mariadb/Docs/internals.texi

\input texinfo  @c -*-texinfo-*-
@c Copyright 2002 MySQL AB, TcX AB, Detron HB and Monty Program KB
@c
@c %**start of header
@setfilename internals.info

@c We want the types in the same index
@synindex cp fn

@iftex
@afourpaper
@end iftex

@c Get version and other info
@include include.texi

@ifclear tex-debug
@c This removes the black squares in the right margin
@finalout
@end ifclear

@c Set background for HTML
@set _body_tags BGCOLOR=#FFFFFF TEXT=#000000 LINK=#101090 VLINK=#7030B0
@settitle @strong{MySQL} Internals Manual for version @value{mysql_version}.
@setchapternewpage odd
@paragraphindent 0

@c %**end of header

@ifinfo
@format
START-INFO-DIR-ENTRY
* mysql-internals: (mysql-internals).               @strong{MySQL} internals.
END-INFO-DIR-ENTRY
@end format
@end ifinfo

@titlepage
@sp 10
@center @titlefont{@strong{MySQL} Internals Manual}
@sp 10
@center Copyright @copyright{} 1998-2002 MySQL AB
@page
@end titlepage

@node Top, caching, (dir), (dir)

@ifinfo
This is a manual about @strong{MySQL} internals.
@end ifinfo

@menu
* caching::                     How MySQL Handles Caching
* flush tables::                How MySQL Handles @code{FLUSH TABLES}
* filesort::                    How MySQL Does Sorting (@code{filesort})
* coding guidelines::           Coding Guidelines
* mysys functions::             Functions In The @code{mysys} Library
* DBUG::                        DBUG Tags To Use
* protocol::                    MySQL Client/Server Protocol
* Fulltext Search::             Fulltext Search in MySQL
@end menu


@node caching, flush tables, Top, Top
@chapter How MySQL Handles Caching

@strong{MySQL} has the following caches:
(Note that the some of the filename have a wrong spelling of cache. :)

@table @strong

@item Key Cache
A shared cache for all B-tree index blocks in the different NISAM
files. Uses hashing and reverse linked lists for quick caching of the
last used blocks and quick flushing of changed entries for a specific
table. (@file{mysys/mf_keycash.c})

@item Record Cache
This is used for quick scanning of all records in a table.
(@file{mysys/mf_iocash.c} and @file{isam/_cash.c})

@item Table Cache
This holds the last used tables. (@file{sql/sql_base.cc})

@item Hostname Cache
For quick lookup (with reverse name resolving). Is a must when one has a
slow DNS. 
(@file{sql/hostname.cc})

@item Privilege Cache
To allow quick change between databases the last used privileges are
cached for each user/database combination.
(@file{sql/sql_acl.cc})

@item Heap Table Cache
Many use of @code{GROUP BY} or @code{DISTINCT} caches all found rows in 
a @code{HEAP} table. (This is a very quick in-memory table with hash index.)

@item Join Row Cache
For every full join in a @code{SELECT} statement (a full join here means
there were no keys that one could use to find the next table in a list), 
the found rows are cached in a join cache.  One @code{SELECT} query can 
use many join caches in the worst case.
@end table


@node flush tables, filesort, caching, Top
@chapter How MySQL Handles @code{FLUSH TABLES}

@itemize @bullet

@item
Flush tables is handled in @file{sql/sql_base.cc::close_cached_tables()}.

@item
The idea of flush tables is to force all tables to be closed. This
is mainly to ensure that if someone adds a new table outside of
@strong{MySQL} (for example with @code{cp}) all threads will start using 
the new table. This will also ensure that all table changes are flushed 
to disk (but of course not as optimally as simple calling a sync on
all tables)!

@item
When one does a @code{FLUSH TABLES}, the variable @code{refresh_version} 
will be incremented. Every time a thread releases a table it checks if
the refresh version of the table (updated at open) is the same as
the current @code{refresh_version}.  If not it will close it and broadcast
a signal on @code{COND_refresh} (to wait any thread that is waiting for
all instanses of a table to be closed).

@item
The current @code{refresh_version} is also compared to the open 
@code{refresh_version} after a thread gets a lock on a table.  If the 
refresh version is different the thread will free all locks, reopen the
table and try to get the locks again;  This is just to quickly get all 
tables to use the newest version.  This is handled by
@file{sql/lock.cc::mysql_lock_tables()} and 
@file{sql/sql_base.cc::wait_for_tables()}.

@item
When all tables has been closed @code{FLUSH TABLES} will return an ok 
to client.

@item
If the thread that is doing @code{FLUSH TABLES} has a lock on some tables,
it will first close the locked tables, then wait until all other threads
have also closed them, and then reopen them and get the locks.
After this it will give other threads a chance to open the same tables.

