show table status
show table status 的功能,实际上查的是information_schema.tables这个表,对于myisam是实时正确的,对于innodb则大部分字段是估值。下面copy来自手册的解释:
1 2 | SHOW TABLE STATUS [{FROM | IN} db_name] [LIKE 'pattern' | WHERE expr] |
SHOW TABLE STATUS
works likes SHOW TABLES
, but provides a lot of information about each non-TEMPORARY
table. You can also get this list using the mysqlshow –status db_name
command. The LIKE
clause, if present, indicates which table names to match. The WHERE
clause can be given to select rows using more general conditions, as discussed in Section 22.33, “Extensions to SHOW Statements”.
This statement also displays information about views.
SHOW TABLE STATUS
output has the following columns:
Name
The name of the table.Engine
The storage engine for the table. See Chapter 16, Alternative Storage Engines.Version
The version number of the table’s.frm
file.Row_format
The row-storage format (Fixed
,Dynamic
,Compressed
,Redundant
,Compact
). ForMyISAM
tables, (Dynamic
corresponds to what myisamchk -dvv reports asPacked
. The format ofInnoDB
tables is reported asRedundant
orCompact
. For theBarracuda
file format of theInnoDB Plugin
, the format may beCompressed
orDynamic
.Rows
The number of rows. Some storage engines, such asMyISAM
, store the exact count. For other storage engines, such asInnoDB
, this value is an approximation, and may vary from the actual value by as much as 40 to 50%. In such cases, useSELECT COUNT(*)
to obtain an accurate count.TheRows
value isNULL
for tables in theINFORMATION_SCHEMA
database.Avg_row_length
The average row length.Data_length
ForMyISAM
,Data_length
is the length of the data file, in bytes.ForInnoDB
,Data_length
is the approximate amount of memory allocated for the clustered index, in bytes. Specifically, it is the clustered index size, in pages, multiplied by theInnoDB
page size.Refer to the notes at the end of this section for information regarding other storage engines.
Max_data_length
ForMyISAM
,Max_data_length
is maximum length of the data file. This is the total number of bytes of data that can be stored in the table, given the data pointer size used.Unused forInnoDB
.Refer to the notes at the end of this section for information regarding other storage engines.
Index_length
ForMyISAM
,Index_length
is the length of the index file, in bytes.ForInnoDB
,Index_length
is the approximate amount of memory allocated for non-clustered indexes, in bytes. Specifically, it is the sum of non-clustered index sizes, in pages, multiplied by theInnoDB
page size.Refer to the notes at the end of this section for information regarding other storage engines.
Data_free
The number of allocated but unused bytes.This information is also shown forInnoDB
tables (previously, it was in theComment
value).InnoDB
tables report the free space of the tablespace to which the table belongs. For a table located in the shared tablespace, this is the free space of the shared tablespace. If you are using multiple tablespaces and the table has its own tablespace, the free space is for only that table. Free space means the number of bytes in completely free extents minus a safety margin. Even if free space displays as 0, it may be possible to insert rows as long as new extents need not be allocated.For partitioned tables, this value is only an estimate and may not be absolutely correct. A more accurate method of obtaining this information in such cases is to query the
INFORMATION_SCHEMA.PARTITIONS
table, as shown in this example:
1 2 3 4 | SELECT SUM(DATA_FREE) FROM INFORMATION_SCHEMA.PARTITIONS WHERE TABLE_SCHEMA = 'mydb' AND TABLE_NAME = 'mytable'; |
- For more information, see Section 22.15, “The INFORMATION_SCHEMA PARTITIONS Table”.
Auto_increment
The nextAUTO_INCREMENT
value.Create_time
When the table was created.Update_time
When the data file was last updated. For some storage engines, this value isNULL
. For example,InnoDB
stores multiple tables in its system tablespace and the data file timestamp does not apply. Even with file-per-table mode with eachInnoDB
table in a separate.ibd
file, change buffering can delay the write to the data file, so the file modification time is different from the time of the last insert, update, or delete. ForMyISAM
, the data file timestamp is used; however, on Windows the timestamp is not updated by updates so the value is inaccurate.Check_time
When the table was last checked. Not all storage engines update this time, in which case the value is alwaysNULL
.Collation
The table’s character set and collation.Checksum
The live checksum value (if any).Create_options
Extra options used withCREATE TABLE
. The original options supplied whenCREATE TABLE
is called are retained and the options reported here may differ from the active table settings and options.Comment
The comment used when creating the table (or information as to why MySQL could not access the table information).
Notes:
- For
MEMORY
tables, theData_length
,Max_data_length
, andIndex_length
values approximate the actual amount of allocated memory. The allocation algorithm reserves memory in large amounts to reduce the number of allocation operations. - For views, all the fields displayed by
SHOW TABLE STATUS
areNULL
except thatName
indicates the view name andComment
saysview
.