MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6
The optimizer_trace system variable has these on/off switches:
enabled: Enables (ON) or
disables (OFF) tracing
one_line: If set to ON,
the trace contains no whitespace, thus conserving space. This
renders the trace difficult to read for humans, still usable
by JSON parsers, since they ignore whitespace.