Disaster Prevention and Recovery with the MySQL Database - 4.6.4 Getting Information About a Table
(Page 7 of 7 )
To obtain a description of a table or statistics about it, use the commands shown here. 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-external-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 is no risk of destroying data.
myisamchk -d -v tbl_name
Adding -v runs myisamchk in verbose mode so that it produces more information about what it is doing.
myisamchk -eis tbl_name
Shows only the most important information from a table. This operation is slow because it must read the entire table.
myisamchk -eiv tbl_name
This is like -eis, but tells you what is being done.
Sample output for some of these commands follows. They are based on a table with these data and index file sizes:
-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
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
Datafile pointer (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
Explanations for the types of information myisamchk produces are given here. "Keyfile" refers to the index file. "Record" and "row" are synonymous.
MyISAM file
Name of the MyISAM (index) file.
File-version
Version of MyISAM 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. See Section 4.6.2.10, "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 unreclaimed deleted data there are. You can optimize your table to minimize this space. See Section 4.6.2.10, "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 data file can become, in bytes.
Max keyfile length
How long the table index file can become, in bytes.
Recordlength
How much space each record takes, in bytes.
Record format
The format used to store table rows. The preceding examples use Fixed length. Other possible values are Compressed and Packed.
table description
A list of all keys in the table. For each key, myisamchk displays some low-level information:
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
Whether a key value can exist multiple times in the index. Values are unique or multip. (multiple).
Type
What data type this index part has. This is a MyISAM 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 when MySQL is built from source.
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.
For the table shown in the examples, there are two table description lines for the ninth index. This indicates that it is a multiple-part index with two parts.
Keyblocks used
What percentage of the keyblocks are used. When a table has just been reorganized with myisamchk, as for the table in the examples, 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 indexes on CHAR, VARCHAR, or DECIMAL columns. For long indexed strings that have similar leftmost parts, this can significantly reduce the space used. In the third example above, the fourth 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 key values get high values.
Records
How many rows are in the table.
M.recordlength
The average record length. This is the exact record length for tables with fixed-length records, because all records have the same 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 1.0 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.
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 Section 7.2, "myisampack, the MySQL Compressed Read-Only Table Generator," for an example of this information and a description of what it means.
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter four of the book MySQL Administrator's Guide, written by Paul Dubois (Sams; ISBN: 0672326345). Check it out today at your favorite bookstore. Buy this book now.
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