This section lists the most common mistakes found in Oracle systems. By following the
Oracle performance improvement methodology, you should be able to avoid these
mistakes altogether. If you find these mistakes in your system, then re-engineer the
application where the performance effort is worthwhile. See "Automatic Performance
Tuning Features" on page 1-5 for information on the features that help diagnose and
tune Oracle systems. See Chapter 10, "Instance Tuning Using Performance Views" for a
discussion on how wait event data reveals symptoms of problems that can be
impacting performance.
1. Bad Connection Management
The application connects and disconnects for each database interaction. This
problem is common with stateless middleware in application servers. It has over
two orders of magnitude impact on performance, and is totally unscalable.
2. Bad Use of Cursors and the Shared Pool
Not using cursors results in repeated parses. If bind variables are not used, then
there is hard parsing of all SQL statements. This has an order of magnitude impact
in performance, and it is totally unscalable. Use cursors with bind variables that
See Also: Oracle Database Reference for more information on
V$SQL and V$SQLSTATS
The Oracle Performance Improvement Method
Performance Improvement Methods 3-5
open the cursor and execute it many times. Be suspicious of applications
generating dynamic SQL.
3. Bad SQL
Bad SQL is SQL that uses more resources than appropriate for the application
requirement. This can be a decision support systems (DSS) query that runs for
more than 24 hours or a query from an online application that takes more than a
minute. SQL that consumes significant system resources should be investigated for
potential improvement. ADDM identifies high load SQL and the SQL tuning
advisor can be used to provide recommendations for improvement. See Chapter 6,
"Automatic Performance Diagnostics" and Chapter 12, "Automatic SQL Tuning".
4. Use of Nonstandard Initialization Parameters
These might have been implemented based on poor advice or incorrect
assumptions. Most systems will give acceptable performance using only the set of
basic parameters. In particular, parameters associated with SPIN_COUNT on
latches and undocumented optimizer features can cause a great deal of problems
that can require considerable investigation.
Likewise, optimizer parameters set in the initialization parameter file can override
proven optimal execution plans. For these reasons, schemas, schema statistics, and
optimizer settings should be managed together as a group to ensure consistency of
performance.
5. Getting Database I/O Wrong
Many sites lay out their databases poorly over the available disks. Other sites
specify the number of disks incorrectly, because they configure disks by disk space
and not I/O bandwidth. See Chapter 8, "I/O Configuration and Design".
6. Redo Log Setup Problems
Many sites run with too few redo logs that are too small. Small redo logs cause
system checkpoints to continuously put a high load on the buffer cache and I/O
system. If there are too few redo logs, then the archive cannot keep up, and the
database will wait for the archive process to catch up. See Chapter 4, "Configuring
a Database for Performance" for information on sizing redo logs for performance.
7. Serialization of data blocks in the buffer cache due to lack of free lists, free list
groups, transaction slots (INITRANS), or shortage of rollback segments.
This is particularly common on INSERT-heavy applications, in applications that
have raised the block size above 8K, or in applications with large numbers of
active users and few rollback segments. Use automatic segment-space
management (ASSM) to and automatic undo management solve this problem.
8. Long Full Table Scans
See Also:
■ Oracle Database Administrator's Guide for information on
initialization parameters and database creation
■ Oracle Database Reference for details on initialization parameters
■ "Performance Considerations for Initial Instance Configuration"
on page 4-1 for information on parameters and settings in an
initial instance configuration
Emergency Performance Methods
3-6 Oracle Database Performance Tuning Guide
Long full table scans for high-volume or interactive online operations could
indicate poor transaction design, missing indexes, or poor SQL optimization. Long
table scans, by nature, are I/O intensive and unscalable.
9. High Amounts of Recursive (SYS) SQL
Large amounts of recursive SQL executed by SYS could indicate space
management activities, such as extent allocations, taking place. This is unscalable
and impacts user response time. Use locally managed tablespaces to reduce
recursive SQL due to extent allocation. Recursive SQL executed under another
user Id is probably SQL and PL/SQL, and this is not a problem.
10. Deployment and Migration Errors
In many cases, an application uses too many resources because the schema
owning the tables has not been successfully migrated from the development
environment or from an older implementation. Examples of this are missing
indexes or incorrect statistics. These errors can lead to sub-optimal execution plans
and poor interactive user performance. When migrating applications of known
performance, export the schema statistics to maintain plan stability using the
DBMS_STATS package.
Although these errors are not directly detected by ADDM, ADDM highlights the
resulting high load SQL.
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