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SLOW_QUERY 表中提供了当前节点的慢查询相关的信息，其内容通过解析当前节点的 TiDB 慢查询日志而来，列名和慢日志中的字段名是一一对应。关于如何使用该表调查和改善慢查询，请参考慢查询日志文档。
USE information_schema;DESC slow_query;
+---------------------------+---------------------+------+------+---------+-------+| Field | Type | Null | Key | Default | Extra |+---------------------------+---------------------+------+------+---------+-------+| Time | timestamp(6) | YES | | NULL | || Txn_start_ts | bigint(20) unsigned | YES | | NULL | || User | varchar(64) | YES | | NULL | || Host | varchar(64) | YES | | NULL | || Conn_ID | bigint(20) unsigned | YES | | NULL | || Query_time | double | YES | | NULL | || Parse_time | double | YES | | NULL | || Compile_time | double | YES | | NULL | || Rewrite_time | double | YES | | NULL | || Preproc_subqueries | bigint(20) unsigned | YES | | NULL | || Preproc_subqueries_time | double | YES | | NULL | || Optimize_time | double | YES | | NULL | || Wait_TS | double | YES | | NULL | || Prewrite_time | double | YES | | NULL | || Wait_prewrite_binlog_time | double | YES | | NULL | || Commit_time | double | YES | | NULL | || Get_commit_ts_time | double | YES | | NULL | || Commit_backoff_time | double | YES | | NULL | || Backoff_types | varchar(64) | YES | | NULL | || Resolve_lock_time | double | YES | | NULL | || Local_latch_wait_time | double | YES | | NULL | || Write_keys | bigint(22) | YES | | NULL | || Write_size | bigint(22) | YES | | NULL | || Prewrite_region | bigint(22) | YES | | NULL | || Txn_retry | bigint(22) | YES | | NULL | || Cop_time | double | YES | | NULL | || Process_time | double | YES | | NULL | || Wait_time | double | YES | | NULL | || Backoff_time | double | YES | | NULL | || LockKeys_time | double | YES | | NULL | || Request_count | bigint(20) unsigned | YES | | NULL | || Total_keys | bigint(20) unsigned | YES | | NULL | || Process_keys | bigint(20) unsigned | YES | | NULL | || DB | varchar(64) | YES | | NULL | || Index_names | varchar(100) | YES | | NULL | || Is_internal | tinyint(1) | YES | | NULL | || Digest | varchar(64) | YES | | NULL | || Stats | varchar(512) | YES | | NULL | || Cop_proc_avg | double | YES | | NULL | || Cop_proc_p90 | double | YES | | NULL | || Cop_proc_max | double | YES | | NULL | || Cop_proc_addr | varchar(64) | YES | | NULL | || Cop_wait_avg | double | YES | | NULL | || Cop_wait_p90 | double | YES | | NULL | || Cop_wait_max | double | YES | | NULL | || Cop_wait_addr | varchar(64) | YES | | NULL | || Mem_max | bigint(20) | YES | | NULL | || Disk_max | bigint(20) | YES | | NULL | || Succ | tinyint(1) | YES | | NULL | || Plan_from_cache | tinyint(1) | YES | | NULL | || Plan | longblob | YES | | NULL | || Plan_digest | varchar(128) | YES | | NULL | || Prev_stmt | longblob | YES | | NULL | || Query | longblob | YES | | NULL | |+---------------------------+---------------------+------+------+---------+-------+54 rows in set (0.00 sec)
CLUSTER_SLOW_QUERY table
CLUSTER_SLOW_QUERY 表中提供了集群所有节点的慢查询相关的信息，其内容通过解析 TiDB 慢查询日志而来，该表使用上和 SLOW_QUERY 表一样。CLUSTER_SLOW_QUERY 表结构上比 SLOW_QUERY 多一列 INSTANCE，表示该行慢查询信息来自的 TiDB 节点地址。关于如何使用该表调查和改善慢查询，请参考慢查询日志文档。
desc cluster_slow_query;
