Top 50 Oracle Performance Tuning Interview Questions You Must Prepare 19.May.2024

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 size of the extents and adjust optimal accordingly.

Analyze table emp compute statistics or analyze table emp estimate statistics.

There are two types of hits, they are

  1. Buffer hit
  2. library hit

As long as they are all the same size this isn’t a problem. In fact, it can even improve performance since Oracle won’t have to create a new extent when a user needs one.

  • CPU Tuning
  • Memory Tuning
  • IO Tuning
  • Application Tuning
  • Database Tuning

Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.

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.

Ensure that users don’t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.

Making optimal use of system using existing resources called performance tuning.

Optimizer is a mechanism which will make the execution plan of an sql statement.

select num_rows,blocks,empty_blocks from dba_tables where tab_name=’emp’

exec dbms_stats.gather_schema_stats(scott)

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.

Multiple extents in and of themselves aren’t bad. However if you also have chained rows this can hurt performance.

using tkprof usage: tkprof allllle.trc llkld.txt

Poor data dictionary or library cache hit ratios, getting error ORA-0403@Another indication is steadily decreasing performance with all other tuning parameters the same.

Its a road map how sql is being executed by oracle db..

These are rows, it sp in multiple blocks.

Analyze table emp delete statistics.

The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.

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.

No there is not a problem. You have 40 extents showing and an average of 40 concurrent users. Since there is plenty of room to grow no action is needed.

  • RBO(Rule Based Optimizer)
  • CBO(Cost Based Optimizer)

When you get excessive contention for the copy latches as shown by the “redo copy” latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.

alter system flush shared_pool

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.

Yes, you should strive for zero reloads if possible. If you see excessive reloads then increase the size of the shared pool.

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.

  • Set the optimizer mode
  • Collect the statistics of an object

You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view.

If you use compute, The FTS will happen, if you use estimate just 10% of the table will be read

A large number of wraps indicates that your extent size for your rollback segments are probably too small. Increase the size of your extents to reduce the number of wraps. You can look at the average traction size in the same view to get the information on traction size.

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.

If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort are parameter is the SORT_AREA_SIZe parameter.