MySQL Reference Manual :: 7.5.5 How MySQL Uses Memory

The key buffer (variable key_buffer_size) is shared by all threads; other buffers used by the server are allocated as needed. See Section 7.5.2, “Tuning Server Parameters”.

Each connection uses some thread-specific space:

The connection buffer and result buffer are dynamically enlarged up to max_allowed_packet when needed. While a query is running, a copy of the current query string is also allocated.

All threads share the same base memory.

Only compressed ISAM and MyISAM tables are memory mapped. This is because the 32-bit memory space of 4GB is not large enough for most big tables. When systems with a 64-bit address space become more common, we may add general support for memory mapping.

Each request that performs a sequential scan of a table allocates a read buffer (variable read_buffer_size).

When reading rows in ``random'' order (for example, after a sort), a random-read buffer may be allocated to avoid disk seeks. (variable read_rnd_buffer_size).

All joins are done in one pass, and most joins can be done without even using a temporary table. Most temporary tables are memory-based (HEAP) tables. Temporary tables with a large record length (calculated as the sum of all column lengths) or that contain BLOB columns are stored on disk.

One problem before MySQL 3.23.2 is that if an internal in-memory heap table exceeds the size of tmp_table_size, the error The table tbl_name is full occurs. From 3.23.2 on, this is handled automatically by changing the in-memory heap table to a disk-based MyISAM table as necessary. To work around this problem for older servers, you can increase the temporary table size by setting the tmp_table_size option to mysqld, or by setting the SQL option SQL_BIG_TABLES in the client program. See Section 13.5.3, “SET Syntax”.

In MySQL 3.20, the maximum size of the temporary table is record_buffer*16; if you are using this version, you have to increase the value of record_buffer. You can also start mysqld with the --big-tables option to always store temporary tables on disk. However, this affects the speed of many complicated queries.

Most requests that perform a sort allocate a sort buffer and zero to two temporary files depending on the result set size. See Section A.4.4, “Where MySQL Stores Temporary Files”.

Almost all parsing and calculating is done in a local memory store. No memory overhead is needed for small items, so the normal slow memory allocation and freeing is avoided. Memory is allocated only for unexpectedly large strings; this is done with malloc() and free().

For each MyISAM and ISAM table that is opened, the index file is opened once and the data file is opened once for each concurrently running thread. For each concurrent thread, a table structure, column structures for each column, and a buffer of size 3 * N are allocated (where N is the maximum row length, not counting BLOB columns). A BLOB column requires five to eight bytes plus the length of the BLOB data. The MyISAM and ISAM storage engines maintain one extra row buffer for internal use.

For each table having BLOB columns, a buffer is enlarged dynamically to read in larger BLOB values. If you scan a table, a buffer as large as the largest BLOB value is allocated.

Handler structures for all in-use tables are saved in a cache and managed as a FIFO. By default, the cache has 64 entries. If a table has been used by two running threads at the same time, the cache contains two entries for the table. See Section 7.4.8, “How MySQL Opens and Closes Tables”.

A FLUSH TABLES statement or mysqladmin flush-tables command closes all tables that are not in use at once and marks all in-use tables to be closed when the currently executing thread finishes. This effectively frees most in-use memory. FLUSH TABLES does not return until all tables have been closed.