August 2015

The old question about truncate and undo (“does a truncate generate undo or not”) appeared on the OTN database forum over the week-end, and then devolved into “what really happens on a truncate”, and then carried on.

The quick answer to the traditional question is essentially this: the actual truncate activity typically generates very little undo (and redo) compared to a full delete of all the data because all it does is tidy up any space management blocks and update the data dictionary; the undo and redo generated is only about the metadata, not about the data itself.

In various programming environments, a common metaphor is to open a cursor on the database (a REF CURSOR in Oracle parlance), return that cursor handle to the calling environment, and then that cursor is used to fetch or “fill” a data object, which may map to a grid on screen, or just to an array in memory.

And that’s totally fine – its an effective means to obtain a result set from the database to the calling environment.

For example, a typical PLSQL routine might look like:

PROCEDURE my_proc(p_resultset IN OUT sys_refcursor) IS
BEGIN
   OPEN p_resultset FOR
     SELECT col1, col2, col3, ...
     FROM   my_table
     WHERE  status = 'Unprocessed';
END;  

That’s fine when you want a set of rows, and luckily, most programming environments have evolved to the state nowadays where they will perform that retrieval with an array fetc

In general, performance tuning is quite straightforward. You work out how much time is spent doing what, and follow the numbers to the things that are taking the most time. If you are fortunate to the have the Diagnostics Pack on Enterprise Edition of Oracle your Active Session History (ASH) will report the active database sessions. If you can measure the overall response time of a batch or application server you have merely to deduct the DB reported in ASH, and the difference is client/middleware time.
While that is an over-simplification, it is substantially true. The rest of this blog is about an exception to the general rule.

August 23, 2015 (Modified August 31, 2015, September 14, 2015) (Back to the Previous Article in this Series) I started using Linux in 1999, specifically Red Hat Linux 6.0, and I recall upgrading to Red Hat Linux 6.1 after downloading the files over a 56k modem – the good old days.  I was a little […]