13.3 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.

isamchk -d tbl_name
Runs isamchk in ``describe mode'' to produce a description of your table. If you start the MySQL server using the --skip-locking option, isamchk may report an error for a table that is updated while it runs. However, since isamchk doesn't change the table in describe mode, there isn't any risk of destroying data.
isamchk -d -v tbl_name
To produce more information about what isamchk is doing, add -v to tell it to run in verbose mode.
isamchk -eis tbl_name
Shows only the most important information from a table. It is slow since it must read the whole table.
isamchk -eiv tbl_name
This is like -eis, but tells you what is being done.

Example of isamchk -d output: 

ISAM file:     company.ISM
Data records:           1403698  Deleted blocks:         0
Recordlength:               226
Record format: Fixed length

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 isamchk -d -v output: 

ISAM file:     company.ISM
Isam-version:  2
Creation time: 1996-08-28 11:44:22
Recover time:  1997-01-12 18:35:29
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
Record format: Fixed length

table description:
Key Start Len Index   Type                      Root Blocksize Rec/key
1   2     8   unique  double                15845376      1024       1
2   15    10  multip. text packed stripped  25062400      1024       2
3   219   8   multip. double                40907776      1024      73
4   63    10  multip. text packed stripped  48097280      1024       5
5   167   2   multip. unsigned short        55200768      1024    4840
6   177   4   multip. unsigned long         65145856      1024    1346
7   155   4   multip. text                  75090944      1024    4995
8   138   4   multip. unsigned long         85036032      1024      87
9   177   4   multip. unsigned long         96481280      1024     178
    193   1           text

Example of isamchk -eis output: 

Checking ISAM file: company.ISM
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
Recordblocks:     1403698    Deleteblocks:         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 isamchk -eiv output: 

Checking ISAM file: company.ISM
Data records: 1403698   Deleted blocks:       0
- check file-size
- check delete-chain
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
Recordblocks:     1403698    Deleteblocks:         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.ISD
-rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.ISM

Explanations for the types of information isamchk 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. 13.4.3 Table 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. 13.4.3 Table 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 (.ISD file) can become, in bytes.
Max keyfile length
How long the table's key file (.ISM 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 NISAM 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 isamchk -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. Since the table used in the examples had just been reorganized with isamchk, 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 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 (i.e., 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 isamchk. 13.4.3 Table 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 pack_isam, isamchk -d prints additional information about each table column. See pack_isam, for an example of this information and a description of what it means.