It is impossible to see if "filesort with small limit" optimization was used

We've ported "filesort with small limit" optimization from mysql-5.6 into MariaDB 10.0.

The problem is, it is impossible to see whether that optimization is used or not.

In MySQL, one can check this as follows

• set optimizer_trace=1
• SELECT // need to run SELECT, not EXPLAIN
• select * from optimizer_trace

And then look for something like this:

      "join_execution": {

...

            "filesort_priority_queue_optimization": {

              "limit": 10,

              "rows_estimate": 198717,

              "row_size": 215,

              "memory_available": 262144,

              "chosen": true

            },

In MariaDB, there is no optimizer_trace, so one can't see it at all.