Hi Michael,

I checked your use case in greater depth, and I found the change in
our code that caused the slowdown [1].

A) The nutshell answer : Just use the attached query!

B) The extensive technical answer:

• In previous versions of BaseX, most paths in FLWOR expressions were
»inlined« in the code to trigger further optimizations, such as index
rewritings.
• The enforced inlining led to cases in which the execution time was
worse than for unoptimized queries.
• As a user cannot prevent variables from being inlined, we have
switched to a more predictive pattern in our inlining heuristics:
Paths will only be moved around anymore if we can ensure that the
execution time will not suffer.

A little example:

  let $nodes := db:open('db')/to/this/only/once
  for $i in 1 to 1000
  return $nodes

If $nodes is inlined by the optimizer (i.e., if the variable reference
$nodes in the last line is replaced by the actual path), the path will
be evaluated 1000 times instead of once. The revised query optimizer
won’t inline such paths anymore.

Your particular query benefited from the offensive rewriting, though.
In the first step, "db:open('gnd-sachbegriff')/collection/record" was
inlined by the optimizer:

  let $recFromExistingData := db:open('gnd-sachbegriff')/
    collection/record[controlfield[@tag = '001'] = $id]

In the second step, the path was rewritten for index access:

  let $recFromExistingData := db:text('gnd-sachbegriff', $id)/
    parent::controlfield[@tag = '001']/parent::record

The index rewriting (which you can spot in the Info View by looking
for "apply text index") led to a much faster evaluation of your query
because it reduces the execution time from quadratic to linear.

If you adopt one of the code lines above, your query will be evaluated
faster again.

In the attached query, db:open is still assigned to variables. As
db:open will only be evaluated once and already at compile time, the
document nodes that will be bound to $sachbegriffe can always be
inlined.

Hope this helps,
Christian

[1] https://github.com/BaseXdb/basex/issues/1722