Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.
   The cost_trace as in EXPLAIN FORMAT="cost_trace" SELECT * FROM t is not doucmented

2. Describe your suggestion or addition.
   something like:

| `cost_trace`    | The `EXPLAIN` statement outputs execution plans including the estimated cost, and the formula for calculating the costs, which can be useful when tuning some cost factors. |

3. Provide some reference materials (such as documents and websites) if you could.
   It was already mentioned in Add missing explain formats #15601, but was never done.