Why does DBA_TAB_MODIFICATIONS sometimes have no values [ID 762738.1]

Why does DBA_TAB_MODIFICATIONS sometimes have no values [ID 762738.1]   

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  Modified 18-MAR-2009     Type HOWTO     Status MODERATED   

In this Document
  Goal
  Solution
  References



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Applies to:
Oracle Server - Enterprise Edition - Version: 10.2.0.3
Information in this document applies to any platform.
Oracle Server Enterprise Edition - Version: 10.2.0.3
Goal
The goal is to explain why the view DBA_TAB_MODIFICATIONS  does sometimes have no values
even when the parameter STATISTICS_LEVEL  is set to TYPICAL and  the specific schema has been analyzed successful using the package DBMS_STATS.GATHER_SCHEMA_STATS.
In addition all the tables in that schema shows MONITORING=YES in the view dba_tables.












Solution
The updates to the table *_tab_modifications are related to the volumne of updates for a table.
There is a need of approximatly 10% of datavolumn changes. Just only on single update of the row for example might not lead to fill the *_tab_modifications .

See example below:



STEP1: *** create a table crc.gs , analyze it and then fill test_gs.gs  with 100 rows and perform some DML
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create user crc identified by crc
default tablespace users temporary tablespace temp;
grant connect,resource to crc;

connect crc/crc

alter session set nls_language = american;
alter session set nls_date_format= 'DD-MM-YY HH24:MI:SS';

create table crc.gs (i number);


begin
dbms_stats.gather_schema_stats(
ownname =>'CRC',
estimate_percent => dbms_stats.auto_sample_size,
method_opt =>'FOR ALL COLUMNS SIZE AUTO',
degree => 1,
granularity => 'ALL',
cascade => true,
options => 'GATHER'
);
end;
/


begin
for i in 1..100 loop
insert into CRC.gs values(i);
end loop;
commit;
end;
/
delete from CRC.gs where i between 40 and 60;
commit;
     
update CRC.gs set i=i+1000 where i between 80 and 100;
commit;






STEP2: *** select and use the procedure DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO
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Note: The procedure DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO flushes in-memory monitoring information for all tables in the dictionary.
Corresponding entries in the *_TAB_MODIFICATIONS, *_TAB_STATISTICS and *_IND_STATISTICS
views are updated immediately, without waiting for the Oracle database to flush them periodically (per default every 3 hours). This procedure is useful when you need up-to-date information in those views.
SQL> select TABLE_OWNER, TABLE_NAME, INSERTS , UPDATES, DELETES, TIMESTAMP from
2 sys.dba_tab_modifications where TABLE_OWNER='CRC';

no rows selected

SQL> execute DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;

PL/SQL procedure successfully completed.

SQL> col table_name format a5
SQL> col table_owner format a10
SQL> select TABLE_OWNER, TABLE_NAME, INSERTS , UPDATES, DELETES, TIMESTAMP from
2 sys.dba_tab_modifications where TABLE_OWNER='CRC';

TABLE_OWNER TABLE INSERTS UPDATES DELETES TIMESTAMP
-------------                -----       ----------     ----------       ----------     -----------------
CRC                         GS         100             21                21              18-03-09 15:34:37

==> Because of the 'high' volumne of DML (100 inserts, 21 updates and 21 deletes)  we have an entry in the table
        sys.dba_tab_modifications for the table 'GS'.




STEP3: *** analyze again the table GS which leads to an empty sys.DBA_TAB_MODIFICATIONS for table 'GS'
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SQL> begin
2 dbms_stats.gather_schema_stats(
3 ownname =>'CRC',
4 estimate_percent => dbms_stats.auto_sample_size,
5 method_opt =>'FOR ALL COLUMNS SIZE AUTO',
6 degree => 1,
7 granularity => 'ALL',
8 cascade => true,
9 options => 'GATHER'
10 );
11 end;
12 /

PL/SQL procedure successfully completed.

SQL> select TABLE_OWNER, TABLE_NAME, INSERTS , UPDATES, DELETES, TIMESTAMP from
2 sys.dba_tab_modifications where TABLE_OWNER='CRC';

no rows selected  ==> which is normal due to the analyze command



STEP4: *** now perform only 1 update on the table, flush the monitoring information out and then check the
entry in DBA_TAB_MODIFICATIONS
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SQL> update crc.gs gs set i=i+100 where i=30;
1 row updated.

SQL> commit;
Commit complete.

SQL> execute DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;
PL/SQL procedure successfully completed.


SQL> select TABLE_OWNER, TABLE_NAME, INSERTS , UPDATES, DELETES, TIMESTAMP from
2 sys.dba_tab_modifications where TABLE_OWNER='CRC';

no rows selected



Note: please be aware that this is only an example. Other values/dml changes may show different results.


References
NOTE:456535.1 - DB Monitoring Automatic