Behaviour of WFS filter query

[Hester Viola]

Hello,

This is a general question rather than an issue I am having.

I am wondering about the behaviour of MapServer in processing a WFS
GetFeature requests, with a filter in the URL, reading data from PostGIS or
ESRI shapefile.
eg
http://www4.bom.gov.au/cgi-bin/mapserver/mapserv?map=/web/htdocs/mapserver/weather.map&version=1.0.0&service=WFS&request=GetFeature&typename=metarrain&filter=<Filter><PropertyIsGreaterThan><PropertyName>rf9am</PropertyName><Literal>0.2</Literal></PropertyIsGreaterThan></Filter>

Does this sort of request :
1. Return all of the data in the layer to MapServer, which then applies the
filter (meaning there is significant Internet traffic involved)

OR

2. Generate an SQL query within MapServer in order to only request the data
defined by the filter from the database or file (thereby reducing the
Internet traffic)

I hope that this makes sense and someone can provide some information

Thanks in advance
Hester