How does MapServer query database

[Jackey Cheung]

Greetings!

I'm wondering, how does MapServer retrieve data from database?

Say, I've got a database table with around 10 million records including points, lines, and polygons. I am drawing a line layer in the scale of 1:100K. In such scale, many lines won't be visible on the map, and there is a field that controls the visible scale of objects in the database. How can I make MapServer retrieves only necessary lines from the database instead of selecting all lines? This shouldn't be tough if I use MapScript, but Java/MapScript keeps crashing tomcat so have to give it up.
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