MySQL Internals Manual  /  Tracing the Optimizer  /  The end_markers_in_json System Variable

8.11 The end_markers_in_json System Variable

If a JSON structure is large, it's difficult to pair its closing bracket with its opening bracket; to help the reader (but this is not JSON-compliant), setting @@end_markers_in_json=on repeats the structure's key (if it has one) near the closing bracket. It also affects 'EXPLAIN FORMAT=JSON' in the same way. Note that before MySQL 5.6.6, this variable didn't exist and was rather a switch in the @@optimizer_trace variable (it was set with 'set optimizer_trace="end_marker=on";').