Merge work:/my/mysql into donna.mysql.fi:/home/my/bk/mysql

* Key space::                   Space needed for keys

* MyISAM table formats::        MyISAM table formats

* MyISAM table problems::       

MyISAM Table Formats

The index is stored in a file with the @code{.MYI} (MYIndex) extension,

and the data is stored in a file with the @code{.MYD} (MYData) extension.

You can check/repair @code{MyISAM} tables with the @code{myisamchk}

utility. @xref{Crash recovery}.

utility. @xref{Crash recovery}. You can compress @code{MyISAM} tables with 

@code{myisampack} to take up much less space. @xref{myisampack}.

The following is new in @code{MyISAM}:

@menu

* Key space::                   Space needed for keys

* MyISAM table formats::        MyISAM table formats

* MyISAM table problems::       

@end menu

@cindex key space, MyISAM

many integer keys that have an identical prefix when the numbers are stored

high-byte first.

@node MyISAM table formats,  , Key space, MyISAM

@node MyISAM table formats, MyISAM table problems, Key space, MyISAM

@subsection MyISAM Table Formats

@strong{MyISAM} supports 3 different table types. Two of them are chosen

Can be uncompressed with @code{myisamchk}.

@end itemize

@node MyISAM table problems,  , MyISAM table formats, MyISAM

@subsection MyISAM table problems.

Each @code{MyISAM} @code{.MYI} file has in the header a counter that can

be used to check if a table has been closed properly.

If you get the following warning from @code{CHECK TABLE} or @code{myisamchk}:

@example

# clients is using or hasn't closed the table properly

@end example

this means that this counter has come out of sync.  This doesn't mean

that the table is corrupted, but means that you should at least do a

check on the table to verify that it's ok.

The counter works as follows:

@itemize @bullet

@item

The first time a table is updated in @strong{MySQL}, a counter in the

header of the index files is incremented.

@item

The counter is not changed during further updates.

@item

When the last instance of a table is closed (because of a @code{FLUSH} or

because there isn't room in the table cache) the counter is

decremented if the table has been updated at any point.

@item

When you repair the table or check the table and it was ok,

the counter is reset to 0.

@item

To avoid problems with interaction with other processes that may do a

check on the table, the counter is not decremented on close if it was 0.

@end itemize

In other words, the only ways this can go out of sync are:

@itemize @bullet

@item

The @code{MyISAM} tables are copied without a @code{LOCK} and

@code{FLUSH TABLES}.

@item

@strong{MySQL} has crashed between an update and the final close

(Note that the table may still be ok, as @strong{MySQL} always issues writes

for everything between each statement).

@item

Someone has done a @code{myisamchk --repair} or @code{myisamchk --update-state}on a table that was in use by @code{mysqld}.

@item

Many @code{mysqld} servers are using the table and one has done a

@code{REPAIR} or @code{CHECK} of the table while it was in use by

another server.  In this setup the @code{CHECK} is safe to do (even if

you will get the warning from other servers), but @code{REPAIR} should

be avoided as it currently replaces the data file with a new one, which

is not signaled to the other servers.

@end itemize

@cindex tables, merging

@cindex MERGE tables, defined

@node MERGE, ISAM, MyISAM, Table types