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operation, the begin and end time of the transaction, and so on. It is these pseudo-columns that tell you exactly when a row was modified and what was done to the row at that time. Here is a brief explanation of each pseudo-column in the Flashback Versions Query output: VERSIONS_STARTSCN and VERSIONS_STARTTIME: These pseudo-columns tell you the SCN and time stamp when this particular row was first created. If the VERSIONS_STARTTIME is null, the row was created before the lower time boundary of the query. VERSIONS_ENDSCN and VERSIONS_ENDTIME: These pseudo-columns tell you when this particular row expired. If the VERSIONS_ENDTIME column value is null, it means that the row is either current or that it has been deleted. VERSIONS_OPERATION: This pseudo-column provides you with information about the type of DML activity that was performed on the particular row. The column has three possible values: I represents an insert, D a delete operation, and U an update operation. VERSIONS_XID: This pseudo-column displays the unique transaction identifier of the transaction that resulted in this row version.

Signals carry information from one object to another. Objects can send signals in two basic ways: 1. Using procedure calls: Execution control is passed from the sender to the receiver. 2. Using a shared resource: With shared resource interactions, signals transfer data, not execution control, from the sender to the receiver. On a wiring diagram, the abstract path over which signals travel from sender to receiver is depicted with lines. The lines are also known as wires. A wire can only carry one type of signal. You must label every wire in a diagram to show the name of the signal associated with it and to indicate the direction of travel of the signal. Figure 7-3 shows how a signal is represented on diagrams.

An index-organized table (IOT) will show an update operation as a delete and an insert operation. Your Flashback Versions Query would produce both the deleted and inserted rows as two independent versions. The first version would show a D for the delete operation under the VERSIONS_OPERATION pseudo-column, and the subsequent insert column would show an I for the same pseudo-column.

If the version of a row was created before the MINVALUE or the beginning of the query, you can t capture the value for the starting time stamp or SCN, and your VERSIONS_STARTSCN and VERSIONS_STARTTIME pseudo-columns will be null there won t be any history for this row in your undo segments. The VERSIONS_ENDSCN and VERSIONS_ENDTIME pseudo-columns tell you when the row version expired. If the row version is still current at the time of your Flashback Versions Query, the VERSIONS_ENDSCN and VERSIONS_ENDTIME pseudo-columns will be null. Similarly, if the row version has been deleted from the table, you ll see a null value for these two pseudo-columns.

To understand the capabilities and power of the Flashback Versions Query feature, let s look at the simple example shown in Listing 8-1. Listing 8-1. Using the Flashback Versions Query Feature SQL> SELECT versions_xid AS XID, versions_startscn AS START_SCN, versions_endscn AS END_SCN, versions_operation AS OPERATION, empname FROM EMPLOYEES VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE AS OF SCN 7920 WHERE emp_id = 222; XID ---------------0003002F00038BA9 0004002D0002B366 000400170002B366 SQL> START_SCN --------2266 0864 0827 END_SCN -------OPERATION ---------I D I EMPNAME ---------Nick Sam Sam SALARY ------19000 20000 20000

Figure 7-3. The representation of a signal on a wiring diagram If a signal with a given name can originate from more than one place, the name of the sender can be included with a signal, using a dot notation. Figure 7-4 shows an example.

The example in Listing 8-1 retrieves three versions of a row for employee number (emp_id) 222. The AS OF SCN of the query is 7920. That is, we want to know what versions of the row existed at this SCN. Although you see three versions in the output, only one of the row versions is true as of the SCN you re interested in. So, which version is it Let s read the query output from top to bottom. Pay particular attention to the START_SCN and the END_SCN columns. All rows will have a START_SCN, but they may have a null value for the END_SCN if the version of the row still exists at the current SCN. The first row, which inserted (operation I) employee name Nick at SCN 2266, is the latest version of the row. Since the END_SCN is null for the first row, you know that this row still existed at SCN 7920. If you look under the OPERATION column, you see the letter D for the second version (START_SCN 0864), indicating that the middle row was deleted (probably accidentally), and the row didn t exist at SCN 7920. The first row thus reflects the fact that the row was reinserted, with a different employee s name. The bottom or third row has an END_SCN number, so clearly this row expired at SCN 0864. This was the originally inserted version of this row, as indicated by the value I (insert) under the OPERATION column.