(Continued from previous part...)
Tuning Questions for DBA (2)
12. Discuss row chaining, how does it happen? How can you reduce it? How do you correct it?
Expected answer: Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and won?t fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.
13. When looking at the estat events report you see that you are getting busy buffer waits. Is this bad? How can you find what is causing it?
Expected answer: Buffer busy waits could indicate contention in redo, rollback or data blocks. You need to check the v$waitstat view to see what areas are causing the problem. The value of the "count" column tells where the problem is, the "class" column tells you with what. UNDO is rollback segments, DATA is data base buffers.
14. If you see contention for library caches how can you fix it?
Expected answer: Increase the size of the shared pool.
15. If you see statistics that deal with "undo" what are they really talking about?
Expected answer: Rollback segments and associated structures.
16. If a tablespace has a default pctincrease of zero what will this cause (in relationship to the smon process)?
Expected answer: The SMON process won?t automatically coalesce its free space fragments.
17. If a tablespace shows excessive fragmentation what are some methods to defragment the tablespace? (7.1,7.2 and 7.3 only)
Expected answer: In Oracle 7.0 to 7.2 The use of the 'alter session set events 'immediate trace name coalesce level ts#';? command is the easiest way to defragment contiguous free space fragmentation. The ts# parameter corresponds to the ts# value found in the ts$ SYS table. In version 7.3 the ?alter tablespace coalesce;? is best. If the free space isn?t contiguous then export, drop and import of the tablespace contents may be the only way to reclaim non-contiguous free space.
18. How can you tell if a tablespace has excessive fragmentation?
If a select against the dba_free_space table shows that the count of a tablespaces extents is greater than the count of its data files, then it is fragmented.
19. You see the following on a status report:
redo log space requests 23
redo log space wait time 0
Is this something to worry about? What if redo log space wait time is high? How can you fix this?
Expected answer: Since the wait time is zero, no. If the wait time was high it might indicate a need for more or larger redo logs.
20. What can cause a high value for recursive calls? How can this be fixed?
Expected answer: A high value for recursive calls is cause by improper cursor usage, excessive dynamic space management actions, and or excessive statement re-parses. You need to determine the cause and correct it By either relinking applications to hold cursors, use proper space management techniques (proper storage and sizing) or ensure repeat queries are placed in packages for proper reuse.
21. If you see a pin hit ratio of less than 0.8 in the estat library cache report is this a problem? If so, how do you fix it?
Expected answer: This indicate that the shared pool may be too small. Increase the shared pool size.
22. If you see the value for reloads is high in the estat library cache report is this a matter for concern?
Expected answer: Yes, you should strive for zero reloads if possible. If you see excessive reloads then increase the size of the shared pool.
23. You look at the dba_rollback_segs view and see that there is a large number of shrinks and they are of relatively small size, is this a problem? How can it be fixed if it is a problem?
Expected answer: A large number of small shrinks indicates a need to increase the size of the rollback segment extents. Ideally you should have no shrinks or a small number of large shrinks. To fix this just increase the