@end itemize

@node filesort, coding guidelines, flush tables, Top
@chapter How MySQL Does Sorting (@code{filesort})

@itemize @bullet

@item
Read all rows according to key or by table scanning.

@item
Store the sort-key in a buffer (@code{sort_buffer}).

@item
When the buffer gets full, run a @code{qsort} on it and store the result
in a temporary file.  Save a pointer to the sorted block.

@item
Repeat the above until all rows have been read.

@item
Repeat the following until there is less than @code{MERGEBUFF2} (15) 
blocks left.

@item
Do a multi-merge of up to @code{MERGEBUFF} (7) regions to one block in
another temporary file.  Repeat until all blocks from the first file
are in the second file.

@item
On the last multi-merge, only the pointer to the row (last part of
the sort-key) is written to a result file.

@item
Now the code in @file{sql/records.cc} will be used to read through them
in sorted order by using the row pointers in the result file.
To optimize this, we read in a big block of row pointers, sort these
and then we read the rows in the sorted order into a row buffer
(@code{record_buffer}).

@end itemize


@node coding guidelines, mysys functions, filesort, Top
@chapter Coding Guidelines

@itemize @bullet

@item
We are using @uref{http://www.bitkeeper.com/, BitKeeper} for source management.

@item
You should use the @strong{MySQL} 4.0 source for all developments.

@item
If you have any questions about the @strong{MySQL} source, you can post these
to @email{dev-public@@mysql.com} and we will answer them.  Please
remember to not use this internal email list in public!

@item
Try to write code in a lot of black boxes that can be reused or use at
least a clean, easy to change interface.

@item
Reuse code;  There is already a lot of algorithms in MySQL for list handling,
queues, dynamic and hashed arrays, sorting, etc. that can be reused.

@item
Use the @code{my_*} functions like @code{my_read()}/@code{my_write()}/
@code{my_malloc()} that you can find in the @code{mysys} library instead 
of the direct system calls;  This will make your code easier to debug and 
more portable.

@item
Try to always write optimized code, so that you don't have to
go back and rewrite it a couple of months later.  It's better to
spend 3 times as much time designing and writing an optimal function than
having to do it all over again later on.

@item
Avoid CPU wasteful code, even where it does not matter, so that
you will not develop sloppy coding habits.

@item
If you can write it in fewer lines, do it (as long as the code will not
be slower or much harder to read).

@item
Don't use two commands on the same line.

@item
Do not check the same pointer for @code{NULL} more than once.

@item
Use long function and variable names in English.  This makes your code
easier to read. 

@item
Use @code{my_var} as opposed to @code{myVar} or @code{MyVar} (@samp{_} 
rather than dancing SHIFT to seperate words in identifiers).

@item
Think assembly - make it easier for the compiler to optimize your code.

@item
Comment your code when you do something that someone else may think
is not ``trivial''.

@item
Use @code{libstring} functions (in the @file{strings} directory)
instead of standard @code{libc} string functions whenever possible.

@item
Avoid using @code{malloc()} (its REAL slow);  For memory allocations 
that only need to live for the lifetime of one thread, one should use
@code{sql_alloc()} instead.

@item
Before making big design decisions, please first post a summary of
what you want to do, why you want to do it, and how you plan to do
it.  This way we can easily provide you with feedback and also
easily discuss it thoroughly if some other developer thinks there is better
way to do the same thing!

@item
Class names start with a capital letter.

@item
Structure types are @code{typedef}'ed to an all-caps identifier.

@item
Any @code{#define}'s are in all-caps.

@item
Matching @samp{@{} are in the same column.

@item
Put the @samp{@{} after a @code{switch} on the same line, as this gives 
better overall indentation for the switch statement:

@example
switch (arg) @{
@end example

@item
In all other cases, @samp{@{} and @samp{@}} should be on their own line, except
if there is nothing inside @samp{@{} and @samp{@}}.

@item
Have a space after @code{if}

@item
Put a space after @samp{,} for function arguments

@item
Functions return @samp{0} on success, and non-zero on error, so you can do:

@example
if(a() || b() || c()) @{ error("something went wrong"); @}
@end example

@item
Using @code{goto} is okay if not abused.

@item
Avoid default variable initalizations, use @code{LINT_INIT()} if the
compiler complains after making sure that there is really no way
the variable can be used uninitialized.

@item
Do not instantiate a class if you do not have to.

@item
Use pointers rather than array indexing when operating on strings.

