对比MySQL5.7版本、Oracle 11g版本、TiDB3.0版本
其中MySQL是基于 InnoDB 引擎的。
MySQL中各种count的行为
参考自：丁奇老师的MySQL 45讲
这里，首先你要弄清楚 count() 的语义。count() 是一个聚合函数，对于返回的结果集，一行行地判断，如果 count 函数的参数不是 NULL，累计值就加 1，否则不加。最后返回累计值。所以，count(*)、count(主键 id) 和 count(1) 都表示返回满足条件的结果集的总行数；而 count(字段），则表示返回满足条件的数据行里面，参数“字段”不为 NULL 的总个数。至于分析性能差别的时候，你可以记住这么几个原则：
- server 层要什么就给什么；
- InnoDB 只给必要的值；
- 现在的优化器只优化了 count(*) 的语义为“取行数”，其他“显而易见”的优化并没有做。
这是什么意思呢？接下来，我们就一个个地来看看。
对于 count(主键 id) 来说，InnoDB 引擎会遍历整张表，把每一行的 id 值都取出来，返回给 server 层。server 层拿到 id 后，判断是不可能为空的，就按行累加。
对于 count(1) 来说，InnoDB 引擎遍历整张表，但不取值。server 层对于返回的每一行，放一个数字“1”进去，判断是不可能为空的，按行累加。
单看这两个用法的差别的话，你能对比出来，count(1) 执行得要比 count(主键 id) 快。因为从引擎返回 id 会涉及到解析数据行，以及拷贝字段值的操作。
对于 count(字段) 来说：
- 如果这个“字段”是定义为 not null 的话，一行行地从记录里面读出这个字段，判断不能为 null，按行累加；
- 如果这个“字段”定义允许为 null，那么执行的时候，判断到有可能是 null，还要把值取出来再判断一下，不是 null 才累加。也就是前面的第一条原则，server 层要什么字段，InnoDB 就返回什么字段。
count(字段)和其它count()有明显的区别就是不会计数该字段中的空值。
对于count(*) 来说：
并不会把全部字段取出来，而是专门做了优化，不取值。count(*) 肯定不是 null，按行累加。看到这里，你一定会说，优化器就不能自己判断一下吗，主键 id 肯定非空啊，为什么不能按照 count(*) 来处理，多么简单的优化啊。当然，MySQL 专门针对这个语句进行优化，也不是不可以。但是这种需要专门优化的情况太多了，而且 MySQL 已经优化过 count(*) 了，你直接使用这种用法就可以了。
所以结论是：按照效率排序的话，count(字段)<count(主键id)<count(1) ≈count(*)，所以我建议你，尽量使用 count(*)。
MySQL中测试下效率
测试下前面丁奇老师的按照效率高低的结论
创建一张1000万数据的测试表，表结构如下：
CREATE TABLE `t` ( `a` int(11) NOT NULL, `name` varchar(32) DEFAULT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
表上没有主键的情况下
执行计划都一样的
explain select count(a) from t; +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ | 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 9750627 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ explain select count(1) from t; +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ | 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 9750627 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ explain select count(*) from t; +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+ | 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 9750627 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+
看看执行时间的差异：多执行几次取稳定的时间
+-------------+ | count(name) | +-------------+ | 10000000 | +-------------+ 1 row in set (3.92 sec) +----------+ | count(a) | +----------+ | 10000000 | +----------+ 1 row in set (3.84 sec) +----------+ | count(1) | +----------+ | 10000000 | +----------+ 1 row in set (3.63 sec) +----------+ | count(*) | +----------+ | 10000000 | +----------+ 1 row in set (3.63 sec)
结论：在表上没有主键和二级索引的时候 count(字段)<count(主键id)<count(1) ≈count(*)，符合前面说的理论规律
给表t添加主键并收集下统计信息再测试下：
alter table t add primary key (a); analyze table t;
先看下执行计划：除了count(字段)，其它三种执行计划都一样
explain select count(name) from t; +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------+ | 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 10042216 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------+ explain select count(a) from t; +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | PRIMARY | 4 | NULL | 10042216 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ explain select count(1) from t; +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | PRIMARY | 4 | NULL | 10042216 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ explain select count(*) from t; +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | PRIMARY | 4 | NULL | 10042216 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+---------+---------+------+----------+----------+-------------+
再看下执行时间：多执行几次，取稳定的时间
+-------------+ | count(name) | +-------------+ | 10000000 | +-------------+ 1 row in set (2.19 sec) +----------+ | count(a) | +----------+ | 10000000 | +----------+ 1 row in set (2.03 sec) +----------+ | count(1) | +----------+ | 10000000 | +----------+ 1 row in set (1.85 sec) +----------+ | count(*) | +----------+ | 10000000 | +----------+ 1 row in set (1.82 sec)
