5.9.4.5. Getting Information About a Table
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 with external locking disabled,
myisamchk may report an error for a
table that is updated while it runs. However, because
myisamchk does not 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.MYI
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 rows 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 5.9.4.4, “Table Optimization”.
-
Datafile parts
For dynamic-row format, this indicates how many data
blocks there are. For an optimized table without
fragmented rows, 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 5.9.4.4, “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 row 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 row 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 row this portion of the index starts.
-
Len
How long this portion of the index 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. Possible values are unique
or multip. (multiple).
-
Type
What data type this portion of the index has. This is
a MyISAM data type with the
possible values 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 rows there are per value for this
index. A unique index 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 key values that have a common suffix.
This can only be used for indexes on
CHAR and VARCHAR
columns. For long indexed strings that have similar
leftmost parts, this can significantly reduce the space
used. In the third of the preceding examples, 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 row length. This is the exact row length for
tables with fixed-length rows, because all rows 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 row (that is, how many links
a fragmented row 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 large, you can reorganize
the table. See Section 5.9.4.4, “Table Optimization”.
-
Recordblocks
How many blocks (links) are used. For fixed-format tables,
this is the same as the number of rows.
-
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 row 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, row 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 8.4, “myisampack — Generate Compressed, Read-Only MyISAM Tables”, for an example of this
information and a description of what it means.
