Oracle Performance Tuning FAQ
$Date: 23-Apr-2000 $
$Revision: 1.10 $
$Author: Frank Naudé $
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Proper Preparation Prevents Piss Poor Performance
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When is cost base optimization triggered?
It's important to have statistics on all tables for the CBO (Cost Based
Optimizer) to work correctly. If one table involved in a statement does not
have statistics, Oracle has to revert to rule-based optimization for that statement.
So you really want for all tables to have statistics right away; it won't
help much to just have the larger tables analyzed.
Generally, the CBO can change the execution plan when you:
- Change statistics of objects by doing an ANALYZE;
- Change some initialization parameters (for example:
hash_join_enabled, sort_area_size, db_file_multiblock_read_count).
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How can one optimize %XYZ% queries?
It is possible to improve %XYZ% queries by forcing the optimizer
to scan all the entries from the index instead of the table.
This can be done by specifying hints.
If the index is physically smaller than the table (which is usually the case)
it will take less time to scan the entire index than to scan the entire
table.
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Where can one find I/O statistics per table?
The UTLESTAT report shows I/O per tablespace but one cannot see what
tables in the tablespace has the most I/O.
The $ORACLE_HOME/rdbms/admin/catio.sql script creates a sample_io
procedure and table to gather the required information. After executing the
procedure, one can do a simple SELECT * FROM io_per_object; to extract
the required information.
For more details, look at the header comments in the $ORACLE_HOME/rdbms/admin/catio.sql script.
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My query was fine last week and now it is slow. Why?
The likely cause of this is because the execution plan has changed. Generate a current explain plan of the
offending query and compare it to a previous one that was taken when the query was performing well.
Usually the previous plan is not available.
Some factors that can cause a plan to change are:
- Which tables are currently analyzed? Were they previously analyzed?
(ie. Was the query using RBO and now CBO?)
- Has OPTIMIZER_MODE been changed in INIT.ORA?
- Has the DEGREE of parallelism been defined/changed on any table?
- Have the tables been re-analyzed? Were the tables analyzed using estimate
or compute? If estimate, what percentage was used?
- Have the statistics changed?
- Has the INIT.ORA parameter DB_FILE_MULTIBLOCK_READ_COUNT
been changed?
- Has the INIT.ORA parameter SORT_AREA_SIZE been changed?
- Have any other INIT.ORA parameters been changed?
What do you think the plan should be? Run the query with hints to see if this produces the required
performance.
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Why is Oracle not using the damn index?
This problem normally only arises when the query plan is being generated by the Cost Based Optimizer. The
usual cause is because the CBO calculates that executing a Full Table Scan would be faster than accessing
the table via the index. Fundamental things that can be checked are:
- USER_TAB_COLUMNS.NUM_DISTINCT - This column defines the number of distinct values the
column holds.
- USER_TABLES.NUM_ROWS - If NUM_DISTINCT = NUM_ROWS then using an index would
be preferable to doing a FULL TABLE SCAN. As the NUM_DISTINCT decreases, the cost of
using an index decreases.
- USER_INDEXES.CLUSTERING_FACTOR - This defines how ordered the rows are in the index.
If CLUSTERING_FACTOR approaches the number of blocks in the table, the rows are ordered. If
it approaches the number of rows in the table, the rows are randomly ordered. In such a case, it is
unlikely that index entries in the same leaf block will point to rows in the same data blocks.
- Decrease the INIT.ORA parameter DB_FILE_MULTIBLOCK_READ_COUNT - A higher
value will make the cost of a FULL TABLE SCAN cheaper.
Remember that you MUST supply the leading column of an index, for the index to be used (unless you use a
FAST FULL SCAN).
There are many other factors taht affect the cost, but sometimes the above can help to show why an
index is not being used by the CBO. If from checking the above you still feel that the query should
be using an index, try specifying an index hint. Obtain an explain plan of the
query either using TKPROF with TIMED_STATISTICS, so that one can see the CPU
utilization, or with AUTOTRACE to see the statistics. Compare this to the
explain plan when not using an index.
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How does one tune Oracle Wait events?
Some wait events from V$SESSION_WAIT and V$SYSTEM_EVENT views:
| Event Name: | Tuning Recomendation:
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| db file sequential read | Tune SQL to do less I/O. Make sure all objects are analyzed. Redistribute I/O across disks.
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| buffer busy waits | Increase DB_BLOCK_BUFFERS/ Alalyze contention from SYS.V$BH
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| log buffer space | Increase LOG_BUFFER parameter or move log files to faster disks
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