XPath: Optimize constant filters/predicates

If a filter/predicate expression is a constant, we don't need to evaluate it
for every nodeset element - we can evaluate it once and pick the right element
or keep/discard the entire collection.

If the expression is 1, we can early out on first node when evaluating the
node set - queries like following::item[1] are now significantly faster.

Additionally this change refactors filters/predicates to have additional
metadata describing the expression type in _test field that is filled during
optimization.

Note that predicate_constant selection right now is very simple (but captures
most common use cases except for maybe [last()]).

0 files changed, 0 insertions, 0 deletions