Filter Types
From InterBase
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Filters can be divided into two types: filters that convert data one segment at a time, and filters that convert data many segments at a time.
The first type of filter reads a segment of data, converts it, and supplies it to the application a segment at a time.
The second type of filter might read all the data and do all the conversion when the Blob read cursor is first opened, and then simulate supplying data a segment at a time to the application.
If timing is an issue for your application, you should carefully consider these two types of filters and which might better serve your purpose.