Added information about MERGE tables

* Manual conventions::          Conventions used in this manual

MySQL mailing lists and how to ask questions or report errors (bugs)

MySQL mailing lists

* Mailing-list::                The @strong{MySQL} mailing lists

* Asking questions::            Asking questions or reporting bugs

* Source install system issues::  System-specific issues

* Windows::                     Windows notes

* OS/2::                        OS/2 notes

* MySQL binaries::              

* MySQL binaries::              MySQL binaries

* Post-installation::           Post-installation setup and testing

* Upgrade::                     Is there anything special to do when upgrading/downgrading @strong{MySQL}?

MySQL table types

* MyISAM::                      MyISAM tables

* MERGE::                       

* ISAM::                        ISAM tables

* HEAP::                        HEAP tables

* BDB::                         BDB or Berkeley_db tables

* example-Maximum-column::      The maximum value for a column

* example-Maximum-row::         The row holding the maximum of a certain column

* example-Maximum-column-group::  Maximum of column: per group: only the values

* example-Maximum-column-group::  Maximum of column per group

* example-Maximum-column-group-row::  The rows holding the group-wise maximum of a certain field

* example-Foreign keys::        Using foreign keys

* ODBC and last_insert_id::     How to get the value of an @code{AUTO_INCREMENT} column in ODBC

* MyODBC bug report::           Reporting problems with MyODBC

Using @strong{MySQL} with some common programs

Using MySQL with some common programs

* Apache::                      Using @strong{MySQL} with Apache

@uref{http://www.softagency.co.jp/mysql/index.en.phtml, Links about using

@strong{MySQL} in Japan/Asia}

@item

@uref{http://abattoir.cc.ndsu.nodak.edu/~nem/mysql/udf/, @strong{MySQL} UDF Registry}

@item

@uref{http://www.open.com.au/products.html, Commercial Web defect tracking

system}

@item

@item

@uref{http://www.aewa.org, Airborne Early Warning Association }

@item

@uref{http://abattoir.cc.ndsu.nodak.edu/~nem/mysql/udf/, @strong{MySQL} UDF Registry}

@item

@uref{http://21ccs.com/~gboersm/y2kmatrix/, Y2K tester}

@end itemize

        RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

table_options:

	TYPE = @{ISAM | MYISAM | HEAP@}

	TYPE = @{ISAM | MYISAM | HEAP | MERGE@}

or	AUTO_INCREMENT = #

or	AVG_ROW_LENGTH = #

or	CHECKSUM = @{0 | 1@}

The different table types are:

@multitable @columnfractions .20 .80

@item BDB or Berkeley_db @tab Transaction safe tables @xref{BDB}.

@item HEAP @tab The data for this table is only stored in memory. @xref{HEAP}.

@item ISAM @tab The original table handler. @xref{ISAM}.

@item MERGE @tab A collection of MyISAM tables used as one table @xref{MERGE}.

@item MyISAM @tab The new binary portable table handler. @xref{MyISAM}.

@item HEAP @tab The data for this table is only stored in memory. @xref{HEAP}.

@item BDB or Berkeley_db @tab Transaction safe tables @xref{BDB}.

@end multitable

@xref{Table types}.

@multitable @columnfractions .30 .70

@item @strong{Column} @tab @strong{Meaning}

@item @code{Name} @tab Name of the table

@item @code{Type} @tab Type of table (BDB, ISAM, MyISAM or HEAP)

@item @code{Type} @tab Type of table (BDB, ISAM, MERGE, MyISAM or HEAP)

@item @code{Row_format} @tab The row storage format (Fixed, Dynamic, or Compressed)

@item @code{Rows} @tab Number of rows

@item @code{Avg_row_length} @tab Average row length

@cindex Berkeley_db table type

@cindex ISAM table type

@cindex HEAP table type

@cindex MERGE table type

@cindex MySQL table types

@cindex MyISAM table type

@node Table types, Tutorial, Reference, Top

Note that @strong{MySQL} supports two different kind of

tables. Transactions safe tables (@code{BDB}) and not transaction safe

tables (@code{ISAM}, @code{MyISAM} and @code{HEAP}).

tables (@code{ISAM}, @code{MERGE}, @code{MyISAM} and @code{HEAP}).

Advantages of transaction safe tables (TST)

@menu

* MyISAM::                      MyISAM tables

* MERGE::                       

* ISAM::                        ISAM tables

* HEAP::                        HEAP tables

* BDB::                         BDB or Berkeley_db tables

@end menu

@node MyISAM, ISAM, Table types, Table types

@node MyISAM, MERGE, Table types, Table types

@section MyISAM tables

@code{MyISAM} is the default table type in @strong{MySQL} 3.23. It's

Can be uncompressed with @code{myisamchk}.

