Much more conservative approach to disk space management - each MyISAM table
is stored in a separate file, which could be compressed then with myisamchk
if needed. With InnoDB the tables are stored in tablespace, and not much
further optimization is possible. All data except for TEXT and BLOB can
occupy 8,000 bytes at most. No full text indexing is available for InnoDB.
TRhe COUNT(*)s execute slower than in MyISAM due to tablespace complexity.
MyISAM tables have the following characteristics:
· All data values are stored with the low byte first. This makes the
data machine and *operating system* independent. The only requirements for
binary portability are that the machine uses two's-complement signed
integers and IEEE floating-point format. These requirements are widely used
among mainstream machines. Binary compatibility might not be applicable to
embedded systems, which sometimes have peculiar processors.
There is no significant speed penalty for storing data low byte first; the
bytes in a table row normally are unaligned and it takes little more
processing to read an unaligned byte in order than in reverse order. Also,
the code in the server that fetches column values is not time critical
compared to other code.
· All numeric key values are stored with the high byte first to
allow better index compression.
· Large files (up to 63-bit file length) are supported on
filesystems and operating systems that support large files.
· The maximum number of indexes per MyISAM table is 64. This can be
changed by recompiling. Beginning with MySQL 5.1.4, you can configure the
build by invoking *configure* with the --with-max-indexes=*N* option, where
*N* is the maximum number of indexes to permit per MyISAM table. *N* must be
less thann or equal to 128. Before MySQL 5.1.4, you must change the source.
The maximum number of columns per index is 16.
· The maximum key length is 1000 bytes. This can also be changed by
changing the source and recompiling. For the case of a key longer than 250
bytes, a larger key block size than the default of 1024 bytes is used.
· When rows are inserted in sorted order (as when you are using an
AUTO_INCREMENT column), the index tree is split so that the high node only
contains one key. This improves space utilization in the index tree.
· Internal handling of one AUTO_INCREMENT column per table is
supported. MyISAM automatically updates this column for INSERT and
UPDATEoperations. This makes
AUTO_INCREMENT columns faster (at least 10%). Values at the top of the
sequence are not reused after being deleted. (When an AUTO_INCREMENT column
is defined as the last column of a multiple-column index, reuse of values
deleted from the top of a sequence does occur.) The AUTO_INCREMENT value can
be reset with ALTER TABLE or *myisamchk*.
· Dynamic-sized rows are much less fragmented when mixing deletes
with updates and inserts. This is done by automatically combining adjacent
deleted blocks and by extending blocks if the next block is deleted.
· If a table has no free blocks in the middle of the data file, you
can INSERT new rows into it at the same time that other threads are reading
from the table. (These are known as concurrent inserts.) A free block can
occur as a result of deleting rows or an update of a dynamic length row with
more data than its current contents. When all free blocks are used up
(filled in), future inserts become concurrent again. See Section 7.3.3,
· You can put the data file and index file on different directories
to get more speed with the DATA DIRECTORY and INDEX DIRECTORY table options
to CREATE TABLE. See Section 13.1.5, “CREATE TABLE
· BLOB and TEXT columns can be indexed.
· NULL values are allowed in indexed columns. This takes 0–1 bytes
· Each character column can have a different character set.
See Chapter 10,
· There is a flag in the MyISAM index file that indicates whether
the table was closed correctly. If *mysqld* is started with the
--myisam-recover option, MyISAM tables are automatically checked when
opened, and are repaired if the table wasn't closed properly.
· *myisamchk* marks tables as checked if you run it with the
--update-state option. *myisamchk --fast* checks only those tables that
don't have this mark.
· *myisamchk --analyze* stores statistics for portions of keys, as
well as for entire keys.
· *myisampack* can pack BLOB and VARCHAR columns.
*MyISAM also supports the following features: *
· Support for a true VARCHAR type; a VARCHAR column starts with a
length stored in one or two bytes.
· Tables with VARCHAR columns may have fixed or dynamic row length.
· The sum of the lengths of the VARCHAR and CHAR columns in a table
may be up to 64KB.
· A hashed computed index can be used for UNIQUE. This allows you to
have UNIQUE on any combination of columns in a table. (However, you cannot
search on a UNIQUE computed index.)
On Fri, Jul 16, 2010 at 11:16 AM, P.R.Karthik <prkart@stripped> wrote:
> I am newbie to mysql can i know the advantages of myisam storage engine
> and some of its special features.
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