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So, if you are modifying the table that you are reading (as in the previous example), you are generating undo information required for your query Your UPDATE generates undo information that your query will probably be making use of to get the read-consistent view of the data it needs to update If you commit, you are allowing the system to reuse the undo segment space you just filled up If it does reuse the undo, wiping out old undo data that your query subsequently needs, you are in big trouble Your SELECT will fail and your UPDATE will stop partway through You have a partly finished logical transaction and probably no good way to restart it (more about this in a moment) Let s see this concept in action with a small demonstration.

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System.Data.SqlClient System.Data.OleDb System.Data.OracleClient System.Data.Odbc Npgsql MySql.Data.MySqlClient

In a small test database, I set up a table: ops$tkyte%ORA11GR2> create table t as select * from all_objects; Table created ops$tkyte%ORA11GR2> create index t_idx on t(object_name); Index created ops$tkyte%ORA11GR2> exec dbms_statsgather_table_stats( user, 'T', cascade=>true ); PL/SQL procedure successfully completed I then created a very small undo tablespace and altered the system to use it Note that by setting AUTOEXTEND off, I have limited the size of all UNDO to be 10MB or less in this system: ops$tkyte%ORA11GR2> create undo tablespace undo_small 2 datafile '/home/ora11gr2/app/ora11gr2/oradata/orcl/undo_smalldbf' 3 size 10m reuse 4 autoextend off 5 / Tablespace created ops$tkyte%ORA11GR2> alter system set undo_tablespace = undo_small; System altered.

Now, with only the small undo tablespace in use, I ran this block of code to do the UPDATE: ops$tkyte%ORA11GR2> 2 for x in ( 3 4 5 loop 6 update 7 set 8 where begin select /*+ INDEX(t t_idx) */ rowid rid, object_name, rownum r from t where object_name > ' ' ) t object_name = lower(xobject_name) rowid = xrid;.

.NET 2.0 .NET 2.0 .NET 2.0 .NET 2.0 http:// npgsql.projects.postgresql.org/ http://dev.mysql.com/downloads/

9 if ( mod(x.r,100) = 0 ) then 10 commit; 11 end if; 12 end loop; 13 commit; 14 end; 15 / begin * ERROR at line 1: ORA-01555: snapshot too old: rollback segment number ORA-06512: at line 2

I get the error I should point out that I added an index hint to the query and a WHERE clause to make sure I was reading the table randomly (together, they caused the cost-based optimizer to read the table sorted by the index key) When we process a table via an index, we tend to read a block for a single row, and then the next row we want will be on a different block Ultimately, we will process all of the rows on block 1, just not all at the same time Block 1 might hold, say, the data for all rows with OBJECT_NAMEs starting with the letters A, M, N, Q, and Z So we would hit the block many times, since we are reading the data sorted by OBJECT_NAME and presumably many OBJECT_NAMEs start with letters between A and M.

The OleDb and ODBC data providers are provided for compatibility with earlier database access technologies. All ADO.NET connection and command classes have the data provider name as the prefix to their class name, as in OdbcConnection and OdbcCommand or OleDbConnection and OleDbCommand.

At first glance, it might appear that you could use .NET serialization to implement undo functionality: what easier way to take a snapshot of an object s state than to serialize it into a byte stream Unfortunately, this isn t as easy as it might sound, at least when it comes to restoring the object s state. Taking a snapshot of a <Serializable()> object is easy, and can be done with code similar to this:

Since we are committing frequently and reusing undo space, we eventually revisit a block where we can simply no longer roll back to the point our query began, and at that point we get the error This was a very artificial example just to show how it happens in a reliable manner My UPDATE statement was generating undo I had a very small undo tablespace to play with (10MB) I wrapped around in my undo segments many times, since they are used in a circular fashion Every time I committed, I allowed Oracle to overwrite the undo data I generated Eventually, I needed some piece of data I had generated, but it no longer existed and I received the ORA-01555 error.