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.
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.
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.
When rows are deleted, space is not automatically freed. The only way to free space without dropping the table is using
ALTER TABLE tbl_name ENGINE = MEMORY.
TRUNCATE TABLE frees the memory too.
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.
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;