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302 lines
8.6 KiB
Text
302 lines
8.6 KiB
Text
\input texinfo @c -*-texinfo-*-
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@c Copyright 1998 TcX AB, Detron HB and Monty Program KB
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@c
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@c %**start of header
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@setfilename internals.info
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@c We want the types in the same index
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@c @synindex tp fn cp
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@synindex cp fn
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@iftex
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@c Well this is normal in Europe. Maybe this should go into the include.texi?
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@afourpaper
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@end iftex
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@c Get version and other info
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@include include.texi
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@ifclear tex-debug
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@c This removes the black squares in the right margin
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@finalout
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@end ifclear
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@c Set background for HTML
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@set _body_tags BGCOLOR=#FFFFFF TEXT=#000000 LINK=#101090 VLINK=#7030B0
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@settitle @strong{MySQL} internals Manual for version @value{mysql_version}.
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@setchapternewpage off
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@paragraphindent 0
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@c %**end of header
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@ifinfo
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@format
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START-INFO-DIR-ENTRY
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* mysql-internals: (mysql-internals). @strong{MySQL} internals.
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END-INFO-DIR-ENTRY
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@end format
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@end ifinfo
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@titlepage
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@sp 10
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@center @titlefont{@strong{MySQL} Internals Manual}
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@sp 10
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@center Copyright @copyright{} 1998 TcX AB, Detron HB and Monty Program KB
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@end titlepage
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@node Top, Introduction, (dir), (dir)
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@ifinfo
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This is a manual about @strong{MySQL} internals.
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@end ifinfo
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@menu
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@end menu
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@node caching
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@chapter How MySQL handles caching
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@strong{MySQL} has the following caches:
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(Note that the some of the filename have a wrong spelling of cache. :)
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@itemize @bullet
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@item Key cache
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A shared cache for all B-tree index blocks in the different NISAM
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files. Uses hashing and reverse linked lists for quick caching of the
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last used blocks and quick flushing of changed entries for a specific
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table. (@file{mysys/mf_keycash.c})
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@item Record cache
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This is used for quick scanning of all records in a table.
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(@file{mysys/mf_iocash.c} and @file{isam/_cash.c})
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@item Table cache
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This holds the last used tables. (@file{sql/sql_base.cc})
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@item Hostname cache
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For quick lookup (with reverse name resolving). Is a must when one has a
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slow DNS.
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(@file{sql/hostname.cc})
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@item Privilege cache
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To allow quick change between databases the last used privileges are
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cached for each user/database combination.
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(@file{sql/sql_acl.cc})
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@item Heap table cache
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Many use of GROUP BY or DISTINCT caches all found
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rows in a HEAP table (this is a very quick in-memory table with hash index)
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@item Join row cache.
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For every full join in a SELECT statement (a full join here means there
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were no keys that one could use to find the next table in a list), the
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found rows are cached in a join cache. One SELECT query can use many
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join caches in the worst case.
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@end itemize
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@node flush tables
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@chapter How MySQL handles flush tables
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@itemize @bullet
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@item
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Flush tables is handled in @code{sql/sql_base.cc::close_cached_tables()}.
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@item
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The idea of flush tables is to force all tables to be closed. This
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is mainly to ensure that if someone adds a new table outside of
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@strong{MySQL} (for example with @code{cp}) all threads will start using
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the new table. This will also ensure that all table changes are flushed
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to disk (but of course not as optimally as simple calling a sync on
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all tables)!
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@item
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When one does a @code{FLUSH TABLES}, the variable @code{refresh_version}
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will be incremented. Every time a thread releases a table it checks if
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the refresh version of the table (updated at open) is the same as
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the current refresh_version. If not it will close it and broadcast
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a signal on COND_refresh (to wait any thread that is waiting for
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all instanses of a table to be closed).
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@item
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The current @code{refresh_version} is also compared to the open
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@code{refresh_version} after a thread gets a lock on a table. If the
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refresh version is different the thread will free all locks, reopen the
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table and try to get the locks again; This is just to quickly get all
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tables to use the newest version. This is handled by
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@code{sql/lock.cc::mysql_lock_tables()} and
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@code{sql/sql_base.cc::wait_for_tables()}.
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@item
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When all tables has been closed @code{FLUSH TABLES} will return an ok
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to client.
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@item
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If the thread that is doing @code{FLUSH TABLES} has a lock on some tables,
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it will first close the locked tables, then wait until all other threads
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have also closed them, and then reopen them and get the locks.