@end itemize

Suggested mode in emacs:

@example
(load "cc-mode")
(setq c-mode-common-hook '(lambda ()
			    (turn-on-font-lock)
			    (setq comment-column 48)))
(setq c-style-alist
      (cons
       '("MY"
	 (c-basic-offset . 2)
	 (c-comment-only-line-offset . 0)
	 (c-offsets-alist . ((statement-block-intro . +)
			     (knr-argdecl-intro . 0)
			     (substatement-open . 0)
			     (label . -)
			     (statement-cont . +)
			     (arglist-intro . c-lineup-arglist-intro-after-paren)
			     (arglist-close . c-lineup-arglist)
			     ))
	 )
       c-style-alist))
(c-set-style "MY")
(setq c-default-style "MY")
@end example


@node mysys functions, DBUG, coding guidelines, Top
@chapter Functions In The @code{mysys} Library

Functions in @code{mysys}: (For flags see @file{my_sys.h})

@table @code
@item int my_copy _A((const char *from, const char *to, myf MyFlags));
Copy file from @code{from} to @code{to}.

@item int my_delete _A((const char *name, myf MyFlags));
Delete file @code{name}.

@item int my_getwd _A((string buf, uint size, myf MyFlags));
@item int my_setwd _A((const char *dir, myf MyFlags));
Get and set working directory.

@item string my_tempnam _A((const char *pfx, myf MyFlags));
Make a unique temporary file name by using dir and adding something after
@code{pfx} to make name unique.  The file name is made by adding a unique
six character string and @code{TMP_EXT} after @code{pfx}.
Returns pointer to @code{malloc()}'ed area for filename. Should be freed by
@code{free()}.

@item File my_open _A((const char *FileName,int Flags,myf MyFlags));
@item File my_create _A((const char *FileName, int CreateFlags, int AccsesFlags, myf MyFlags));
@item int my_close _A((File Filedes, myf MyFlags));
@item uint my_read _A((File Filedes, byte *Buffer, uint Count, myf MyFlags));
@item uint my_write _A((File Filedes, const byte *Buffer, uint Count, myf MyFlags));
@item ulong my_seek _A((File fd,ulong pos,int whence,myf MyFlags));
@item ulong my_tell _A((File fd,myf MyFlags));
Use instead of open, open-with-create-flag, close, read, and write
to get automatic error messages (flag @code{MYF_WME}) and only have
to test for != 0 if error (flag @code{MY_NABP}).

@item int my_rename _A((const char *from, const char *to, myf MyFlags));
Rename file from @code{from} to @code{to}.

@item FILE *my_fopen _A((const char *FileName,int Flags,myf MyFlags));
@item FILE *my_fdopen _A((File Filedes,int Flags,myf MyFlags));
@item int my_fclose _A((FILE *fd,myf MyFlags));
@item uint my_fread _A((FILE *stream,byte *Buffer,uint Count,myf MyFlags));
@item uint my_fwrite _A((FILE *stream,const byte *Buffer,uint Count, myf MyFlags));
@item ulong my_fseek _A((FILE *stream,ulong pos,int whence,myf MyFlags));
@item ulong my_ftell _A((FILE *stream,myf MyFlags));
Same read-interface for streams as for files.

@item gptr _mymalloc _A((uint uSize,const char *sFile,uint uLine, myf MyFlag));
@item gptr _myrealloc _A((string pPtr,uint uSize,const char *sFile,uint uLine, myf MyFlag));
@item void _myfree _A((gptr pPtr,const char *sFile,uint uLine));
@item int _sanity _A((const char *sFile,unsigned int uLine));
@item gptr _myget_copy_of_memory _A((const byte *from,uint length,const char *sFile, uint uLine,myf MyFlag));
@code{malloc(size,myflag)} is mapped to these functions if not compiled 
with @code{-DSAFEMALLOC}.

@item void TERMINATE _A((void));
Writes @code{malloc()} info on @code{stdout} if compiled with 
@code{-DSAFEMALLOC}.

@item int my_chsize _A((File fd, ulong newlength, myf MyFlags));
Change size of file @code{fd} to @code{newlength}.

@item void my_error _D((int nr, myf MyFlags, ...));
Writes message using error number (see @file{mysys/errors.h}) on @code{stdout},
or using curses, if @code{MYSYS_PROGRAM_USES_CURSES()} has been called.

@item void my_message _A((const char *str, myf MyFlags));
Writes @code{str} on @code{stdout}, or using curses, if 
@code{MYSYS_PROGRAM_USES_CURSES()} has been called.

@item void my_init _A((void ));
Start each program (in @code{main()}) with this.

@item void my_end _A((int infoflag));
Gives info about program.
If @code{infoflag & MY_CHECK_ERROR}, prints if some files are left open.
If @code{infoflag & MY_GIVE_INFO}, prints timing info and malloc info
about program.

@item int my_redel _A((const char *from, const char *to, int MyFlags));
Delete @code{from} before rename of @code{to} to @code{from}.  Copies state
from old file to new file. If @code{MY_COPY_TIME} is set, sets old time.

