GNU Info
(mysql.info)Table-info
Getting Information About a Table
---------------------------------
To get a description of a table or statistics about it, use the
commands shown below. We explain some of the information in more detail
later:
`myisamchk -d tbl_name'
Runs `myisamchk' in "describe mode" to produce a description of
your table. If you start the MySQL server using the
`--skip-locking' option, `myisamchk' may report an error for a
table that is updated while it runs. However, because `myisamchk'
doesn't change the table in describe mode, there isn't any risk of
destroying data.
`myisamchk -d -v tbl_name'
To produce more information about what `myisamchk' is doing, add
`-v' to tell it to run in verbose mode.
`myisamchk -eis tbl_name'
Shows only the most important information from a table. It is slow
because it must read the whole table.
`myisamchk -eiv tbl_name'
This is like `-eis', but tells you what is being done.
Example of `myisamchk -d' output:
MyISAM file: company.MYI
Record format: Fixed length
Data records: 1403698 Deleted blocks: 0
Recordlength: 226
table description:
Key Start Len Index Type
1 2 8 unique double
2 15 10 multip. text packed stripped
3 219 8 multip. double
4 63 10 multip. text packed stripped
5 167 2 multip. unsigned short
6 177 4 multip. unsigned long
7 155 4 multip. text
8 138 4 multip. unsigned long
9 177 4 multip. unsigned long
193 1 text
Example of `myisamchk -d -v' output:
MyISAM file: company
Record format: Fixed length
File-version: 1
Creation time: 1999-10-30 12:12:51
Recover time: 1999-10-31 19:13:01
Status: checked
Data records: 1403698 Deleted blocks: 0
Datafile parts: 1403698 Deleted data: 0
Datafilepointer (bytes): 3 Keyfile pointer (bytes): 3
Max datafile length: 3791650815 Max keyfile length: 4294967294
Recordlength: 226
table description:
Key Start Len Index Type Rec/key Root Blocksize
1 2 8 unique double 1 15845376 1024
2 15 10 multip. text packed stripped 2 25062400 1024
3 219 8 multip. double 73 40907776 1024
4 63 10 multip. text packed stripped 5 48097280 1024
5 167 2 multip. unsigned short 4840 55200768 1024
6 177 4 multip. unsigned long 1346 65145856 1024
7 155 4 multip. text 4995 75090944 1024
8 138 4 multip. unsigned long 87 85036032 1024
9 177 4 multip. unsigned long 178 96481280 1024
193 1 text
Example of `myisamchk -eis' output:
Checking MyISAM file: company
Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4
Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4
Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4
Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3
Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3
Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3
Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3
Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3
Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4
Total: Keyblocks used: 98% Packed: 17%
Records: 1403698 M.recordlength: 226 Packed: 0%
Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00
Record blocks: 1403698 Delete blocks: 0
Recorddata: 317235748 Deleted data: 0
Lost space: 0 Linkdata: 0
User time 1626.51, System time 232.36
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 639, Involuntary context switches 28966
Example of `myisamchk -eiv' output:
Checking MyISAM file: company
Data records: 1403698 Deleted blocks: 0
- check file-size
- check delete-chain
block_size 1024:
index 1:
index 2:
index 3:
index 4:
index 5:
index 6:
index 7:
index 8:
index 9:
No recordlinks
- check index reference
- check data record references index: 1
Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4
- check data record references index: 2
Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4
- check data record references index: 3
Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4
- check data record references index: 4
Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3
- check data record references index: 5
Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3
- check data record references index: 6
Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3
- check data record references index: 7
Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3
- check data record references index: 8
Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3
- check data record references index: 9
Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4
Total: Keyblocks used: 9% Packed: 17%
- check records and index references
[LOTS OF ROW NUMBERS DELETED]
Records: 1403698 M.recordlength: 226 Packed: 0%
Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00
Record blocks: 1403698 Delete blocks: 0
Recorddata: 317235748 Deleted data: 0
Lost space: 0 Linkdata: 0
User time 1639.63, System time 251.61
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
Blocks in 4 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 10604, Involuntary context switches 122798
Here are the sizes of the data and index files for the table used in the
preceding examples:
-rw-rw-r-- 1 monty tcx 317235748 Jan 12 17:30 company.MYD
-rw-rw-r-- 1 davida tcx 96482304 Jan 12 18:35 company.MYM
Explanations for the types of information `myisamchk' produces are
given below. The "keyfile" is the index file. "Record" and "row" are
synonymous:
`ISAM file'
Name of the ISAM (index) file.
`Isam-version'
Version of ISAM format. Currently always 2.
`Creation time'
When the data file was created.
`Recover time'
When the index/data file was last reconstructed.
`Data records'
How many records are in the table.
`Deleted blocks'
How many deleted blocks still have reserved space. You can
optimize your table to minimize this space. Note: Optimization.
`Datafile: Parts'
For dynamic record format, this indicates how many data blocks
there are. For an optimized table without fragmented records, this
is the same as `Data records'.
`Deleted data'
How many bytes of non-reclaimed deleted data there are. You can
optimize your table to minimize this space. Note: Optimization.
`Datafile pointer'
The size of the data file pointer, in bytes. It is usually 2, 3,
4, or 5 bytes. Most tables manage with 2 bytes, but this cannot be
controlled from MySQL yet. For fixed tables, this is a record
address. For dynamic tables, this is a byte address.
`Keyfile pointer'
The size of the index file pointer, in bytes. It is usually 1, 2,
or 3 bytes. Most tables manage with 2 bytes, but this is calculated
automatically by MySQL. It is always a block address.
`Max datafile length'
How long the table's data file (`.MYD' file) can become, in bytes.
`Max keyfile length'
How long the table's key file (`.MYI' file) can become, in bytes.
`Recordlength'
How much space each record takes, in bytes.
`Record format'
The format used to store table rows. The examples shown above use
`Fixed length'. Other possible values are `Compressed' and
`Packed'.
`table description'
A list of all keys in the table. For each key, some low-level
information is presented:
`Key'
This key's number.
`Start'
Where in the record this index part starts.
`Len'
How long this index part is. For packed numbers, this should
always be the full length of the column. For strings, it may
be shorter than the full length of the indexed column,
because you can index a prefix of a string column.
`Index'
`unique' or `multip.' (multiple). Indicates whether or not
one value can exist multiple times in this index.
`Type'
What data-type this index part has. This is an ISAM data-type
with the options `packed', `stripped' or `empty'.
`Root'
Address of the root index block.
`Blocksize'
The size of each index block. By default this is 1024, but
the value may be changed at compile time.
`Rec/key'
This is a statistical value used by the optimizer. It tells
how many records there are per value for this key. A unique
key always has a value of 1. This may be updated after a
table is loaded (or greatly changed) with `myisamchk -a'. If
this is not updated at all, a default value of 30 is given.
`'
In the first example above, the 9th key is a multi-part key with
two parts.
`Keyblocks used'
What percentage of the keyblocks are used. Because the table used
in the examples had just been reorganized with `myisamchk', the
values are very high (very near the theoretical maximum).
`Packed'
MySQL tries to pack keys with a common suffix. This can only be
used for `CHAR'/`VARCHAR'/`DECIMAL' keys. For long strings like
names, this can significantly reduce the space used. In the third
example above, the 4th key is 10 characters long and a 60%
reduction in space is achieved.
`Max levels'
How deep the B-tree for this key is. Large tables with long keys
get high values.
`Records'
How many rows are in the table.
`M.recordlength'
The average record length. For tables with fixed-length records,
this is the exact record length.
`Packed'
MySQL strips spaces from the end of strings. The `Packed' value
indicates the percentage of savings achieved by doing this.
`Recordspace used'
What percentage of the data file is used.
`Empty space'
What percentage of the data file is unused.
`Blocks/Record'
Average number of blocks per record (that is, how many links a
fragmented record is composed of). This is always 1 for
fixed-format tables. This value should stay as close to 1.0 as
possible. If it gets too big, you can reorganize the table with
`myisamchk'. Note: Optimization.
`Recordblocks'
How many blocks (links) are used. For fixed format, this is the
same as the number of records.
`Deleteblocks'
How many blocks (links) are deleted.
`Recorddata'
How many bytes in the data file are used.
`Deleted data'
How many bytes in the data file are deleted (unused).
`Lost space'
If a record is updated to a shorter length, some space is lost.
This is the sum of all such losses, in bytes.
`Linkdata'
When the dynamic table format is used, record fragments are linked
with pointers (4 to 7 bytes each). `Linkdata' is the sum of the
amount of storage used by all such pointers.
If a table has been compressed with `myisampack', `myisamchk -d' prints
additional information about each table column. See Note:
`myisampack', for an example of this information and a
description of what it means.
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