Is it possible to listen to relation database update? For example, my web app want to send data update to client through Comet technology. I can have the program to poll the database periodically, but that would not be performant and scalable.
If app can hood to a "event handler" of database, then app can get notification every time given database table data is updated. This sounds more promising, but I didn't find any concrete example for it. This is listener pattern.
Does common relational database support such feature?
MS SQL Server has had query change notifications since 2005. Here's more information on how to use the feature: http://msdn.microsoft.com/en-us/library/ms175110.aspx
Oracle has had this since version 10g, I think. Here's more information: http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14251/adfns_dcn.htm
I don't know of any other common RDBMSes that support this.