Code review comment for lp:~zorba-coders/zorba/dataguide

I've done some additional testing, and these are the results:

For the xray query, the largest that we have in the testsuite, compilation time with --compile-only is pretty much the same with and without the dataguide computaiton, at around ~0.08 sec.

With a specially constructed query that looks like this: (see dataguide-29.jq test)

let $col := dml:collection()
let $col2 := ($col.cat1, $col.cat2, ... , $col.cat10)
return $col2.category.category.category ... category (repeated ~2000 times)

the compilation time goes from ~0.7s without the dataguide to ~10s with the dataguide enabled, so it is significant. But this is a worst-case scenario. The resulting dataguide is an object 2000-levels deep.

The compilation can be improved significantly by:
- keeping track of the "leaves" nodes in the dataguide tree
- rewriting a bit the dataguide structure to store the trees incrementally instead of cloning them
- adding a depth cutoff

What do you think?