Under which conditions could a completion suggestion deplete thread pool and heap memory?

Elastic Stack Elasticsearch
1

Situation: Elasticsearch cluster crashed (thread pool and soon after heap memory depleted).

When investigating the heap dump it showed that the suggester was processing unusual queries like this:

+"\uffab-\uffaa/(+-\uffaa\uffab+(-\uffaa\uffab+(-\uffab\uffaa+(-+(\uffaa-\uffab"+(-\uffaa\uffab"+(+((-"\uffaa\uffab+(-\uffaa\uffab"+-(\uffaa\uffab/"*(-/\uffaa"+(\uffaa\uffab-"/-+(\uffaa-/\uffab"-(-\uffaa\uffab("+1

Now the questions are:

  1. Could it really be that a few of such queries lead to the described crash?
  2. If yes, why and how would it lead to a crash?
  3. Similar to question 2 but in more generality: what are the characteristics of queries that cause performance problems with the completion suggester?
  4. Additionally, how does the structure of the completion index relate to potential performance issues?

Any thoughts are appreciated :pray:t2: