MEMORY Storage Engine

Contents of the MEMORY storage engine (previously known as HEAP) are stored in memory rather than on disk.

It is best-used for read-only caches of data from other tables, or for temporary work areas.

Since the data is stored in memory, it is highly vulnerable to power outages or hardware failure, and is unsuitable for permanent data storage. In fact, after a server restart, MEMORY tables will be recreated (because the definition file is stored on disk), but they will be empty. It is possible to re-populate them with a query using the --init-file server startup option.

Variable-length types like VARCHAR can be used in MEMORY tables. BLOB or TEXT columns are not supported for MEMORY tables.

Memory Usage

The maximum total size of MEMORY tables cannot exceed the max_heap_table_size system server variable. When a table is created this value applies to that table, and when the server is restarted this value applies to existing tables. Changing this value has no effect on existing tables. However, executing a ALTER TABLE ... ENGINE=MEMORY statement applies the current value of max_heap_table_size to the table. Also, it is possible to change the session value of max_heap_table_size before creating a table, to make sure that tables created by other sessions are not affected.

The MAX_ROWS table option provides a hint about the number of rows you plan to store in them. This is not a hard limit that cannot be exceeded, and does not allow to exceed max_heap_table_size. The storage engine uses max_heap_table_size and MAX_ROWS to calculate the maximum memory that could be allocated for the table.

Memory allocated to a MEMORY table is freed by running DROP TABLE or TRUNCATE TABLE, or rebuilding with ALTER TABLE tbl_name ENGINE = MEMORY. When rows are deleted, space is not automatically freed.

Index Type

The MEMORY storage engine permits indexes to be either B-tree or Hash. Hash is the default type for MEMORY. See Storage Engine index types for more on their characteristics.

A MEMORY table can have up to 64 indexes, 16 columns for each index and a maximum key length of 3072 bytes.

Replication

MEMORY tables are lost when a server restarts. In order to achieve this result in replication, the first time a primary uses a MEMORY table after a restart, it writes a DELETE statement for that table to the binary log, so that replicas are also emptied.

Example

The following example shows how to create a MEMORY table with a given maximum size, as described above.

SET max_heap_table_size = 1024*516;

CREATE TABLE t (a VARCHAR(10), b INT) ENGINE = MEMORY;

SET max_heap_table_size = @@max_heap_table_size;

See Also

• Performance of MEMORY tables