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CHAPTER 16 LOCKING-RELATED ISSUES
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Let s look at a simple example. We create a table, t1, and insert three numbers in it: scott@ORA10G> create Table created. scott@ORA10G> insert 1 row created. scott@ORA10G> insert 1 row created. scott@ORA10G> insert 1 row created. table t1 ( x number ); into t1 values ( 1 ); into t1 values ( 1 ); into t1 values ( 22);
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We then select the column x and the pseudo column ora_rowscn from table t1. Since the transaction is not committed yet, the SCN has not been assigned; hence ora_rowscn returns null for all rows. scott@ORA10G> select x, ora_rowscn from t1; X ORA_ROWSCN ---------- ---------1 1 22 Let s commit and rerun our query: scott@ORA10G> commit; Commit complete. scott@ORA10G> select x, ora_rowscn from t1; X ORA_ROWSCN ---------- ---------1 4919036 1 4919036 22 4919036 As you can see, all rows that were committed in this transaction got the same ora_rowscn value. Let s insert another row, issue a commit, and rerun our query: scott@ORA10G> insert into t1 values ( 3 ); 1 row created. scott@ORA10G> commit; Commit complete. scott@ORA10G> select x, ora_rowscn from t1; X ORA_ROWSCN ---------- ---------1 4919036 1 4919036 22 4919036 3 4919038
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As shown, a different ora_rowscn value is set for the last row since it was committed in a different transaction. There is one issue, though. By default, ora_rowscn is maintained at a database block level. The problem is that one database block can contain multiple rows. This will not work for us from the optimistic locking point of view, because our pseudo column has to be changed only for the row we update regardless of whether it has other rows in the block it resides in. To demonstrate this issue, we will update one row of table t1 and see how it affects the ora_rowscn values selected from the table: scott@ORA10G> update t1 set x = 3 where x =22; 1 row updated. scott@ORA10G> commit; Commit complete. We select the ora_rowscn values again along with the block number of each row: scott@ORA10G> select x, ora_rowscn, dbms_rowid.rowid_block_number( rowid ) block_number from t1; X ORA_ROWSCN BLOCK_NUMBER ---------- ---------- -----------1 4919041 508 1 4919041 508 3 4919041 508 3 4919041 508 Notice how the ora_rowscn value changed for all the rows of the table (since they are all in the same database block), even though we updated only one of them. Fortunately, Oracle does provide a way to maintain ora_rowscn values at a row level. For this, we need to create the underlying table with the rowdependencies option. Let s drop and re-create table t1 and insert some data into it, followed by a select of the ora_rowscn values: scott@ORA10G> drop table t1; Table dropped. scott@ORA10G> create table t1 ( x number ) rowdependencies; Table created. scott@ORA10G> insert into t1 values ( 1 ); 1 row created. scott@ORA10G> insert into t1 values ( 2 ); 1 row created. scott@ORA10G> insert into t1 values ( 22); 1 row created. scott@ORA10G> commit; Commit complete. scott@ORA10G> select x, ora_rowscn from t1; X ORA_ROWSCN ---------- ---------1 4920043 2 4920043 22 4920043
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All rows have the same ora_rowscn values, since they were inserted in the same transaction. Let s now update one row as before and rerun the select to query ora_rowscn and the block numbers: scott@ORA10G> update t1 set x = 3 where x =22; 1 row updated. scott@ORA10G> commit; Commit complete. scott@ORA10G> select x, ora_rowscn, dbms_rowid.rowid_block_number( rowid ) block_number from t1; X ORA_ROWSCN BLOCK_NUMBER ---------- ---------- -----------1 4920043 532 2 4920043 532 3 4920046 532 Notice how the ora_rowscn value of the third row is different from the rest (since it was changed), even though it is in the same block as the other rows. Thus, we can safely use the ora_rowscn column to proxy for the values in our row as long as the underlying tables are created with the rowdependencies option. Note that using the rowdependencies option does cause the row size to increase by 6 bytes due to additional information maintained at the row level. As of 10g Release 1, the use of ora_rowscn has a few restrictions you should be aware of. You cannot use ora_rowscn in a query to a view. However, you can use it to refer to the underlying table when creating a view. To understand this, create a view, v1, on table t1: scott@ORA10G> create or replace view v1 as 2 select x 3 from t1; View created. If you try to select ora_rowscn from it, you will get an error: scott@ORA10G> select ora_rowscn from v1; select ora_rowscn from v1 * ERROR at line 1: ORA-00904: "ORA_ROWSCN": invalid identifier However, you can refer to ora_rowscn in the view definition itself to overcome this restriction. The view v2 is created with the ora_rowscn pseudo column values as one of its columns: scott@ORA10G> create or replace view v2 as 2 select x, ora_rowscn 3 from t1; View created.
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