@Enrico Parisi
See my answer above.

That's weird,  ##class(loggpro.IndexTest).%BuildIndices() indeed recreates the index properly. But the way I do it, with Portal's SQL, Rebuild Index just wipes the index out. Please check how Portal works for you.

Now all my fields are of type %Library.Integer but Rebuild Index still just deletes ^mytest("idx"). Does Rebuild Index work for you?