+---------------------------+---------------------+------+------+---------+-------+| Field | Type | Null | Key | Default | Extra |+---------------------------+---------------------+------+------+---------+-------+| INSTANCE | varchar(64) | YES | | NULL | || Time | timestamp(6) | YES | | NULL | || Txn_start_ts | bigint(20) unsigned | YES | | NULL | || User | varchar(64) | YES | | NULL | || Host | varchar(64) | YES | | NULL | || Conn_ID | bigint(20) unsigned | YES | | NULL | || Query_time | double | YES | | NULL | || Parse_time | double | YES | | NULL | || Compile_time | double | YES | | NULL | || Rewrite_time | double | YES | | NULL | || Preproc_subqueries | bigint(20) unsigned | YES | | NULL | || Preproc_subqueries_time | double | YES | | NULL | || Optimize_time | double | YES | | NULL | || Wait_TS | double | YES | | NULL | || Prewrite_time | double | YES | | NULL | || Wait_prewrite_binlog_time | double | YES | | NULL | || Commit_time | double | YES | | NULL | || Get_commit_ts_time | double | YES | | NULL | || Commit_backoff_time | double | YES | | NULL | || Backoff_types | varchar(64) | YES | | NULL | || Resolve_lock_time | double | YES | | NULL | || Local_latch_wait_time | double | YES | | NULL | || Write_keys | bigint(22) | YES | | NULL | || Write_size | bigint(22) | YES | | NULL | || Prewrite_region | bigint(22) | YES | | NULL | || Txn_retry | bigint(22) | YES | | NULL | || Cop_time | double | YES | | NULL | || Process_time | double | YES | | NULL | || Wait_time | double | YES | | NULL | || Backoff_time | double | YES | | NULL | || LockKeys_time | double | YES | | NULL | || Request_count | bigint(20) unsigned | YES | | NULL | || Total_keys | bigint(20) unsigned | YES | | NULL | || Process_keys | bigint(20) unsigned | YES | | NULL | || DB | varchar(64) | YES | | NULL | || Index_names | varchar(100) | YES | | NULL | || Is_internal | tinyint(1) | YES | | NULL | || Digest | varchar(64) | YES | | NULL | || Stats | varchar(512) | YES | | NULL | || Cop_proc_avg | double | YES | | NULL | || Cop_proc_p90 | double | YES | | NULL | || Cop_proc_max | double | YES | | NULL | || Cop_proc_addr | varchar(64) | YES | | NULL | || Cop_wait_avg | double | YES | | NULL | || Cop_wait_p90 | double | YES | | NULL | || Cop_wait_max | double | YES | | NULL | || Cop_wait_addr | varchar(64) | YES | | NULL | || Mem_max | bigint(20) | YES | | NULL | || Disk_max | bigint(20) | YES | | NULL | || Succ | tinyint(1) | YES | | NULL | || Plan_from_cache | tinyint(1) | YES | | NULL | || Plan | longblob | YES | | NULL | || Plan_digest | varchar(128) | YES | | NULL | || Prev_stmt | longblob | YES | | NULL | || Query | longblob | YES | | NULL | |+---------------------------+---------------------+------+------+---------+-------+55 rows in set (0.00 sec)
查询集群系统表时，TiDB 也会将相关计算下推给其他节点执行，而不是把所有节点的数据都取回来，可以查看执行计划，如下：
desc SELECT count(*) FROM cluster_slow_query WHERE user = 'u1';
+--------------------------+----------+-----------+--------------------------+------------------------------------------------------+| id | estRows | task | access object | operator info |+--------------------------+----------+-----------+--------------------------+------------------------------------------------------+| StreamAgg_20 | 1.00 | root | | funcs:count(Column#53)->Column#51 || └─TableReader_21 | 1.00 | root | | data:StreamAgg_9 || └─StreamAgg_9 | 1.00 | cop[tidb] | | funcs:count(1)->Column#53 || └─Selection_19 | 10.00 | cop[tidb] | | eq(information_schema.cluster_slow_query.user, "u1") || └─TableFullScan_18 | 10000.00 | cop[tidb] | table:CLUSTER_SLOW_QUERY | keep order:false, stats:pseudo |+--------------------------+----------+-----------+--------------------------+------------------------------------------------------+
上面执行计划表示，会将 user = u1 条件下推给其他的 (cop) TiDB 节点执行，也会把聚合算子（即图中的 StreamAgg 算子）下推。
目前由于没有对系统表收集统计信息，所以有时会导致某些聚合算子不能下推，导致执行较慢，用户可以通过手动指定聚合下推的 SQL HINT 来将聚合算子下推，示例如下：
SELECT /*+ AGG_TO_COP() */ count(*) FROM cluster_slow_query GROUP BY user;
