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MERGE Tables
MERGE tables were introduced in MySQL Version 3.23.25. The code
is now reasonably stable.
A MERGE table (also known as a MRG_MyISAM table) is a
collection of identical MyISAM tables that can be used as one.
You can only SELECT, DELETE, and UPDATE from the
collection of tables. If you DROP the MERGE table, you
are only dropping the MERGE specification.
Note that DELETE FROM merge_table used without a WHERE
will clear only the mapping for the table, not delete everything in the
mapped tables. (We plan to fix this in 4.1).
With identical tables we mean that all tables are created with identical
column and key information. You can't merge tables in which the
columns are packed differently, doesn't have exactly the same columns,
or have the keys in different order. However, some of the tables can be
compressed with myisampack. See section myisampack.
When you create a MERGE table, you will get a `.frm' table
definition file and a `.MRG' table list file. The `.MRG' just
contains a list of the index files (`.MYI' files) that should
be used as one. Before 4.1.1 all used tables had to be in the same
database as the MERGE table itself.
For the moment, you need to have SELECT, UPDATE, and
DELETE privileges on the tables you map to a MERGE table.
MERGE tables can help you solve the following problems:
myisampack, and then create a MERGE to use these as one.
MERGE table on this could be much faster than using
the big table. (You can also use a RAID to get the same
kind of benefits.)
MERGE table for others. You can even have many
different MERGE tables active, with possible overlapping files.
MERGE file than trying to repair a really big file.
MERGE table uses the
index of the individual tables. It doesn't need to maintain an index of
its one. This makes MERGE table collections VERY fast to make or
remap. Note that you must specify the key definitions when you create
a MERGE table!.
MERGE table on them on demand.
This is much faster and will save a lot of disk space.
MERGE
over one table. There shouldn't be any really notable performance
impacts of doing this (only a couple of indirect calls and memcpy()
calls for each read).
The disadvantages with MERGE tables are:
MyISAM tables for a MERGE table.
REPLACE doesn't work.
MERGE tables uses more file descriptors. If you are using a
MERGE table that maps over 10 tables and 10 users are using this, you
are using 10*10 + 10 file descriptors. (10 datafiles for 10 users
and 10 shared index files.)
MERGE
storage engine will need to issue a read on all underlying tables to check
which one most closely matches the given key. If you then do a "read-next"
then the MERGE storage engine will need to search the read buffers
to find the next key. Only when one key buffer is used up, the storage engine
will need to read the next key block. This makes MERGE keys much slower
on eq_ref searches, but not much slower on ref searches.
See section 7.2.1 EXPLAIN Syntax (Get Information About a SELECT).
DROP TABLE,
ALTER TABLE,
DELETE FROM table_name without a WHERE clause,
REPAIR TABLE,
TRUNCATE TABLE,
OPTIMIZE TABLE, or
ANALYZE TABLE
on any of the table that is
mapped by a MERGE table that is "open". If you do this, the
MERGE table may still refer to the original table and you will
get unexpected results. The easiest way to get around this deficiency
is to issue the FLUSH TABLES command, ensuring no MERGE
tables remain "open".
When you create a MERGE table you have to specify with
UNION=(list-of-tables) which tables you want to use as
one. Optionally you can specify with INSERT_METHOD if you want
insert for the MERGE table to happen in the first or last table
in the UNION list. If you don't specify INSERT_METHOD or
specify NO, then all INSERT commands on the MERGE
table will return an error.
The following example shows you how to use MERGE tables:
CREATE TABLE t1 (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY, message CHAR(20));
CREATE TABLE t2 (a INT NOT NULL 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 AUTO_INCREMENT, message CHAR(20), KEY(a))
TYPE=MERGE UNION=(t1,t2) INSERT_METHOD=LAST;
SELECT * FROM total;
|
Note that we didn't create a UNIQUE or PRIMARY KEY in the
total table as the key isn't going to be unique in the total
table.
Note that you can also manipulate the `.MRG' file directly from the outside of the MySQL server:
shell> cd /mysql-data-directory/current-database shell> ls -1 t1.MYI t2.MYI > total.MRG shell> mysqladmin flush-tables |
Now you can do things like:
mysql> SELECT * FROM total; +---+---------+ | a | message | +---+---------+ | 1 | Testing | | 2 | table | | 3 | t1 | | 1 | Testing | | 2 | table | | 3 | t2 | +---+---------+ |
Note that the a column, though declared as PRIMARY KEY,
is not really unique, as MERGE table cannot enforce uniqueness
over a set of underlying MyISAM tables.
To remap a MERGE table you can do one of the following:
DROP the table and re-create it
ALTER TABLE table_name UNION=(...)
FLUSH TABLE on the
MERGE table and all underlying tables to force the storage engine to
read the new definition file.
If you use ALTER TABLE to change a MERGE table to another
table type, the mapping to the underlying tables is lost. Instead, the rows
from the underlying MyISAM tables are copied into the altered table,
which then is assigned the new type.
14.2.1 MERGE Table Problems |
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