Why is using a WHERE clause on the reader typically faster than filtering data with a Tester in an FME workflow?

• A. It is not faster.
• B. The Tester is particularly slow when working with PostGIS and geodatabase formats.
• C. With the WHERE clause you filter the data using a published parameter, while the Tester uses a fixed value.
• D. With the WHERE clause you don't read data only to immediately filter it out.

Answer: (D)

Filtering as early as possible minimizes data movement and processing. When you put a WHERE clause on the Reader, the database applies the filter while retrieving data, often using indexes to return only the matching features. In contrast, using a Tester later means FME reads all features from the source, then evaluates the condition and discards the non-matches, which adds unnecessary I/O and CPU work. This extra work becomes especially costly with large datasets or remote databases, so the WHERE clause saves time by avoiding reading data that would be filtered out.