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Oracle Metric Concurrency

Oracle Consulting Tips by Burleson


The Concurrency Oracle metric allows simultaneous access of the same data by many users. A multi-user database management system must provide adequate concurrency controls, so that data cannot be updated or changed improperly, compromising data integrity.

While Oracle manages concurrent updates efficiently, within end-user application we must always remember that once data has been released from Oracle (and displayed on a screen) it may sit there for hours before the end-user changes a row.  During the time between map-out and update, other users may have updated the row,.  This can cause two types of concurrency anomalies:


1 - Data overlay - If user 123 updates column A while user 456 is viewing column A, user 456 might overlay the changes made by user 123.


2 - Incorrect update decisions - The data on a screen may be used by the end-user to make their update decision, and the end-user must be informed whenever a displayed column value has been changed since screen map-out.  The end-user will be asked to re-retrieve the screen and re-apply their updates, based on the new column values.


It is critical to develop a generic, reusable mechanism to validate that no displayed columns have been updated by another user, causing a data overlay or incorrect update decisions.


Oracle provides several ways of preventing data overlaying:


  • The 'select for update' method - The 'select for update' approach can e problematic because SQL*Net timeouts often do not release the locks, causing DBA-related problems.  We also cannot use 'select for update' with applications (e.g. web-based HTML) that do not hold sessions open after screen map-out.

  • The data re-read method - Here we simply re-read a row to ensure that it had not been changed between screen map-out time and update time.

  • Automatic Row Processing - Oracle APEX provides default mechanism to prevent concurrency or 'lost update' issues within an application.  This mechanism is called 'Automatic Row (DML) Processing'.    The mechanism can automatically be created during the 'Create Page' Wizard or can be added manually afterward.   This approach also provides 2 other advantages: do not have to provide SQL coding for DML actions and APEX will perform the DML actions automatically.  

For my notes on Oracle concurrency management, see:

This, and many other Oracle performance metrics are discussed in my book "
Oracle Tuning" by Rampant TechPress.  You can buy it directly from the publisher and save 30% at this link:



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