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After this it will give other threads a chance to open the same tables.
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@end itemize
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@node Filesort
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@chapter How MySQL does sorting (filesort)
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@itemize @bullet
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@item
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Read all rows according to key or by table scanning.
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@item
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Store the sort-key in a buffer (@code{sort_buffer}).
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@item
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When the buffer gets full, run a qsort on it and store the result
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in a temporary file. Save a pointer to the sorted block.
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@item
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Repeat the above until all rows have been read.
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@item
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Repeat the following until there is less than @code{MERGEBUFF2} (15)
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blocks left.
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@item
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Do a multi-merge of up to @code{MERGEBUFF} (7) regions to one block in
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another temporary file. Repeat until all blocks from the first file
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are in the second file.
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@item
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On the last multi-merge, only the pointer to the row (last part of
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the sort-key) is written to a result file.
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@item
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Now the code in @file{sql/records.cc} will be used to read through them
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in sorted order by using the row pointers in the result file.
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To optimize this, we read in a big block of row pointers, sort these
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and then we read the rows in the sorted order into a row buffer
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(@code{record_buffer}) .
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@end itemize
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@node Coding guidelines
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@chapter Coding guidelines
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@itemize @bullet
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@item
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We are using @uref{http://www.bitkeeper.com/, BitKeeper} for source management.
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@item
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You should use the @strong{MySQL} 3.23 or 4.0 source for all developments.
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@item
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If you have any questions about the @strong{MySQL} source, you can post these
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to @email{developers@@mysql.com} and we will answer them.
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Note that we will shortly change the name of this list to
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@email{internals@@mysql.com}, to more accurately reflect what should be
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posted to this list.
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@item
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Try to write code in a lot of black boxes that can be reused or at
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least have a clean interface.
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@item
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Reuse code; There is already a lot of algorithms in MySQL for list handling,
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queues, dynamic and hashed arrays, sorting, etc. that can be reused.
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@item
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Try to always write optimized code, so that you don't have to
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go back and rewrite it a couple of months later. It's better to
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spend 3 times as much time designing and writing an optimal function than
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having to do it all over again later on.
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@item
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Avoid CPU wasteful code, even where it does not matter, so that
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you will not develop sloppy coding habits.
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@item
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If you can write it in fewer lines, do it (as long as the code will not
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be slower or much harder to read).
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@item
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Do not check the same pointer for @code{NULL} more than once.
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@item
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Use long function and variable names in English; This makes your
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code easier to read.
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@item
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Think assembly - make it easier for the compiler to optimize your code.
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@item
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Comment your code when you do something that someone else may think
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is not ''trivial''.
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@item
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Use the @code{my_*} functions like @code{my_read()}/@code{my_write()}/
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@code{my_malloc()} that you can find in the @code{mysys} library instead
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of the direct system calls; This will make your code easier to debug and
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more portable.
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@item
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Use @code{libstring} functions instead of standard libc string functions
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whenever possible.
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@item
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Avoid using @code{malloc()} (its REAL slow); For memory allocations
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that only need to live for the lifetime of one thread, one should use
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@code{sql_alloc()} instead.
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@item
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Before making big design decisions, please first post a summary of
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what you want to do, why you want to do it, and how you plan to do
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it. This way we can easily provide you with feedback and also
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easily discuss it thoroughly if some other developer thinks there is better
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way to do the same thing!
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@item
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Use my_var as opposed to myVar or MyVar (@samp{_} rather than dancing SHIFT
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to seperate words in identifiers).
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@item
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Class names start with a capital letter.
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@item
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Structure types are @code{typedef}'ed to an all-caps identifier.
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@item
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Any @code{#define}'s are in all-caps.
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@item
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Matching @samp{@{} are in the same column.
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@item
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Functions return 0 on success, and non-zero on error, so you can do:
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@example
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if(a() || b() || c()) { error("something went wrong"); }
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@end example
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@item
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Using @code{goto} is okay if not abused.
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@item
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Avoid default variable initalizations, use @code{LINT_INIT()} if the
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compiler complains after making sure that there is really no way
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the variable can be used uninitialized.
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@item
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Do not instantiate a class if you do not have to.
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@item
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Use pointers rather than array indexing when operating on strings.
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@end itemize
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@c The Index was empty, and ugly, so I removed it. (jcole, Sep 7, 2000)
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@c @node Index
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@c @unnumbered Index
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@c @printindex fn
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@summarycontents
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@contents
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@bye
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