Cause

Oracle

decides what rows need to be purged based on the retention policy. There is a special mechanism which is used in the case of the large AWR tables where we store the snapshot data in partitions. One method of purging data from these tables is by removing partitions that only contain rows that have exceeded the retention criteria. During the nightly purge task, we only drop the partition if all the data in the partition has expired. If the partition contains at least one row which, according to the retention policy shouldn't be removed,  then  the partition won't be dropped and as such the table will contain old data.

If partition splits do not occur (for whatever reason),

then we can end up with a situation where we have to wait for the latest entries to expire before the partition that they sit in can be removed. This can mean that some of the older entries can be retained significantly past their expiry date. The result of this is that the data is not purged as expected.

Solution

A potential solution to this issue is to manually split the partitions of the partitioned AWR objects such that there is more chance of the split partition being purged.You will still have to wait for all the rows in the new partitions to reach their retention time but with split partitions there is more chance of this happening. you can manually split the partitions using the following undocumented command:

alter session set "_swrf_test_action" = 72;

To perform a single split of all the AWR partitions.

Check the partition details for the offending table before the split:

SELECT owner,

segment_name,

partition_name,

segment_type,

bytes/1024/1024/1024 Size_GB

FROM dba_segments

WHERE segment_name='WRH$_ACTIVE_SESSION_HISTORY';

Split the partitions so that there is more chance of the smaller partition being purged:

alter session set "_swrf_test_action" = 72;

NOTE:

This command will split partitions for ALL partitioned AWR objects. It also initiates a single split; it does not need to be disabled and will need to be repeated if multiple splits are required.

Check the partition details for the offending table after the split:

SELECT owner,

segment_name,

partition_name,

segment_type,

bytes/1024/1024/1024 Size_GB

FROM dba_segments

WHERE segment_name='WRH$_ACTIVE_SESSION_HISTORY';

With smaller partitions it is expected that some will be automatically removed when the retention period of all the rows within each partition is reached.

As an alternative, you could purge data based upon a snapshot range. Depending on the snapshots chosen, this may remove data that has not yet reached the retention limit so this may not be suitable for all cases. The following output shows the min and max snapshot_id in each partition.

set serveroutput on

declare

CURSOR cur_part IS

SELECT partition_name from dba_tab_partitions

WHERE table_name = 'WRH$_ACTIVE_SESSION_HISTORY';

query1 varchar2(200);

query2 varchar2(200);

TYPE partrec IS RECORD (snapid number, dbid number);

TYPE partlist IS TABLE OF partrec;

Outlist partlist;

begin

dbms_output.put_line('PARTITION NAME SNAP_ID DBID');

dbms_output.put_line('--------------------------- ------- ----------');

for part in cur_part loop

query1 := 'select min(snap_id), dbid from sys.WRH$_ACTIVE_SESSION_HISTORY partition ('||part.partition_name||') group by dbid';

execute immediate query1 bulk collect into OutList;

if OutList.count > 0 then

for i in OutList.first..OutList.last loop

dbms_output.put_line(part.partition_name||' Min '||OutList(i).snapid||' '||OutList(i).dbid);

end loop;

end if;

query2 := 'select max(snap_id), dbid from sys.WRH$_ACTIVE_SESSION_HISTORY partition ('||part.partition_name||') group by dbid';

execute immediate query2 bulk collect into OutList;

if OutList.count > 0 then

for i in OutList.first..OutList.last loop

dbms_output.put_line(part.partition_name||' Max '||OutList(i).snapid||' '||OutList(i).dbid);

dbms_output.put_line('---');

end loop;

end if;

end loop;

end;

/

Once you have split the partitions and identified a partition with a range of snap ids that can be deleted, you can free up the memory by dropping a snapshot range than matches the high and low snap_ids for the partition:

DBMS_WORKLOAD_REPOSITORY.DROP_SNAPSHOT_RANGE(

low_snap_id IN NUMBER,

high_snap_id IN NUMBER

dbid IN NUMBER DEFAULT NULL);