@end itemize

@node ISAM, HEAP, MyISAM, Table types

@node MERGE, ISAM, MyISAM, Table types

@section MERGE tables

@code{MERGE} tables are new in @strong{MySQL} 3.23.25; The code is still

in alpha, but should stabilize soon!  The one thing that is currently

missing is a way from the SQL prompt to say which tables are part of the

@code{MERGE} table.

A @code{MERGE} table is a collection of identical @code{MyISAM} tables

that can be used as one.  You can only @code{SELECT} from the collection

of tables. If you @code{DROP} the @code{MERGE} table, you are only

dropping the @code{MERGE} specification.

With identical tables we mean that all tables are created with identical

column information.  Some of the tables can be compressed with

@code{myisampack}. @xref{myisampack}.

When you create a @code{MERGE} table, you will get a @code{.frm} table

definition file and a @code{.MRG} table list file.  The @code{.MRG} just

contains a list of the index files (@code{.MYI} files) that should

be used as one.

@code{MERGE} tables helps you solve the following problems:

@itemize @bullet

@item

Easily manage a set of log tables; For example you can put data from

different months into separate files, compress some of them with

@code{myisampack} and then create a @code{MERGE} to use these as one.

@item

Give you more speed;  You can split a big read-only table based on some

criteria and then put the different table part on different disks.

A @code{MERGE} table on this could be much faster than using

the big table. (You can of course also use a RAID to get the same

kind of benefits).

@item

Do more efficient searches: If you know exactly what you are looking

after, you can search in just one of the split tables for some queries

and use @strong{MERGE} table for others.  You can even have many

different @code{MERGE} tables active, which possible overlapping files.

@item

More efficient repairs; It's easier to repair the individual files that

are mapped to a @code{MERGE} file than trying to repair a real big file.

@item

Instant mapping of many files as one;  A @code{MERGE} table uses the

index of the individual tables;  It doesn't need an index of its one.

This makes @code{MERGE} table collections VERY fast to make or remap.

@end itemize

The disadvantages with @code{MERGE} tables are:

@itemize @bullet

@item

@code{MERGE} tables are read-only.

@item

@code{MERGE} tables uses more file descriptors: If you are using a

@strong{MERGE} that maps over 10 tables and 10 users are using this, you

are using 10*10 + 10 file descriptors.  (10 data files for 10 users

and 10 shared index files).

@item

Key reads are slower;  When you do a read on a key, the @code{MERGE}

handler will need to issue a read on all underlying tables to check

which on most closely matches the given key.  If you then do a 'read-next'

then the merge table handler will need to search the read buffers

to find the next key;  Only when one key buffer is used up, the handler

will need to read the next key block. This makes @code{MERGE} keys much slower

on @code{eq_ref} searches, but not much slower on @code{ref} searches.

@xref{EXPLAIN}.

@item

You can't yet easily map the @code{MERGE} table from withing @strong{MySQL}.

@end itemize

The following example shows you how to use @code{MERGE} tables:

@example

CREATE TABLE t1 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));

CREATE TABLE t2 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));

INSERT INTO t1 (message) VALUES ("Testing"),("table"),("t1");

INSERT INTO t2 (message) VALUES ("Testing"),("table"),("t2");

CREATE TABLE total (a INT NOT NULL, message CHAR(20), KEY(a)) TYPE=MERGE;

@end example

Note that we didn't create an @code{UNIQUE} or @code{PRIMARY KEY} in the

@code{total} table as the key isn't going to be unique in the @code{total}

table.

(We plan to in the future add the information in the @code{MERGE} handler

that unique keys are not necessarily unique in the @code{MERGE} table.)

Now you have to use tool (editor, unix command...) to insert the file

names into the 'total' table:

@example

shell> cd /mysql-data-directory/current-database

shell> ls -1 t1.MYI t2.MYI > total.MRG

@end example

Now you can do things like:

@example

mysql> select * from total;

+---+---------+

| a | message |

+---+---------+

| 1 | Testing |

| 2 | table   |

| 3 | t1      |

| 1 | Testing |

| 2 | table   |

| 3 | t2      |

+---+---------+

@end example

To remap a @code{MERGE} table you must either @code{DROP} it and recreate it

or change the @code{.MRG} file and issue a @code{FLUSH TABLE} on the

@code{MERGE} table to force the handler to read the new definition file.

You can also put full paths to the index files in the @code{.MRG} file;  If

you don't do this, the @code{MERGE} handler assumes that the index files

are in the same directory as the @code{.MRG} file.

@node ISAM, HEAP, MERGE, Table types

@section ISAM tables

You can also use the deprecated ISAM table type. This will disappear

@code{myisampack}}.

Another solution can be the included MERGE library, which allows you to

handle a collection of identical tables as one.  (Identical in this case

means that all tables are created with identical column information.)

Currently MERGE can only be used to scan a collection of tables because it

doesn't support indexes. We will add indexes to this in the near future.

handle a collection of identical tables as one. @xref{MERGE, MERGE

tables}. 

@cindex Replication

@node Replication, Performance, Server, Top