Hi, Christian: I can confirm that the BaseX944-20200913.183608 snapshot corrects the issue, for both my toy example and the original full query. Thanks for your quick response. Take care, Mark On 2020-09-13 10:42, Christian Grün wrote:

Hi Mark,

With BaseX 9.4.3, your FLWOR expression will be rewritten to a simple map expression – for which no TCOs detection was implemented so far. It works with the latest snapshot [1].

If you want to stick with 9.4.3, you can manually inline $x:

declare function local:f($n, $xs) { if ($n = 0) then $xs else local:f($n - 1, ($xs, copy $node := <a/> modify rename node $node as 'x' return $node )) }; local:f(2000, ())

Hope this helps Christian

[1] https://files.basex.org/releases/latest/

On 9/13/20, Mark McSweeny <mcsweeny@shaw.ca> wrote:

...

Hello,

I have encountered a difference in behaviour between versions 9.3.3 and 9.4.3 for a certain query involving tail recursion. In 9.3.3 the compiled query plan correctly applied tail call optimization. But in 9.4.3 tail call optimization is not applied, resulting in a stack overflow during execution.

Here is a minimal query that demonstrates the issue:

declare function local:f($n, $xs) { if ($n = 0) then $xs else let $x := copy $node := <a/> modify ( rename node $node as 'x') return $node return local:f($n - 1, ($xs, $x)) }; local:f(2000, ())

Mark