-- 临时关闭mysql查询缓存,为了查看sql多次执行的真实时间 set global query_cache_size=0; set global query_cache_type=0; EXPLAIN select count(1) from employees; EXPLAIN select count(id) from employees; EXPLAIN select count(name) from employees; EXPLAIN select count(*) from employees; 1. 2. 3...
querycachesize:缓存 MySQL 中的 ResultSet,也就是一条 SQL 语句执行的结果集,所以仅仅只能针对 select 语句。当某个表的数据有任何任何变化,都会导致所有引用了该表的select 语句在 Query Cache 中的缓存数据失效。所以,当我们的数据变化非常频繁的情况下,使用 Query Cache 可能会得不偿失。根据命中率(Qcache_hi...
The index will be decided in find_shortest_key(), called from optimize_aggregated_query(). */ bool JOIN::select_count{false}; (2)SQL引擎层在生成执行计划阶段,判断变量“JOIN::select_count”的值,如果变量值为TRUE,则生成并行COUNT算子“UnqualifiedCountIterator”,用户可以通过“EXPLAIN FORMAT=TREE”...
【8】But it works well and very fast in slave server, all are almost 2+ seconds. Please help, how to tuning the performance? Subject Views Written By Posted MySQL 5.7 simple count query sometimes fast and sometimes very slow 1369
1 原因 mysql的sql_mode默认开启了only_full_group_by模式 2 解决办法 2.1 命令解决(临时生效) 查看sql_mode show variableslike'%sql_mode'; show session variableslike'%sql_mode'; show global variableslike'%sql_mode'; 修改sql_mode setglobal sql_mode='STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION';se...
SELECT COUNT(*)从 MySQL 5.7.18 开始,通过遍历最小的可用二级索引来InnoDB处理SELECT COUNT(*)语句,除非索引或优化器提示指示优化器使用不同的索引。如果二级索引不存在,则扫描聚集索引。大概意思就是有二级索引的情况下就使用二级索引,如果有多个二级索引优先选择最小的那个二级索引来降低成本,没有二级索引使用聚集...
As of MySQL 8.0.13,SELECT COUNT(*)FROM tbl_name query performance for InnoDB tables is optimized for single-threaded workloads if there are no extra clauses such as WHERE or GROUP BY. InnoDB processesSELECT COUNT(*)statements by traversing the smallest available secondary index unless an index...
mysql>callsp_name();QueryOK,0rows affected mysql>selectcount(*)from t;+---+|count(*)|+---+|10000000|+---+1rowinset 验证执行效果 cout(*)和count(1) 没区别,但是cout(列名) 不统计为 null 的 代码语言:javascript 代码运行次数:0 运行 AI代码解释 ...
首先 count函数是sql(Structured Query Language)中的一个聚合函数，用来统计满足“条件”数据的行数。
mysql> create index idx_rowguid on api_runtime_log(rowguid);Query OK, 0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> select count(*) from api_runtime_log;+---+| count(*) |+---+| 5718952 |+---+1 row in set (0.89 sec)mysql> explain select count...