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MySQL Reference Manual by Kaj Arno, David Axmark, Michael Widenius

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MERGE Tables

MERGE tables are new in MySQL Version 3.23.25. The code is still in gamma, but should be reasonable 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 only clear 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 put a MERGE over tables where the columns are packed differently, don’t have exactly the same columns, or have the keys in a different order. Some of the tables can, however, be compressed with myisampack. See Section 4.7.4.

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. All used tables must 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:

  • Easily manage a set of log tables. For example, you can put data from different months into separate files, compress some of them with myisampack, and then create a MERGE to use these as one.

  • 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 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.)

  • Do more efficient searches. If you know exactly what you are looking for, you can search in just one of the split tables for some queries and use MERGE table for others. You can even have many different MERGE tables active, with possible overlapping files.

  • More efficient repairs. It’s easier to repair the individual files that are mapped to a MERGE file than trying to repair a real big file.

  • Instant mapping of many files as one. A MERGE table uses the index of the individual tables. It doesn’t need to maintain an index of 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!

  • If you have a set of tables that you join to a big table on demand or batch, you should instead create a MERGE table on them on demand. This is much faster and will save a lot of disk space.

  • Go around the file-size limit for the operating system.

  • You can create an alias/synonym for a table by just using MERGE over one table. There shouldn’t be any really notable performance impacts of doing this (only a couple of indirect calls and memcpys for each read).

The disadvantages with MERGE tables are:

  • You can only use identical MyISAM tables for a MERGE table.

  • AUTO_INCREMENT columns are not automatically updated on INSERT.

  • REPLACE doesn’t work.

  • MERGE tables uses more file descriptors. If you are using a 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.)

  • Key reads are slower. When you do a read on a key, the MERGE handler 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,’ the merge table handler will need to search the read buffers to find the next key. Only when one key buffer is used up will the handler 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 5.2.1.

  • You can’t do DROP TABLE, ALTER TABLE, or DELETE FROM table_name without a WHERE clause on any of the tables that are 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.

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, 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 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 UNION=(t1,t2) INSERT_METHOD=LAST;

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      |
+---+---------+

To remap a MERGE table you can do one of the following:

  • DROP the table and re-create it.

  • Use ALTER TABLE table_name UNION(...).

  • Change the .MRG file and issue a FLUSH TABLE on the MERGE table and all underlying tables to force the handler to read the new definition file.

MERGE Table Problems

The following are the known problems with MERGE tables:

  • MERGE tables cannot maintain UNIQUE constraints over the whole table. When you do INSERT, the data goes into the first or last table (according to INSERT_METHOD=xxx) and this MyISAM table ensures that the data is unique, but it knows nothing about the first MyISAM table.

  • DELETE FROM merge_table used without a WHERE will only clear the mapping for the table, not delete everything in the mapped tables.

  • RENAME TABLE on a table used in an active MERGE table may corrupt the table. This will be fixed in MySQL 4.0.x.

  • Creation of a table of type MERGE doesn’t check if the underlying tables are of compatible types. If you use MERGE tables in this fashion, you are very likely to run into strange problems.

  • If you use ALTER TABLE to first add a UNIQUE index to a table used in a MERGE table and then use ALTER TABLE to add a normal index on the MERGE table, the key order will be different for the tables if there was an old non-unique key in the table. This is because ALTER TABLE puts UNIQUE keys before normal keys to be able to detect duplicate keys as early as possible.

  • The range optimizer can’t yet use MERGE table efficiently and may sometimes produce non-optimal joins. This will be fixed in MySQL 4.0.x.

  • DROP TABLE on a table that is in use by a MERGE table will not work on Windows because the MERGE handler does the table mapping hidden from the upper layer of MySQL. Because Windows doesn’t allow you to drop files that are open, you first must flush all MERGE tables (with FLUSH TABLES) or drop the MERGE table before dropping the table. We will fix this at the same time we introduce VIEWs.

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