check if a node has already been determined to evaluate to a static,
precalculated value, and if so, use this value for the node instead
of attempting to compile the actual contents of the node itself
If we are certain that a node is static and will never change,
the this potentially allows us to short-cut a large part of the
filter expressions content. We already use this short-cut when
evaluating expressions on the QGIS side since years, and its
proven to be stable and reliable. By respecting this during
expression compilation we can offer a huge speed up to certain
filter expressions, especially those which utilise QGIS variables
which are known to be static (such as atlas variables, map scales,
etc). Previously ANY use of a qgis variable would always cause
expression compilation to fail and require a full set of feature
fetching from the provider.
(Resulted in orders of magnitude faster atlas export for a complex
atlas.)
for a request. Most requests should reserve more
than a single connection (default: 3) and only
requests that are executed as nested expressions
should specify 1.
