What is "ignore in comparison" port in lookup

Could anyone please explain what is "ignore in comparison" port in lookup?

Thanks In Advance

[B]Question asked by visitor AR[/B]

When you run a session that uses a dynamic lookup cache, the PowerCenter Server compares the values in all lookup ports with the values in their associated input ports by default. It compares the values to determine whether or not to update the row in the lookup cache. When a value in an input port differs from the value in the lookup port, the PowerCenter Server updates the row in the cache.

If you do not want to compare all ports, you can choose the ports you want the PowerCenter Server to ignore when it compares ports. The Designer only enables this property for lookup/output ports when the port is not used in the lookup condition. You can improve performance by ignoring some ports during comparison.

You might want to do this when the source data includes a column that indicates whether or not the row contains data you need to update. Select the Ignore in Comparison property for all lookup ports except the port that indicates whether or not to update the row in the cache and target table.
@sriram_kumar@oneindia.in