Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




33.2. Visibility of Data Changes

If you execute SQL commands in your trigger function, and these commands access the table that the trigger is for, then you need to be aware of the data visibility rules, because they determine whether these SQL commands will see the data change that the trigger is fired for. Briefly:

  • Statement-level triggers follow simple visibility rules: none of the changes made by a statement are visible to statement-level triggers that are invoked before the statement, whereas all modifications are visible to statement-level after triggers.

  • The data change (insertion, update, or deletion) causing the trigger to fire is naturally not visible to SQL commands executed in a row-level before trigger, because it hasn't happened yet.

  • However, SQL commands executed in a row-level before trigger will see the effects of data changes for rows previously processed in the same outer command. This requires caution, since the ordering of these change events is not in general predictable; a SQL command that affects multiple rows may visit the rows in any order.

  • When a row-level after trigger is fired, all data changes made by the outer command are already complete, and are visible to the invoked trigger function.

Further information about data visibility rules can be found in Section 40.4. The example in Section 33.4 contains a demonstration of these rules.

  Published courtesy of The PostgreSQL Global Development Group Design by Interspire