CHANGES TO DATA BASE TABLES

CHANGES TO DATA BASE TABLES:


Correct access by ABAP programs to a database table is only possible if the runtime object of the table is consistent with the structure of the table in the database. Each time the table is changed in the ABAP Dictionary, you must check if the database structure of the table must be adjusted to the changed ABAP Dictionary definition of the table when it is activated (when the runtime object is rewritten).


The database structure does not have to be altered for certain changes to the ABAP Dictionary. For example, you do not have to change the database structure when the order of the fields in the ABAP Dictionary is changed (other than for key fields). In this case the changed structure is simply activated in the ABAP Dictionary and the database structure remains unchanged.


The database table can be adjusted to the changed definition in the ABAP Dictionary in three different ways:


By deleting the database table and creating it again. The table on the database is deleted, the inactive table is activated in the ABAP Dictionary, and the table is created again on the database. Data existing in the table is lost.


By changing the database catalog (ALTER TABLE). The definition of the table on the database is simply changed. Existing data is retained. However, indexes on the table might have to be built again.

By converting the table. This is the most time-consuming way to adjust a structure.

If the table does not contain any data, it is deleted in the database and created again with its new structure. If data exists in the table, there is an attempt to adjust the structure with ALTER TABLE. If the database system used is not able to do so, the structure is adjusted by converting the table.

EXAMPLE:


CONVERSION PROCESS :

The following example shows the steps necessary during conversion.

Starting situation: Table TAB was changed in the ABAP Dictionary. The length of field 3 was reduced from 60 to 30 places.

The ABAP Dictionary therefore has an active (field 3 has a length of 60 places) and an inactive (field 3 still has 30 places) version of the table.

The active version of the table was created in the database, which means that field 3 currently has 60 places in the database. A secondary index with the ID A11, which was also created in the database, is defined for the table in the ABAP Dictionary.

The table already contains data.

Step 1: The table is locked against further structure changes. If the conversion terminates due to an error, the table remains locked. This lock mechanism prevents further structure changes from being made before the conversion has been completed correctly. Data could be lost in such a case.

Step 2: The table in the database is renamed. All the indexes on the table are deleted. The name of the new (temporary) table is defined by the prefix QCM and the table name. The name of the temporary table for table TAB is therefore QCMTAB.

Step 3: The inactive version of table TAB is activated in the ABAP Dictionary. The table is created in the database with its new structure and with the name QCM8TAB. The primary index on the table is also created in the database. The structure of database table QCM8TAB is the same as the structure of the active table in the ABAP Dictionary after this step. The database table, however, still does not contain any data.

The table therefore does not exist in the database with its original name during conversion. Programs that access this table therefore cannot be executed. You should therefore always make sure that no applications access the table to be converted during conversion.

Step 4: The data is loaded back from table QCMTAB to table QCM8TAB (with MOVE-CORRESPONDING). The data is in both tables after this step. When you reduce the size of fields, for example, the extra places are truncated when you reload the data.

Since the data is in both tables QCM8TAB and QCMTAB during the conversion, there are greater storage requirements during conversion. You should therefore check whether there is sufficient space available in the corresponding tablespace before converting large tables.

There is a database commit after 16 MB when you copy the data from table QCMTAB to table QCM8TAB. A conversion process therefore needs 16 MB resources in the rollback segment. The existing database lock is released with the Commit and then requested again before the next data area to be converted is edited.

When you reduce the size of keys, only one record can be reloaded if there are several records whose key cannot be distinguished. It is not possible to say which record this will be. In such a case you should clean up the data of the table before converting.

Step 5: Table QCM8TAB is renamed to TAB. The secondary indexes defined in the ABAP Dictionary for the table are created again. The views on the table deleted in the first step of the conversion are also created again in the database.

Step 6: Table QCMTAB is deleted.

Step 7: The lock set at the beginning of the conversion is deleted.

If the conversion terminates, the table remains locked and a restart log is written.

Caution: The table does not exist in the database with its original name during conversion. Programs therefore may not access the table during conversion. Conversions therefore should not run during production! You must at least deactivate all the applications that use the tables to be converted.


You must clean up terminated conversions. Programs that access the table will otherwise not run correctly. In this case you must find out why the conversion terminated (for example overflow of the corresponding tablespace) and correct it. Then continue the terminated conversion.


POSSIBLE PROBLEMS DURING CONVERSIONS:


Since the data exists in both the original table and temporary table during conversion, the storage requirements increase during conversion. If the tablespace overflows when you reload the data from the temporary table, the conversion will terminate. In this case you must extend the tablespace and start the conversion in the database utility again.


If you shorten the key of a table (for example when you remove or shorten the field length of key fields), you cannot distinguish between the new keys of existing records of the table. When you reload the data from the temporary table, only one of these records can be loaded back into the table. It is not possible to say which record this will be. If you want to copy certain records, you have to clean up the table before the conversion.


During a conversion, the data is copied back to the database table from the temporary table with the ABAP statement MOVE-CORRESPONDING. Therefore only those type changes that can be executed with MOVE-CORRESPONDING are allowed. All other type changes cause the conversion to be terminated when the data is loaded back into the original table. In this case you have to recreate the old state prior to conversion. Using database tools, you have to delete the table, rename the QCM table to its old name, reconstruct the runtime object (in the database utility), set the table structure in the Dictionary back to its old state and then activate the table.


RESUMING TERMINATED CONVERSIONS:

If a conversion terminates, the lock entry for the table set in the first step is retained. The table can no longer be edited with the maintenance tools of the ABAP Dictionary (Transaction SE11).


A terminated conversion can be analyzed with the database utility (Transaction SE14) and then resumed. The database utility provides an analysis tool with which you can find the cause of the error and the current state of all the tables involved in the conversion.


You can usually find the precise reason for termination in the object log. If the object log does not provide any information about the cause of the error, you have to analyze the syslog or the short dumps.


If there is a terminated conversion, two options are displayed as pushbuttons in the database utility:


After correcting the error, you can resume the conversion where it terminated with the Continue adjustment option.


There is also the Unlock table option. This option only deletes the existing lock entry for the table . You should never choose Unlock table for a terminated conversion if the data only exists in the temporary table, i.e. if the conversion terminated in steps 3 or 4.


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