结论：在表上只有一个主键的时候 count(字段)<count(主键id)<count(1) ≈count(*)
给表添加测试列以及二级索引
alter table t add c1 int,add c2 int,add c3 varchar(4); alter table t add index idx_c1(c1),add index idx_name(name); analyze table t; show create table t; CREATE TABLE `t` ( `a` int(11) NOT NULL, `name` varchar(32) DEFAULT NULL, `c1` int(11) DEFAULT NULL, `c2` int(11) DEFAULT NULL, `c3` varchar(4) DEFAULT NULL, PRIMARY KEY (`a`), KEY `idx_c1` (`c1`), KEY `idx_name` (`name`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 | show index from t; +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | t | 0 | PRIMARY | 1 | a | A | 10042216 | NULL | NULL | | BTREE | | | | t | 1 | idx_c1 | 1 | c1 | A | 1 | NULL | NULL | YES | BTREE | | | | t | 1 | idx_name | 1 | name | A | 9987441 | NULL | NULL | YES | BTREE | | | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
看下各语句的执行计划：
explain select count(name) from t; +----+-------------+-------+------------+-------+---------------+----------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+----------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx_name | 131 | NULL | 10042216 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+----------+---------+------+----------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) explain select count(a) from t; +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx_c1 | 5 | NULL | 10042216 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) explain select count(1) from t; +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx_c1 | 5 | NULL | 10042216 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) explain select count(*) from t; +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+ | 1 | SIMPLE | t | NULL | index | NULL | idx_c1 | 5 | NULL | 10042216 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+--------+---------+------+----------+----------+-------------+
从执行计划中可以看出，select count(name) from t 因为逻辑语义的不同，所以会走自己字段上的索引。其它三种被优化为走idx_c1索引。不走name上的索引是因为name列上索引比较大。
测试各自的执行时间：
+-------------+ | count(name) | +-------------+ | 10000000 | +-------------+ 1 row in set (2.12 sec) select count(a) from t; +----------+ | count(a) | +----------+ | 10000000 | +----------+ 1 row in set (1.87 sec) select count(1) from t; +----------+ | count(1) | +----------+ | 10000000 | +----------+ 1 row in set (1.70 sec) select count(*) from t; +----------+ | count(*) | +----------+ | 10000000 | +----------+ 1 row in set (1.72 sec) 补充测试：因为count(字段) name列上有索引，count(*) 被优化为走idx_c1索引，不在同一个对比维度，可以再测试下count(c1)，也是同样的执行计划 +-----------+ | count(c1) | +-----------+ | 0 | +-----------+ 1 row in set (1.92 sec)
总结：还是符合规律count(字段)<count(主键id)<count(1) ≈count(*)
测试下Oracle中的情况
同样是1000万级别的数据：
CREATE TABLE "SCOTT"."T"
( "A" NUMBER(*,0) NOT NULL ENABLE,
"NAME" VARCHAR2(32),
"C1" NUMBER(*,0),
"C2" NUMBER(*,0),
"C3" VARCHAR2(40),
PRIMARY KEY ("A")
USING INDEX ENABLE) ;
CREATE INDEX "SCOTT"."IDX_C1" ON "SCOTT"."T" ("C1");
CREATE INDEX "SCOTT"."IDX_NAME" ON "SCOTT"."T" ("NAME");
同样对表收集统计信息：
begin dbms_stats.gather_table_stats ( OWNNAME => 'SCOTT', TABNAME => 'T', ESTIMATE_PERCENT =>dbms_stats.AUTO_SAMPLE_SIZE, METHOD_OPT=>'FOR ALL INDEXED COLUMNS', degree=>4, cascade=> TRUE); end;
查看执行计划：
