18.7.1 MERGE Table Advantages and Disadvantages

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 tables, compress some of them with myisampack, and then create a MERGE table to use them as one.

• Obtain more speed. You can split a large read-only table based on some criteria, and then put individual tables on different disks. A MERGE table structured this way could be much faster than using a single large table.

• Perform more efficient searches. If you know exactly what you are looking for, you can search in just one of the underlying tables for some queries and use a MERGE table for others. You can even have many different MERGE tables that use overlapping sets of tables.

• Perform more efficient repairs. It is easier to repair individual smaller tables that are mapped to a MERGE table than to repair a single large table.

• Instantly map many tables as one. A MERGE table need not maintain an index of its own because it uses the indexes of the individual tables. As a result, MERGE table collections are very fast to create or remap. (You must still specify the index definitions when you create a MERGE table, even though no indexes are created.)

• If you have a set of tables from which you create a large table on demand, you can instead create a MERGE table from them on demand. This is much faster and saves a lot of disk space.

• Exceed the file size limit for the operating system. Each MyISAM table is bound by this limit, but a collection of MyISAM tables is not.

• You can create an alias or synonym for a MyISAM table by defining a MERGE table that maps to that single table. There should be no really notable performance impact from doing this (only a couple of indirect calls and memcpy() calls for each read).

The disadvantages of MERGE tables are:

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

• Some MyISAM features are unavailable in MERGE tables. For example, you cannot create FULLTEXT indexes on MERGE tables. (You can create FULLTEXT indexes on the underlying MyISAM tables, but you cannot search the MERGE table with a full-text search.)

• If the MERGE table is nontemporary, all underlying MyISAM tables must be nontemporary. If the MERGE table is temporary, the MyISAM tables can be any mix of temporary and nontemporary.

• MERGE tables use more file descriptors than MyISAM tables. If 10 clients are using a MERGE table that maps to 10 tables, the server uses (10 × 10) + 10 file descriptors. (10 data file descriptors for each of the 10 clients, and 10 index file descriptors shared among the clients.)

• Index reads are slower. When you read an index, the MERGE storage engine needs to issue a read on all underlying tables to check which one most closely matches a given index value. To read the next index value, the MERGE storage engine needs to search the read buffers to find the next value. Only when one index buffer is used up does the storage engine need to read the next index block. This makes MERGE indexes much slower on eq_ref searches, but not much slower on ref searches. For more information about eq_ref and ref, see Section 15.8.2, “EXPLAIN Statement”.