Change Tracking

Posting from the UKOUG (UK Oracle User Group) conference. The most interesting detail I picked up today came from a presentation by Alex Gorbachev of the Pythian Group, a company that specialises in remote DBA operations – and probably does it very well judging from this blog entry from Doug Burns

I’ll try to find a public link to the presentation, but the critcal point is this. The Change Tracking log (produced by CTWR) is a bitmap representing the entire database – (probably) using one bit for each 32KB chunk of the database. The bitmap has an associated SCN, which is the SCN as at the last backup, and a bit gets set in the bitmap to show that the corresponding chunk (32KB) of the database has changed since that SCN.

There are 7 versions of the bitmap; and each time you take a backup of the database using rman, you starting using the next available bitmap and stamp it with the latest backup SCN. So if you take 8 backups, of the database the first bitmap has to be overwritten.

dbms_xplan pt.2

[More on dbms_xplan.display_cursor()]

A comment on my previous posting about dbms_xplan pointed out that in 10g you also get the function dbms_xplan.display_awr that allows you to report historical execution plans from the AWR (automatic workload repository).

This is true, and there is yet another function dbms_xplan.display_sqlset which allows you to report execution plans from SQL Tuning Sets if you have been using the Automatic SQL Tuning Tools. But to use these functions you do need to purchase the Tuning Pack licence and the Diagnostic Pack licence.