As a follow up to my previous post on SCN_TO_TIMESTAMP, here is a demo of the ORA_ROWSCN function.
I have two different sessions and two different tables where I insert one row each. I then delay the COMMIT in each session.
This is the first session :
oracle19c>sqlplus hemant/hemant@orclpdb1
SQL*Plus: Release 19.0.0.0.0 - Production on Wed Aug 12 18:02:47 2020
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Wed Aug 12 2020 18:02:31 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
18:02:47 SQL> create table table_a
18:02:51 2 (table_name varchar2(32), insert_scn number, insert_timestamp timestamp);
Table created.
18:03:10 SQL> insert into table_a
18:03:14 2 select 'TABLE_A', current_scn, systimestamp from v$database
18:03:32 3 /
1 row created.
18:03:33 SQL> select * from table_a
18:03:37 2 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP
-------------------------------- ---------- ---------------------------------------------------------------------------
TABLE_A 6580147 12-AUG-20 06.03.33.263180 PM
18:03:38 SQL>
18:05:16 SQL> !sleep 120
18:07:21 SQL>
18:07:26 SQL> commit;
Commit complete.
18:07:28 SQL>
And this is the second session :
oracle19c>sqlplus hemant/hemant@orclpdb1
SQL*Plus: Release 19.0.0.0.0 - Production on Wed Aug 12 18:04:27 2020
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Wed Aug 12 2020 18:03:32 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
18:04:27 SQL> create table table_b
18:04:36 2 (table_name varchar2(32), insert_scn number, insert_timestamp timestamp);
Table created.
18:04:46 SQL> insert into table_b
18:04:51 2 select 'TABLE_B',current_scn, systimestamp from v$database;
1 row created.
18:05:03 SQL> select * from table_b
18:05:09 2 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP
-------------------------------- ---------- ---------------------------------------------------------------------------
TABLE_B 6581390 12-AUG-20 06.05.03.813011 PM
18:05:10 SQL>
18:05:24 SQL> !sleep 30
18:06:00 SQL>
18:06:07 SQL>
18:06:13 SQL> commit;
Commit complete.
18:06:16 SQL>
So, the second session, against TABLE_B did the INSERT after the first session but issued a COMMIT before the first session. (TABLE_B has a higher INSERT_SCN and INSERT_TIMESTAMP than TABLE_A).
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SQL> select table_name, insert_scn, insert_timestamp, scn_to_timestamp(ora_rowscn)
2 from table_a
3 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP
-------------------------------- ---------- ---------------------------------------------------------------------------
SCN_TO_TIMESTAMP(ORA_ROWSCN)
---------------------------------------------------------------------------
TABLE_A 6580147 12-AUG-20 06.03.33.263180 PM
12-AUG-20 06.07.26.000000000 PM
SQL> select table_name, insert_scn, insert_timestamp, scn_to_timestamp(ora_rowscn)
2 from table_b
3 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP
-------------------------------- ---------- ---------------------------------------------------------------------------
SCN_TO_TIMESTAMP(ORA_ROWSCN)
---------------------------------------------------------------------------
TABLE_B 6581390 12-AUG-20 06.05.03.813011 PM
12-AUG-20 06.06.14.000000000 PM
SQL>
The actual INSERT into TABLE_B was after that in TABLE_A (higher INSERT_SCN and INSERT_TIMESTAMP) but SCN_TO_TIMESTAMP of the ORA_ROWSCN implies that the row in TABLE_B is earlier than that in TABLE_A !
SQL> select table_name, insert_scn, insert_timestamp, ora_rowscn
2 from table_a
3 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP ORA_ROWSCN
-------------------------------- ---------- --------------------------------------------------------------------------- ----------
TABLE_A 6580147 12-AUG-20 06.03.33.263180 PM 6586905
SQL> select table_name, insert_scn, insert_timestamp, ora_rowscn
2 from table_b
3 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP ORA_ROWSCN
-------------------------------- ---------- --------------------------------------------------------------------------- ----------
TABLE_B 6581390 12-AUG-20 06.05.03.813011 PM 6584680
SQL>
The actual SCN recorded is that of the COMMIT time, *not* the INSERT time.
A database session gets an SCN for the Transaction it does when it COMMITs.
So, even though the INSERT into TABLE_A was earlier, it has a higher SCN simply because the COMMIT was issued later.
Does it matter if I use the ROWDEPENDENCIES extended attribute for the table ? Without ROWDEPENDENCIES, ORA_ROWSCN actually uses the SCN in the block header -- irrespective of when each row in the block was inserted / updated.
In my scenario, I had a new table with only 1 row, so there would be no difference.
Nevertheless, I repeat the experiment with ROWDEPENDENCIES.
oracle19c>sqlplus hemant/hemant@orclpdb1
SQL*Plus: Release 19.0.0.0.0 - Production on Wed Aug 12 18:17:57 2020
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Wed Aug 12 2020 18:17:47 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
18:17:57 SQL> create table table_a
18:18:10 2
18:18:10 SQL> create table table_a
18:18:14 2 (table_name varchar2(32), insert_scn number, insert_timestamp timestamp) rowdependencies;
Table created.
18:18:31 SQL> insert into table_a
18:18:40 2 select 'TABLE_A', current_scn, systimestamp from v$database
18:18:50 3 /
1 row created.
18:18:51 SQL>
18:20:11 SQL> !sleep 60
18:21:13 SQL>
18:21:15 SQL> commit;
Commit complete.
18:21:16 SQL> exit
Disconnected from Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
oracle19c>
and
oracle19c>sqlplus hemant/hemant@orclpdb1
SQL*Plus: Release 19.0.0.0.0 - Production on Wed Aug 12 18:19:30 2020
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Wed Aug 12 2020 18:19:04 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
18:19:30 SQL> create table table_b
18:19:33 2 (table_name varchar2(32), insert_scn number, insert_timestamp timestamp) rowdependencies;
Table created.
18:19:40 SQL> insert into table_b
18:19:52 2 select 'TABLE_B',current_scn, systimestamp from v$database;
1 row created.
18:20:00 SQL>
18:20:16 SQL> !sleep 30
18:20:49 SQL>
18:20:51 SQL> commit;
Commit complete.
18:20:52 SQL> exit
Disconnected from Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
oracle19c>
resulting in :
SQL> select table_name, insert_scn, insert_timestamp, ora_rowscn
2 from table_a
3 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP ORA_ROWSCN
-------------------------------- ---------- --------------------------------------------------------------------------- ----------
TABLE_A 6612380 12-AUG-20 06.18.51.562927 PM 6618886
SQL> select table_name, insert_scn, insert_timestamp, ora_rowscn
2 from table_b
3 /
TABLE_NAME INSERT_SCN INSERT_TIMESTAMP ORA_ROWSCN
-------------------------------- ---------- --------------------------------------------------------------------------- ----------
TABLE_B 6614592 12-AUG-20 06.20.00.141122 PM 6617807
SQL>
Bottom line : A row that is inserted (or updated) earlier can still have a higher SCN (and, therefore, show a higher SCN_TO_TIMESTAMP) simply because the user or the application program issued the COMMIT later. Even an application or batch job may run multiple queries or DMLs before finally issuing a COMMIT.