Re: [Exist-development] optimizing positional predicates for fast paging

[Wolfgang M. <wol...@ex...>]

> I wonder whether eXist is currently optimizing xquery evaluations that use
> positional predicates, or similar.

I did implement shortcuts for the expensive axes: following and
preceding. For example, LocationStep.getFollowing checks the property
hasPositionalPredicate. If set to true, the method evaluates the
predicate in advance to get its numeric value. This is then forwarded
to the NodeSet method which calculates the join:

return currentSet.selectFollowing(contextSet, position, contextId);

We could apply similar optimizations to other axes, though their
execution paths are more complex as you have more possibilities to
choose from.

However, my positional predicate optimization would only work for
//elem[1], not //elem[position() = 1]. But I guess it would be
possible to add a rewrite rule here to automatically simplify the
expression.

> collection("/")/ (//elem) [position() = (1 to 10)]

Note that using subsequence instead of the predicate can be a lot
faster sometimes.

Wolfgang