Added information about MERGE tables
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monty@donna.mysql.com
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Docs/manual.texi
159
Docs/manual.texi
@ -131,7 +131,7 @@ About this manual
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* Manual conventions:: Conventions used in this manual
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MySQL mailing lists and how to ask questions or report errors (bugs)
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MySQL mailing lists
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* Mailing-list:: The @strong{MySQL} mailing lists
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* Asking questions:: Asking questions or reporting bugs
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@ -183,7 +183,7 @@ Installing MySQL
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* Source install system issues:: System-specific issues
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* Windows:: Windows notes
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* OS/2:: OS/2 notes
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* MySQL binaries::
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* MySQL binaries:: MySQL binaries
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* Post-installation:: Post-installation setup and testing
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* Upgrade:: Is there anything special to do when upgrading/downgrading @strong{MySQL}?
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@ -426,6 +426,7 @@ Functions for use in @code{SELECT} and @code{WHERE} clauses
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MySQL table types
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* MyISAM:: MyISAM tables
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* MERGE::
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* ISAM:: ISAM tables
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* HEAP:: HEAP tables
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* BDB:: BDB or Berkeley_db tables
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@ -456,7 +457,7 @@ Examples of common queries
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* example-Maximum-column:: The maximum value for a column
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* example-Maximum-row:: The row holding the maximum of a certain column
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* example-Maximum-column-group:: Maximum of column: per group: only the values
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* example-Maximum-column-group:: Maximum of column per group
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* example-Maximum-column-group-row:: The rows holding the group-wise maximum of a certain field
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* example-Foreign keys:: Using foreign keys
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@ -608,7 +609,7 @@ MySQL ODBC Support
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* ODBC and last_insert_id:: How to get the value of an @code{AUTO_INCREMENT} column in ODBC
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* MyODBC bug report:: Reporting problems with MyODBC
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Using @strong{MySQL} with some common programs
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Using MySQL with some common programs
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* Apache:: Using @strong{MySQL} with Apache
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@ -2136,6 +2137,8 @@ support @strong{MySQL}}
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@uref{http://www.softagency.co.jp/mysql/index.en.phtml, Links about using
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@strong{MySQL} in Japan/Asia}
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@item
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@uref{http://abattoir.cc.ndsu.nodak.edu/~nem/mysql/udf/, @strong{MySQL} UDF Registry}
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@item
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@uref{http://www.open.com.au/products.html, Commercial Web defect tracking
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system}
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@item
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@ -2168,8 +2171,6 @@ which mentions @strong{MySQL} in the right company}
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@item
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@uref{http://www.aewa.org, Airborne Early Warning Association }
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@item
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@uref{http://abattoir.cc.ndsu.nodak.edu/~nem/mysql/udf/, @strong{MySQL} UDF Registry}
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@item
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@uref{http://21ccs.com/~gboersm/y2kmatrix/, Y2K tester}
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@end itemize
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@ -16432,7 +16433,7 @@ reference_option:
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RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT
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table_options:
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TYPE = @{ISAM | MYISAM | HEAP@}
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TYPE = @{ISAM | MYISAM | HEAP | MERGE@}
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or AUTO_INCREMENT = #
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or AVG_ROW_LENGTH = #
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or CHECKSUM = @{0 | 1@}
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@ -16662,10 +16663,11 @@ implemented in @strong{MySQL} 3.23 and above.
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The different table types are:
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@multitable @columnfractions .20 .80
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@item ISAM @tab The original table handler. @xref{ISAM}.
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@item MyISAM @tab The new binary portable table handler. @xref{MyISAM}.
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@item HEAP @tab The data for this table is only stored in memory. @xref{HEAP}.
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@item BDB or Berkeley_db @tab Transaction safe tables @xref{BDB}.
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@item HEAP @tab The data for this table is only stored in memory. @xref{HEAP}.
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@item ISAM @tab The original table handler. @xref{ISAM}.
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@item MERGE @tab A collection of MyISAM tables used as one table @xref{MERGE}.
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@item MyISAM @tab The new binary portable table handler. @xref{MyISAM}.
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@end multitable
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@xref{Table types}.
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@ -18627,7 +18629,7 @@ The following columns are returned:
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@multitable @columnfractions .30 .70
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@item @strong{Column} @tab @strong{Meaning}
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@item @code{Name} @tab Name of the table
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@item @code{Type} @tab Type of table (BDB, ISAM, MyISAM or HEAP)
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@item @code{Type} @tab Type of table (BDB, ISAM, MERGE, MyISAM or HEAP)
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@item @code{Row_format} @tab The row storage format (Fixed, Dynamic, or Compressed)
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@item @code{Rows} @tab Number of rows
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@item @code{Avg_row_length} @tab Average row length
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@ -20275,6 +20277,7 @@ used them.
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@cindex Berkeley_db table type
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@cindex ISAM table type
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@cindex HEAP table type
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@cindex MERGE table type
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@cindex MySQL table types
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@cindex MyISAM table type
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@node Table types, Tutorial, Reference, Top
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@ -20296,7 +20299,7 @@ TABLE} statement. @xref{ALTER TABLE, , @code{ALTER TABLE}}.
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Note that @strong{MySQL} supports two different kind of
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tables. Transactions safe tables (@code{BDB}) and not transaction safe
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tables (@code{ISAM}, @code{MyISAM} and @code{HEAP}).
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tables (@code{ISAM}, @code{MERGE}, @code{MyISAM} and @code{HEAP}).
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Advantages of transaction safe tables (TST)
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@ -20332,12 +20335,13 @@ of both worlds.
