SpirirHost Support Centre: MySQL Reference Manual p.367

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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

Chapter 5: MySQL Optimisation 367 The MySQL-Linux distribution provided by MySQL AB used to be compiled with pgcc, but we had to go back to regular gcc because of a bug in pgcc that would generate the code that does not run on AMD. We will continue using gcc until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with pgcc. The standard MySQL Linux binary is linked statically to get it faster and more portable. 5.5.4 How MySQL Uses Memory The following list indicates some of the ways that the mysqld server uses memory. Where applicable, the name of the server variable relevant to the memory use is given: The key buer (variable key_buffer_size) is shared by all threads; other buers used by the server are allocated as needed. See Section 5.5.2 [Server parameters], page 363. Each connection uses some thread-specic space: A stack (default 64K, variable thread_stack), a connection buer (variable net_buffer_length), and a result buer (variable net_buffer_length). The connection buer and result buer are dynamically enlarged up to max_allowed_packet when needed. When a query is running, a copy of the current query string is also allocated. All threads share the same base memory. Only the compressed ISAM / 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 doing a sequential scan over a table allocates a read buer (variable record_buffer). When reading rows in 'random' order (for example after a sort) a random-read buer is allocated to avoid disk seeks. (variable record_rnd_buffer). All joins are done in one pass, and most joins can be done without even using a tem- porary table. Most temporary tables are memory-based (HEAP) tables. Temporary tables with a big record length (calculated as the sum of all column lengths) or that contain BLOB columns are stored on disk. One problem in MySQL versions before Version 3.23.2 is that if a HEAP table exceeds the size of tmp_table_size, you get the error The table tbl_name is full. In newer versions this is handled by automatically changing the in-memory (HEAP) table to a disk-based (MyISAM) table as necessary. To work around this problem, you can increase the temporary table size by setting the tmp_table_size option to mysqld, or by setting the SQL option BIG_TABLES in the client program. See Section 5.5.6 [SET], page 369. In MySQL Version 3.20, the maximum size of the temporary table was record_buffer*16, so 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 will aect the speed of many complicated queries. Most requests doing a sort allocates a sort buer and 0-2 temporary les depending on the result set size. See Section A.4.4 [Temporary les], page 643.

MySQL Reference Manual

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