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during preparation wherever possible
In many situations we are able to optimize a case when expression
and replace it with a simpler expression node during the preparation
stage. Specifically, if the WHEN conditions are known to be
static values (such as those coming from certain expression context
variables) then we can often replace the whole condition node
with the THEN node of the first static true condition.
E.g.
CASE
WHEN @variable=1 THEN "first_field"
WHEN @variable=2 THEN "second_field"
ELSE "third_field"
END
If @variable is static and '1', then the whole expression node will ALWAYS
be identical to "first_field". Similiarly if @variable='2', then the
whole expression will ALWAYS be "second_field".
If we're able to apply this optimization, then we use the simplified
effective node which represents the whole node during evaluation
time and save a bunch of unnecessary work.
TODO: If we use the effective node during expression compilation
for providers we would be able to handoff more expressions involving
QGIS-side variables and other components to the backend, resulting
in increased use of backend provider indices, etc....