大结果集查询 fetch and wait 耗时高，如何优化数据回传效率

fetch and wait 高 = TiDB 结果集回传流速跟不上 JDBC 拉取速度，只需调大 chunk 批次、流式并发、流水线队列、关闭服务端流控 四个方向，就能大幅降低结果集等待耗时。可以试试：
SET GLOBAL tidb_max_chunk_size = 512;
SET GLOBAL tidb_init_chunk_size = 128;
SET GLOBAL tidb_distsql_scan_concurrency = 32;
SET GLOBAL tidb_executor_concurrency = 8;
SET GLOBAL pipeline_queue_size = 512;
SET GLOBAL tidb_flow_control_enable = OFF;