A MERGE table is a logical collection of MyISAM tables that have the same structure. You can use this to manage the storage properties of the underlying MyISAM tables:

  • MERGE tables provide a workaround when you run up against filesystem file-size limitations for MyISAM tables. If a given MyISAM table is as large as it can be because its data or index files have reach a file-size limit, split the table into multiple smaller MyISAM tables and then create a MERGE table that groups them into a single logical table. The maximum effective size of the MERGE table is the maximum size of each constituent MyISAM table times the number of tables.
  • A MERGE table can contain a mix of compressed and uncompressed MyISAM tables. If some of the MyISAM tables contain archival data that will not be modified, we can compress them to save disk space.
  • If we use MyISAM table symlinking, we can place different tables on different physical devices, which allows their contents to be read in parallel.
  • If a large MyISAM table is split into smaller tables, you can still access the combined contents by defining a MERGE table. But you also can gain some advantages for certain operations when it is possible to access only particular individual files. A table repair is faster on a smaller table than a larger one. If you happen to have searches where you know all the records needed are contained in a particular table, you can search just that one table rather than the entire collection.
  • It is very fast to create or drop MERGE tables, so applications that need to work with dynamically changing sets of tables can easily instantiate MERGE tables on the fly as necessary.
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