Large arel tree causes ActiveRecord preloader to throw SystemStackError about rails HOT 2 CLOSED

Interesting.

So this limitation isn't very surprising when you know that Arel compilation uses the common visitor pattern, so Arel will commonly use K * tree_depth stack frames to compile an AST, and Ruby the Ruby stack is finite, hence Arel has an implicit max depth limit.

Interestingly, in your stacktraces, it's the hash method that raises, but it's because it similarly recursively digest child nodes, so same problem. #41597 is what started calling hash, but I suspect compiling to SQL would similarly run out of stack space (if not the limit probably isn't that much further).

One way we may be able to improve this is by changing the Or node, so it's no longer a binary node, but can contain any number of conditions.

That would fix your specific problem, and generally probably reduce the depth of trees a bit, but the same problem may exist with other types of nodes. And I'm also not sure yet whether this solution is even doable, I'd need to try to see.

And also, even if it works, it would be a big enough change that I don't think it would be realistic to backport it as a bug fix to be honest.

from rails.

Thank you @casperisfine - I'll try patching this locally and see if it solves our specific use-case!

from rails.