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Oracle wait event bottlenecks

Oracle Tips by Burleson Consulting

How Does A Wait Bottleneck Get Fixed?

High waits on events do not always indicate a bottleneck or a problem. As database users say: “time takes time.” In addition, every Oracle database, no matter how well-tuned, spends time performing activities. High waits may indicate a bottleneck, but some waits are a normal part of database operations.

In general, the system wide wait events will show where the database spends most of its time. For example, high waits on db file sequential reads events may indicate a disk bottleneck, but the average disk queue length for each disk spindle must be checked to be sure that these waits are abnormal.

In one case, an Oracle shop ran the script for system-wide wait events and discovered that their RAID-5 configuration was causing a huge amount of disk enqueues. The shop reorganized their disk to RAID 0+1 and experienced a 3x performance improvement for the whole database. The following is just a small sample of some common wait bottlenecks:

  • SQL*Net waits: High SQL*Net waits could be due to poor encapsulation of SQL statements within the application. For example, a screen may need data from six different tables, and there is much less network traffic and database overhead if all of the information for an online screen is captured in a single trip to the database. High SQL*Net waits can also signify an error in the application programming logic or a serious network problem.
  • parallel query dequeue waits: The default degree of parallelism for database objects need to be checked, and parallelism at the system level should be turned off using specific parallel hints. The value of parallel_threads_per_cpu should be checked and adjusted to reduce automatic parallel query and its influence on the CBO.
  • db file scattered reads waits: These are caused by competing demands for large-table full-table scans and are the most common in data warehouse and decision support systems.
  • db file sequential reads waits: These are sometimes due to segment header contention on hot rows or indexes, but it could also be due to disk-level contention. The first step in this process is to increase the number of freelists on the indexes. If the waits persist, the offending index should be striped across multiple disk spindles. The DBA should check for segment header contention/waits on index headers or create multiple segment header blocks for stressed indexes using alter index xxx storage(freelists 4). The DBA could also distribute heavy impact tables and indexes onto a faster disk or stripe the rows across more data blocks by setting a high pctfree for a table and reorganizing the table.

These are very general wait conditions, but they can sometimes be fixed by changing parameters or object characteristics.


If you like Oracle tuning, you may enjoy the book Oracle Tuning: The Definitive Reference , with over 900 pages of BC's favorite tuning tips & scripts. 

You can buy it directly from the publisher and save 30%, and get instant access to the code depot of Oracle tuning scripts.


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