Home arrow MySQL arrow Page 7 - Disaster Prevention and Recovery with the MySQL Database

4.6.4 Getting Information About a Table - MySQL

If you need to administer MySQL, this article gets you off to a good start. In this section, we discuss the issues of disaster prevention and recovery. The fifth of a multi-part series, it is excerpted from chapter four of the book MySQL Administrator's Guide, written by Paul Dubois (Sams; ISBN: 0672326345).

TABLE OF CONTENTS:
  1. Disaster Prevention and Recovery with the MySQL Database
  2. 4.6.2.2 General Options for myisamchk
  3. 4.6.2.4 Repair Options for myisamchk
  4. 4.6.2.7 Using myisamchk for Crash Recovery
  5. 4.6.2.9 How to Repair Tables
  6. 4.6.3 Setting Up a Table Maintenance Schedule
  7. 4.6.4 Getting Information About a Table
By: Sams Publishing
Rating: starstarstarstarstar / 7
June 22, 2006

print this article
SEARCH DEV SHED

TOOLS YOU CAN USE

advertisement

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.



 
 
>>> More MySQL Articles          >>> More By Sams Publishing
 

blog comments powered by Disqus
escort Bursa Bursa escort Antalya eskort
   

MYSQL ARTICLES

- Oracle Unveils MySQL 5.6
- MySQL Vulnerabilities Threaten Databases
- MySQL Cloud Options Expand with Google Cloud...
- MySQL 5.6 Prepped to Handle Demanding Web Use
- ScaleBase Service Virtualizes MySQL Databases
- Oracle Unveils MySQL Conversion Tools
- Akiban Opens Database Software for MySQL Use...
- Oracle Fixes MySQL Bug
- MySQL Databases Vulnerable to Password Hack
- MySQL: Overview of the ALTER TABLE Statement
- MySQL: How to Use the GRANT Statement
- MySQL: Creating, Listing, and Removing Datab...
- MySQL: Create, Show, and Describe Database T...
- MySQL Data and Table Types
- McAfee Releases Audit Plugin for MySQL Users

Developer Shed Affiliates

 


Dev Shed Tutorial Topics: