MySQL 5.7 Release Notes
Each stored trace is a string, which is extended (using
realloc()) as optimization progresses by
appending more data to it. The
optimizer_trace_max_mem_size
server system variable sets a limit on the total amount of memory
used by all traces currently being stored. If this limit is
reached, the current trace is not extended, which means the trace
is incomplete; in this case the
MISSING_BYTES_BEYOND_MAX_MEM_SIZE column shows
the number of bytes missing from the trace.