一尘不染

查询架构时,MySQL的行数会波动吗?

mysql

在这里,我一遍又一遍地在笔记本电脑上的dev DB上运行相同的命令;

mysql> select count(*) from tblTraceOutput;
+----------+
| count(*) |
+----------+
|   300175 |
+----------+
1 row in set (0.42 sec)

mysql> select count(*) from tblTraceOutput;
+----------+
| count(*) |
+----------+
|   300175 |
+----------+
1 row in set (0.35 sec)

mysql> select count(*) from tblTraceOutput;
+----------+
| count(*) |
+----------+
|   300175 |
+----------+
1 row in set (0.45 sec)

在这里,我做同样的事情,按下“
up”并再次运行最后一个命令,但是输出改变了。这里发生了什么?没有使用这个数据库,因为它是我本地笔记本电脑上的副本,可以自己修改。为什么表的表行计数发生变化tblTraceOutput

mysql> SELECT table_name, table_rows FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'smoketrace';
+----------------+------------+
| table_name     | table_rows |
+----------------+------------+
| tblCategories  |          9 |
| tblResults     |      32463 |
| tblRoutes      |        300 |
| tblSettings    |          2 |
| tblTraceOutput |     303463 |
| tblTraces      |         12 |
+----------------+------------+
6 rows in set (0.01 sec)

mysql> SELECT table_name, table_rows FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'smoketrace';
+----------------+------------+
| table_name     | table_rows |
+----------------+------------+
| tblCategories  |          9 |
| tblResults     |      32948 |
| tblRoutes      |        246 |
| tblSettings    |          2 |
| tblTraceOutput |     297319 |
| tblTraces      |         12 |
+----------------+------------+
6 rows in set (0.00 sec)

mysql> SELECT table_name, table_rows FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'smoketrace';
+----------------+------------+
| table_name     | table_rows |
+----------------+------------+
| tblCategories  |          9 |
| tblResults     |      32948 |
| tblRoutes      |        451 |
| tblSettings    |          2 |
| tblTraceOutput |     302127 |
| tblTraces      |         12 |
+----------------+------------+
6 rows in set (0.02 sec)

刷新页面时,我在phpMyAdmin中看到了此行为,因此我想在CLI上进行自我检查,正如您所看到的,它确实正在改变!

mysql --version
./bin/mysql  Ver 14.14 Distrib 5.5.8, for Linux (i686) using  EditLine wrapper
free -m
             total       used       free     shared    buffers     cached
Mem:          1880       1830         49          0         51        600
-/+ buffers/cache:       1179        701
Swap:         1027          0       1026
uname -a
Linux laptop 3.4.11 #1 SMP Sun Sep 23 15:03:21 BST 2012 i686 i686 i386 GNU/Linux

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2020-05-17

共1个答案

一尘不染

假设您使用的是InnoDB,根据MySQL INFORMATION_SCHEMA
TABLES文档
,这是5.5.x中的默认设置。

并注意:

如果表位于INFORMATION_SCHEMA数据库中,则TABLE_ROWS列为NULL。

对于InnoDB表,行数只是SQL优化中使用的粗略估计。(如果InnoDB表已分区,则也是如此。)

2020-05-17