set autotrace traceonly exp; select count(name) from t; Execution Plan ---------------------------------------------------------- Plan hash value: 4041003673 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 12 | 8589 (1)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 12 | | | | 2 | TABLE ACCESS FULL| T | 10M| 114M| 8589 (1)| 00:00:01 | --------------------------------------------------------------------------- select count(a) from t; Execution Plan ---------------------------------------------------------- Plan hash value: 4041003673 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 5310 (1)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| SYS_C0014859 | 10M| 5310 (1)| 00:00:01 | ------------------------------------------------------------------------------ select count(1) from t; Execution Plan ---------------------------------------------------------- Plan hash value: 4041003673 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 5310 (1)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| SYS_C0014859 | 10M| 5310 (1)| 00:00:01 | ------------------------------------------------------------------------------ select count(*) from t; Execution Plan ---------------------------------------------------------- Plan hash value: 4041003673 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 5310 (1)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| SYS_C0014859 | 10M| 5310 (1)| 00:00:01 | ------------------------------------------------------------------------------ 补充： select count(c1) from t; Execution Plan ---------------------------------------------------------- Plan hash value: 1226206340 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 13 | 0 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 13 | | | | 2 | INDEX FULL SCAN| IDX_C1 | 10M| 123M| 0 (0)| 00:00:01 | ---------------------------------------------------------------------------
执行计划说明：1、oracle会对所有count()情况做出优化
2、Oracle中的索引是不会存储空值的，mysql中会把null值做个标志位。所以count(*)的时候会优化为扫描主键上索引
3、select count(name) from t; 优化器优化为全表扫描并不奇怪，因为name列上的索引比较大，并且还存储rowid的值。全表扫描可以利用多块读、直接路径读等特性提高效率。但不能走主键快速扫，因为我统计的是name列上的非空值个数。
执行时间上的对比：多执行几次，把数据块缓存起来
select count(name) from t; Elapsed: 00:00:00.49 select count(a) from t; Elapsed: 00:00:00.34 select count(1) from t; Elapsed: 00:00:00.29 select count(*) from t; Elapsed: 00:00:00.28 --补充 select count(c1) from t; Elapsed: 00:00:00.00
总结：Oracle中虽然对count()行为有不同的优化措施，但是也符合规律count(主键id)<count(1) ≈count(*)
count(字段)的效率这个要取决于该段上面索引的大小，并且和其它三种count逻辑语义上不等价，不能放一起对比。
测试TiDB中的情况
同样是1000万级别的数据：
show create table t; +-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | t | CREATE TABLE `t` ( `a` int(11) NOT NULL, `name` varchar(32) DEFAULT NULL, `c1` int(11) DEFAULT NULL, `c2` int(11) DEFAULT NULL, `c3` varchar(40) DEFAULT NULL, PRIMARY KEY (`a`), KEY `idx_name` (`name`), KEY `idx_c1` (`c1`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin analyze table t; show index from t; +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | t | 0 | PRIMARY | 1 | a | A | 0 | NULL | NULL | | BTREE | | | | t | 1 | idx_name | 1 | name | A | 0 | NULL | NULL | YES | BTREE | | | | t | 1 | idx_c1 | 1 | c1 | A | 0 | NULL | NULL | YES | BTREE | | | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
执行计划：
explain analyze select count(name) from t; +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | id | count | task | operator info | execution info | memory | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | StreamAgg_16 | 1.00 | root | funcs:count(col_0) | time:575.797216ms, loops:2, rows:1 | N/A | | └─IndexReader_17 | 1.00 | root | index:StreamAgg_8 | time:575.789969ms, loops:2, rows:12 | 257 Bytes | | └─StreamAgg_8 | 1.00 | cop | funcs:count(mtest.t.name) | proc max:575ms, min:47ms, p80:548ms, p95:575ms, rows:12, iters:9771, tasks:12 | N/A | | └─IndexScan_15 | 10000000.00 | cop | table:t, index:name, range:[NULL,+inf], keep order:false | proc max:498ms, min:42ms, p80:473ms, p95:498ms, rows:10000000, iters:9771, tasks:12 | N/A | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ 4 rows in set (0.58 sec) 