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" 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.
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.
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 open the cursor and execute it many times. Be suspicious of applications generating dynamic 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".
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.
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".
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.
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.
Long Full Table Scans
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.
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.
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.