每个存储的跟踪都是一个字符串，随着优化的进行，通过附加更多数据来扩展（使用 realloc()）。optimizer_trace_max_mem_size 服务器系统变量设置了当前存储的所有跟踪所使用的总内存的上限。如果达到此上限，则当前跟踪不会被扩展，这意味着跟踪不完整；在这种情况下，MISSING_BYTES_BEYOND_MAX_MEM_SIZE 列显示跟踪中缺失的字节数。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.