Oracle Openworld Directory Index

The two kinds of DBAs
Which kind are you?
Part I: Backup and Recovery
Misconception:
Backup mode stops writing to the datafiles
Misconception has several permutations
Representative example
Actually, the mechanism is not “complex”
How can we back up a file that is being written?
How can we back up a file that is being written?
Hot Backup Datafile Write Proof
Hot Backup Datafile Write Proof
Misconception: Media recovery will be required after instance failure during backup mode
No recovery is necessary
Part II: Redo and Rollback
Misconception:
‘shutdown abort’ is bad
Abort – dangerous?
Abort: Just as slow as immediate?
When not to shutdown abort
Always use a script for shutdown/startup
Misconception:
You can avoid “Snapshot too old” using “set transaction use rollback segment
You don’t “fill up” an RBS with a query
ORA-01555: Queries are the victims of other sessions’ transactions
Ways to avoid ORA-01555
Ways to avoid ORA-01555
Misconception: Batch jobs should use a “dedicated” rollback segment
Part III: General Practices
Misconception:
select count(primary_key) is faster than count(*).
Many so-called proofs are flawed
Proof using timed statistics
count (*)
count (rowid)
count (PK column)
Comparison
Misconception: Lots of extents are bad
Unintended problem resolution

Rebuild undertaken to “reduce extents” Unwittingly improves performance
Improves block density
Resolves row chaining
Makes indexes that are unbalanced balanced
These are unrelated to number of extents and should be separately inspected and resolved.
A look at the “sequential reads” argument
A look at the “head movement” argument
A look at “fragmentation”
Extent deallocation time
Additional Misconceptions covered in the paper
Always ‘switch logfile’ before/after online backups.
Cold backup once a week as a “baseline.”
Export is a good way to back up the database.
Add more/bigger logfiles if you get ‘checkpoint not complete.’
NOLOGGING turns off redo logging.
After activating a standby, the primary requires re-copying to become the standby.
Listeners must be started before the instance.