINE which displays information about quarantine configurations.

Good news also for admin users of the Autonomous Database: you have full access to the feature:

And note that a DBA can also transfer quarantine configurations from one database to another database using the DBMS_SQLQ package subprograms: CREATE_STGTAB_QUARANTINE, PACK_STGTAB_QUARANTINE, and UNPACK_STGTAB_QUARANTINE.

If you plan to visit Oracle OpenWorld this year (September 16-19, 2019), as of now, there are a couple of presentations on SQL Quarantine:

– Oracle Database 19c: SQL Tuning Using Plan Stability Methods SPM/SQL Quarantine: by Soumendra Paik, Senior Principal Technical Support Engineer, Oracle
– What’s New in Oracle Optimizer, by Nigel Bayliss, Senior Principal Product Manager, Oracle

Question: how do you quarantine a statement based on a sub-string of the query? Like, how can you quarantine statements starting with ‘select *‘?

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