当我查询模式时,MySQL给出了波动的行数？

Question

在这里,我在我的笔记本电脑上的dev DB上按下并运行相同的命令,一遍又一遍;

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,我正在做同样的事情,按'向上'并再次运行最后一个命令,但输出是chaning.这里发生了什么？没有什么可以使用这个数据库,因为它是我本地笔记本电脑上的副本,用于我自己的修修补补.为什么表格的行数会发生变化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中看到了这种行为,所以我想在CLI上查看自己,正如您所看到的,它确实在变化!

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

Answer 1

假设您正在使用InnoDB,因为根据MySQL INFORMATION_SCHEMA TABLES文档,这是5.5.x中的默认值.

而且这个说明:

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

对于InnoDB表,行计数只是SQL优化中使用的粗略估计.(如果对InnoDB表进行了分区,也是如此.)

Answer 2

这不是一个错误.table_rows列中报告的值information_schema.tables不保证是准确的,我们也不期望它.

Answer 3

如果您使用InnoDB作为存储引擎,则行数是估计值.

For InnoDB tables, the row count is only a rough estimate used in SQL optimization. 

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