18:50:40[5.7.25-TiDB-v3.0.16]root->127.0.0.1[mtest]> explain analyze select count(a) from t; +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | id | count | task | operator info | execution info | memory | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | StreamAgg_20 | 1.00 | root | funcs:count(col_0) | time:605.49247ms, loops:2, rows:1 | N/A | | └─IndexReader_21 | 1.00 | root | index:StreamAgg_8 | time:605.486666ms, loops:2, rows:12 | 257 Bytes | | └─StreamAgg_8 | 1.00 | cop | funcs:count(mtest.t.a) | proc max:604ms, min:41ms, p80:495ms, p95:604ms, rows:12, iters:9771, tasks:12 | N/A | | └─IndexScan_18 | 10000000.00 | cop | table:t, index:name, range:[NULL,+inf], keep order:false | proc max:600ms, min:41ms, p80:492ms, p95:600ms, rows:10000000, iters:9771, tasks:12 | N/A | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ 4 rows in set (0.60 sec) 18:50:41[5.7.25-TiDB-v3.0.16]root->127.0.0.1[mtest]> explain analyze select count(1) from t; +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | id | count | task | operator info | execution info | memory | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | StreamAgg_20 | 1.00 | root | funcs:count(col_0) | time:676.323439ms, loops:2, rows:1 | N/A | | └─IndexReader_21 | 1.00 | root | index:StreamAgg_8 | time:676.318048ms, loops:2, rows:12 | 259 Bytes | | └─StreamAgg_8 | 1.00 | cop | funcs:count(1) | proc max:674ms, min:41ms, p80:575ms, p95:674ms, rows:12, iters:9771, tasks:12 | N/A | | └─IndexScan_18 | 10000000.00 | cop | table:t, index:name, range:[NULL,+inf], keep order:false | proc max:674ms, min:41ms, p80:575ms, p95:674ms, rows:10000000, iters:9771, tasks:12 | N/A | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ 4 rows in set (0.68 sec) 18:50:42[5.7.25-TiDB-v3.0.16]root->127.0.0.1[mtest]> explain analyze select count(*) from t; +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | id | count | task | operator info | execution info | memory | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ | StreamAgg_20 | 1.00 | root | funcs:count(col_0) | time:587.30061ms, loops:2, rows:1 | N/A | | └─IndexReader_21 | 1.00 | root | index:StreamAgg_8 | time:587.293109ms, loops:2, rows:12 | 257 Bytes | | └─StreamAgg_8 | 1.00 | cop | funcs:count(1) | proc max:587ms, min:55ms, p80:527ms, p95:587ms, rows:12, iters:9771, tasks:12 | N/A | | └─IndexScan_18 | 10000000.00 | cop | table:t, index:name, range:[NULL,+inf], keep order:false | proc max:586ms, min:55ms, p80:527ms, p95:586ms, rows:10000000, iters:9771, tasks:12 | N/A | +------------------------+-------------+------+----------------------------------------------------------+-------------------------------------------------------------------------------------+-----------+ 4 rows in set (0.59 sec)
其中count(*)和cunt(1)的执行计划一样，count(字段)稍微不一样
看看执行时间
18:53:25[5.7.25-TiDB-v3.0.16]root->127.0.0.1[mtest]> select count(name) from t; +-------------+ | count(name) | +-------------+ | 10000000 | +-------------+ 1 row in set (0.57 sec) 18:53:26[5.7.25-TiDB-v3.0.16]root->127.0.0.1[mtest]> select count(a) from t; +----------+ | count(a) | +----------+ | 10000000 | +----------+ 1 row in set (0.52 sec) 18:53:27[5.7.25-TiDB-v3.0.16]root->127.0.0.1[mtest]> select count(1) from t; +----------+ | count(1) | +----------+ | 10000000 | +----------+ 1 row in set (0.53 sec) 18:53:28[5.7.25-TiDB-v3.0.16]root->127.0.0.1[mtest]> select count(*) from t; +----------+ | count(*) | +----------+ | 10000000 | +----------+ 1 row in set (0.56 sec)
多次执行后发现tidb中执行时间并不稳定，因为分布式数据库中 tidb server和tikv server之间还有网络的消耗。
总结：tidb中几种count执行时间上没有多少差别，也没有找到官方关于count(*)的优化说明。tidb中也是建议使用count(*)来统计数据行数。
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