Tuesday, April 29, 2014

[ ORACLE ] - Top 10 Reasons for poor Oracle performance

Top 10 Reasons for poor Oracle performance

Oracle Tips by Burleson Consulting

What are the most common root-causes of poor Oracle performance?  Every expert will give you a different opinion, but I’ve prepared my list based on our hundreds of Oracle tuning engagements:
  1. Bad Design – The number one offender to poor performance is over-normalization of Oracle tables, excessive (unused indexes) and 15-way table joins for what should be a simple fetch.
  1. Poor server optimization – Setting the server kernel parameters and I/O configuration (e.g. direct I/O) has a profound impact on Oracle performance
  1. Bad disk I/O configuration – Inappropriate use of RAID5, disk channel bottlenecks and poor disk striping.
  1. Poor Optimizer Statistics – Prior to Oracle 10g (automatic statistics), a common cause of poor SQL performance was missing/stale CBO statistics and missing histograms.
  1. Object contention – Failing to set ASSM, freelists or freelist_groups for DML-active tables and indexes can cause very slow DML performance.
  1. Under-allocated RAM regions – Not allocating enough RAM for shared_pool_size, pga_aggregate_target and db_cache_size can cause an I/O-bound database.
  1. Non-reentrant SQL – All SQL should use host variables/cursor_sharing=force to make SQL reusable within the library cache.
  1. Un-set initialization parameters – Many of the initialization parameters are made to be set by the DBA (db_file_multiblock_read_count, optimizer_index_caching) and failing to set these parameters properly results in poorly optimized execution plans.
  1. Excessive nested loop joins – In 64-bit Oracle systems we have gigabytes available for RAM sorts and hash joins.  Failing to set pga_aggregate_target to allow the CBO to choose hash joins can result in very slow SQL performance.
  1. Human Misfeasance – The DBA’s failure to monitor their database (STATSPACK/AWR), set-up exception reporting alerts (OEM) and adjusting their instance to match changing workloads is a major cause of poor performance.

The Oracle Documentation lists these ten reasons for poor performance.  The BC list is similar, but our top-10 list is based on what we see with our clients:
  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 it is totally unscalable.
    1. 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.
    1. 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.
    1. 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.
    1. 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 to 8K or 16K, or in applications with large numbers of active users and few rollback segments.
    1. 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.
    1. In Disk Sorting
In disk sorts for online operations could indicate poor transaction design, missing indexes, or poor SQL optimization. Disk sorts, by nature, are I/O-intensive and unscalable.
    1. 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. Recursive SQL executed under another user ID is probably SQL and PL/SQL, and this is not a problem.
    1. Schema Errors and Optimizer Problems
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.
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.
    1. Use of Nonstandard Initialization Parameters
These might have been implemented based on poor advice or incorrect assumptions. In particular, parameters associated with SPIN_COUNTon latches and undocumented optimizer features can cause a great deal of problems that can require considerable investigation.

Diambil daripada : http://www.dba-oracle.com/t_top_reasons_poor_performance.htm

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