I have 1-1/2 years experience as a DBA and I have an interview next week. What are the top performance questions I should be aware of?
11. Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio; which should be used for tuning?
Expected answer: Hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.
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 may 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 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 wait time is zero, no. If wait time was high it might indicate a need for more or larger redo logs.
Dig Deeper on Oracle database performance problems and tuning
Have a question for an expert?
Please add a title for your question
Get answers from a TechTarget expert on whatever's puzzling you.