MySQL 5.7 Release Notes
When reading a very large JSON document, it can be difficult to
pair its closing bracket and opening brackets; setting
end_markers_in_json=ON repeats the
structure's key, if it has one, near the closing bracket.
This variable affects both optimizer traces and the output of
EXPLAIN FORMAT=JSON.
Note
If end_markers_in_json is enabled, the
repetition of the key means the result is not a valid JSON
document, and causes JSON parsers to throw an error.