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@menu
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* MyISAM:: MyISAM tables
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* MERGE::
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* ISAM:: ISAM tables
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* HEAP:: HEAP tables
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* BDB:: BDB or Berkeley_db tables
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@end menu
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@node MyISAM, ISAM, Table types, Table types
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@node MyISAM, MERGE, Table types, Table types
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@section MyISAM tables
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@code{MyISAM} is the default table type in @strong{MySQL} 3.23. It's
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@ -20620,7 +20624,128 @@ columns.
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Can be uncompressed with @code{myisamchk}.
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@end itemize
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@node ISAM, HEAP, MyISAM, Table types
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@node MERGE, ISAM, MyISAM, Table types
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@section MERGE tables
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@code{MERGE} tables are new in @strong{MySQL} 3.23.25; The code is still
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in alpha, but should stabilize soon! The one thing that is currently
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missing is a way from the SQL prompt to say which tables are part of the
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@code{MERGE} table.
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A @code{MERGE} table is a collection of identical @code{MyISAM} tables
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that can be used as one. You can only @code{SELECT} from the collection
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of tables. If you @code{DROP} the @code{MERGE} table, you are only
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dropping the @code{MERGE} specification.
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With identical tables we mean that all tables are created with identical
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column information. Some of the tables can be compressed with
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@code{myisampack}. @xref{myisampack}.
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When you create a @code{MERGE} table, you will get a @code{.frm} table
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definition file and a @code{.MRG} table list file. The @code{.MRG} just
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contains a list of the index files (@code{.MYI} files) that should
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be used as one.
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@code{MERGE} tables helps you solve the following problems:
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@itemize @bullet
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@item
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Easily manage a set of log tables; For example you can put data from
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different months into separate files, compress some of them with
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@code{myisampack} and then create a @code{MERGE} to use these as one.
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@item
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Give you more speed; You can split a big read-only table based on some
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criteria and then put the different table part on different disks.
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A @code{MERGE} table on this could be much faster than using
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the big table. (You can of course also use a RAID to get the same
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kind of benefits).
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@item
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Do more efficient searches: If you know exactly what you are looking
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after, you can search in just one of the split tables for some queries
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and use @strong{MERGE} table for others. You can even have many
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different @code{MERGE} tables active, which possible overlapping files.
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@item
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More efficient repairs; It's easier to repair the individual files that
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are mapped to a @code{MERGE} file than trying to repair a real big file.
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@item
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Instant mapping of many files as one; A @code{MERGE} table uses the
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index of the individual tables; It doesn't need an index of its one.
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This makes @code{MERGE} table collections VERY fast to make or remap.
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@end itemize
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The disadvantages with @code{MERGE} tables are:
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@itemize @bullet
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@item
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@code{MERGE} tables are read-only.
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@item
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@code{MERGE} tables uses more file descriptors: If you are using a
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@strong{MERGE} that maps over 10 tables and 10 users are using this, you
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are using 10*10 + 10 file descriptors. (10 data files for 10 users
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and 10 shared index files).
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@item
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Key reads are slower; When you do a read on a key, the @code{MERGE}
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handler will need to issue a read on all underlying tables to check
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which on most closely matches the given key. If you then do a 'read-next'
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then the merge table handler will need to search the read buffers
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to find the next key; Only when one key buffer is used up, the handler
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will need to read the next key block. This makes @code{MERGE} keys much slower
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on @code{eq_ref} searches, but not much slower on @code{ref} searches.
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@xref{EXPLAIN}.
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@item
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You can't yet easily map the @code{MERGE} table from withing @strong{MySQL}.
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@end itemize
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The following example shows you how to use @code{MERGE} tables:
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@example
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CREATE TABLE t1 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));
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CREATE TABLE t2 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));
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INSERT INTO t1 (message) VALUES ("Testing"),("table"),("t1");
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INSERT INTO t2 (message) VALUES ("Testing"),("table"),("t2");
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CREATE TABLE total (a INT NOT NULL, message CHAR(20), KEY(a)) TYPE=MERGE;
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@end example
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Note that we didn't create an @code{UNIQUE} or @code{PRIMARY KEY} in the
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@code{total} table as the key isn't going to be unique in the @code{total}
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table.
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(We plan to in the future add the information in the @code{MERGE} handler
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that unique keys are not necessarily unique in the @code{MERGE} table.)
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Now you have to use tool (editor, unix command...) to insert the file
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names into the 'total' table:
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@example
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shell> cd /mysql-data-directory/current-database
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shell> ls -1 t1.MYI t2.MYI > total.MRG
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@end example
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Now you can do things like:
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@example
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mysql> select * from total;
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+---+---------+
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| a | message |
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+---+---------+
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| 1 | Testing |
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| 2 | table |
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| 3 | t1 |
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| 1 | Testing |
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| 2 | table |
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| 3 | t2 |
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+---+---------+
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@end example
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To remap a @code{MERGE} table you must either @code{DROP} it and recreate it
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or change the @code{.MRG} file and issue a @code{FLUSH TABLE} on the
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@code{MERGE} table to force the handler to read the new definition file.
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You can also put full paths to the index files in the @code{.MRG} file; If
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you don't do this, the @code{MERGE} handler assumes that the index files
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are in the same directory as the @code{.MRG} file.
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@node ISAM, HEAP, MERGE, Table types
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@section ISAM tables
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You can also use the deprecated ISAM table type. This will disappear
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@ -23382,10 +23507,8 @@ have, in effect, much bigger tables. @xref{myisampack, ,
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@code{myisampack}}.
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Another solution can be the included MERGE library, which allows you to
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handle a collection of identical tables as one. (Identical in this case
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means that all tables are created with identical column information.)
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Currently MERGE can only be used to scan a collection of tables because it
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doesn't support indexes. We will add indexes to this in the near future.
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handle a collection of identical tables as one. @xref{MERGE, MERGE
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tables}.
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@cindex Replication
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@node Replication, Performance, Server, Top
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