1. Oracle Database 12c features for
DBA
By:
Karan Kukreja

2. Points to note before the presentation
• This slide is self made
• I don’t know everything

3. Features
• Adaptive execution plans during runtime
• PGA_AGGREGATE_LIMIT
• Enhanced Statistics options
• Renaming a datafile online
• Fetch First X Rows only
• Table restoration using rman
• Invisible columns
• Sql statements in rman directly
• preupgrd.sql and parallel upgrade utility
• Real-time ADDM analysis

4. Adaptive execution plans during
runtime
• Execution of a statement can start with one
plan, and (during execution) switch to
another.
• Correct it in flight.
• It is enabled during the hard parse, and used
during execution.
• As the statement executes, if the embedded
counters for actual cardinality cross those
limits, it will switch plan.

5. • Estimated outcome

6. • Actual number of rows

7. • Look what it did ?

8. PGA_AGGREGATE_LIMIT
• So far we have been using
PGA_AGGREGATE_TARGET which is like a soft
limit.
• PGA memory consumption can go beyond
PGA_AGGREGATE_TARGET at times.
• PGA_AGGREGATE_LIMIT is a hard limit. If this
limit is reached, Oracle will terminate sessions
consuming the most untunable PGA.

9. • Jobs that are killed end with a notification : ORA-
04036: PGA memory used by the instance exceeds
PGA_AGGREGATE_LIMIT
• Default value is calculated as below :
• 1. 2 GB
2. Two times of PGA_AGGREGATE_TARGET parameter
• 3. The value of PROCESS parameter * 3M
• In oracle apps , this is set to 0 , which means the
default value, which could be any fro above 3.

10. • How to find which process caused ORA-04036 ?
• Use below steps :
• dba_hist_active_sess_history and sort by PGA
usage in Desc order.
• The top in the list is the sql_id which has issues.
• Use query in reference URL to find the rows
returned / plan changed when it got bad.

11. Enhanced Statistics options
• DBMS_STATS.GATHER_SYSTEM_STATS has now an option to gather system
stats for an Exadata machine. ( It actually came in 11.2.0.4 onwards).
• Allows concurrent statistics collection using job scheduler, AQ and
resource manager. Can reduce time to gather stats overall however the
system should have the capacity to take the load.
• Has to be explicitly enabled.
• Cardinality feedback is now renamed to Statistic feedback. *
• Dynamic Sampling has now been renamed to Dynamics Statistics.*
• 2 new types of histograms are introduced , top frequency and hybrid. (
other being frequency and height based).
• Online stats gathering during CTAS and IAS and during partition creation. (
has some restrictions though).
• It is possible to have session-private statistics for global temporary tables.
• * will be covered later

12. Renaming a datafile online
• Release 12c made is more easy , all steps
taken care by just giving the below command :
• ALTER DATABASE MOVE DATAFILE
'/u01/app/oracle/oradata/cdb1/system01.dbf'
TO '/tmp/system01.dbf';

13. Fetch First X Rows only
• Prior to 12 c , to get the top 5 the query would be :
SELECT ename, sal FROM
( SELECT ename, sal, RANK() OVER (ORDER BY sal DESC) sal_rank FROM emp )
WHERE sal_rank <= 10;
• IN 12c , that’s taken care using FETCH FIRST N ROWS
ONLY as below :
SELECT val FROM
rownum_order_test
ORDER BY val DESC FETCH FIRST 5 ROWS ONLY;
• 12c also supports queries for pagination , fetching last
20% etc. ( refer to references for more details).

14. Table restoration using rman
• Table Point In Time Recovery (PITR)
• Table Point In Time Recovery (PITR) to Dump
File
• Table Point In Time Recovery (PITR) in a
Pluggable Database (PDB)

15. • General syntax :
• RECOVER TABLE 'TEST'.'T1' UNTIL SCN
1853267 AUXILIARY DESTINATION '/u01/aux'
REMAP TABLE 'TEST'.'T1':'T1_PREV';
• Auxiliary is where it will generate an auxiliary
database only with the file it has to extract.

16. Sql statements in rman directly

17. preupgrd.sql and parallel upgrade
utility
• Earlier used was utlu121s.sql however that has
been changed to preupgrd.sql
• Apart from verification , it gives a list of changes
to be done as part of fixup scripts.
In Earlier releases there was no option to do an
upgrade in parallel. catupgrd.sql is now replaced
with catctl.pl utility.
Syntax :
cd $ORACLE_12_HOME/perl/bin $ ./perl catctl.pl –n 3 -catupgrd.sql

18. Real-time ADDM analysis
• Needs OEM 12 c
• Helpful during hung/unresponsive DB time
• Steps :
• Select the Emergency Monitoring option from
the Performance menu on the Access the Database Home page.
• This will show the top blocking sessions in the Hang Analysis table.
• Select the Real-Time ADDM option from the Performance to
perform Real-time ADDM analysis.
• After collecting the performance data, click on the Findings tab to
get the interactive summary of all the findings.

19. • What is dynamic Sampling ?
• Oracle Database 10g introduced Dynamic
Sampling to allow the optimizer to gather
additional information at parse time if
database statistics were missing, stale or
insufficient to produce a good execution plan.
• Value from 1 to 10 , default value is 2.

20. • So what is cardinality feedback ?
• When the optimizer generates an execution plan the presence of
missing statistics, stale statistics, complex predicates or complex
operators may trigger the optimizer to monitor the cardinality of
operations in the plan. Once the execution is complete, if there is a
significant difference between the estimated and actual
cardinalities, the actual cardinalities are stored in the SGA for later
use and the statement is marked as reoptimizable. On next
execution the statement is reoptimized using the stored
cardinalities, allowing a better plan to be determined. Cardinality
feedback is statement specific and is lost if the instance is restarted
or the statement is aged out of the shared pool. In Oracle Database
12c, cardinality feedback has been renamed to statistics feedback.

21. Questions ?

22. References
• www.support.oracle.com
• https://weidongzhou.wordpress.com/2016/02/25/pga
_aggregate_target-vs-pga_aggregate_limit/
• https://oracle-base.com/articles/12c/concurrent-
statistics-collection-12cr1
• https://oracle-base.com/articles/12c/histograms-
enhancements-12cr1
• https://oracle-base.com/articles/12c/online-statistics-
gathering-for-bulk-loads-12cr1
• https://oracle-base.com/articles/12c/row-limiting-
clause-for-top-n-queries-12cr1