@item int my_copystat _A((const char *from, const char *to, int MyFlags));
Copy state from old file to new file.  If @code{MY_COPY_TIME} is set,
sets old time.

@item string my_filename _A((File fd));
Returns filename of open file.

@item int dirname _A((string to, const char *name));
Copy name of directory from filename.

@item int test_if_hard_path _A((const char *dir_name));
Test if @code{dir_name} is a hard path (starts from root).

@item void convert_dirname _A((string name));
Convert dirname according to system.
In MSDOS, changes all characters to capitals and changes @samp{/} to @samp{\}.

@item string fn_ext _A((const char *name));
Returns pointer to extension in filename.

@item string fn_format _A((string to,const char *name,const char *dsk,const char *form,int flag));
	format a filename with replace of library and extension and
	converts between different systems.
	params to and name may be identicall
	function dosn't change name if name != to
	Flag may be:	1   force replace filnames library with 'dsk'
			2   force replace extension with 'form' */
			4   force Unpack filename (replace ~ with home)
			8   Pack filename as short as possibly for output to
			    user.
	All open requests should allways use at least:
	"open(fn_format(temp_buffe,name,"","",4),...)" to unpack home and
	convert filename to system-form.

@item string fn_same _A((string toname, const char *name, int flag));
Copys directory and extension from @code{name} to @code{toname} if neaded.
Copying can be forced by same flags used in @code{fn_format()}.

@item int wild_compare _A((const char *str, const char *wildstr));
Compare if @code{str} matches @code{wildstr}. @code{wildstr} can contain 
@samp{*} and @samp{?} as wildcard characters.
Returns 0 if @code{str} and @code{wildstr} match.

@item void get_date _A((string to, int timeflag));
Get current date in a form ready for printing.

@item void soundex _A((string out_pntr, string in_pntr))
Makes @code{in_pntr} to a 5 char long string.  All words that sound
alike have the same string.

@item int init_key_cache _A((ulong use_mem, ulong leave_this_much_mem));
Use caching of keys in MISAM, PISAM, and ISAM.
@code{KEY_CACHE_SIZE} is a good size.
Remember to lock databases for optimal caching.

@item void end_key_cache _A((void));
End key caching.
@end table 



@node DBUG, protocol, mysys functions, Top
@chapter DBUG Tags To Use

Here is some of the tags we now use:
(We should probably add a couple of new ones)

@table @code 
@item enter
Arguments to the function.

@item exit
Results from the function.

@item info
Something that may be interesting.

@item warning
When something doesn't go the usual route or may be wrong.

@item error
When something went wrong.

@item loop
Write in a loop, that is probably only useful when debugging
the loop.  These should normally be deleted when one is
satisfied with the code and it has been in real use for a while.
@end table

Some specific to mysqld, because we want to watch these carefully:

@table @code
@item trans
Starting/stopping transactions.

@item quit
@code{info} when mysqld is preparing to die.

@item query
Print query.
@end table


@node protocol, Fulltext Search, DBUG, Top
@chapter MySQL Client/Server Protocol

@menu
* raw packet without compression::  
* raw packet with compression::  
* basic packets::               
* communication::               
* fieldtype codes::             
@end menu

@node raw packet without compression, raw packet with compression, protocol, protocol
@section Raw Packet Without Compression

@example
+-----------------------------------------------+
| Packet Length | Packet no     | Data          |
| 3 Bytes       | 1 Byte        | n Bytes       |
+-----------------------------------------------+
@end example

@table @asis
@item 3 Byte packet length
The length is calculated with int3store
See include/global.h for details.
The max packetsize can be 16 MB.

@item 1 Byte packet no
If no compression is used the first 4 bytes of each packet is the header 
of the packet. The packet number is incremented for each sent packet.
The first packet starts with 0.
@item n Byte data 

@end table

The packet length can be recalculated with:

@example
length = byte1 + (256 * byte2) + (256 * 256 * byte3)
@end example


@node raw packet with compression, basic packets, raw packet without compression, protocol
@section Raw Packet With Compression

@example
+---------------------------------------------------+
| Packet Length | Packet no | Uncomp. Packet Length |
| 3 Bytes       | 1 Byte    | 3 Bytes               |
+---------------------------------------------------+
@end example

@table @asis
@item 3 Byte packet length
The length is calculated with int3store
See include/global.h for details.
The max packetsize can be 16 MB.

@item 1 Byte packet no
@item 3 Byte uncompressed packet length
@end table

If compression is used the first 7 bytes of each packet
is the header of the packet.


@node basic packets, communication, raw packet with compression, protocol
@section Basic Packets

@menu
* ok packet::                   
* error packet::                
@end menu


@node ok packet, error packet, basic packets, basic packets
@subsection OK Packet

For details, see @file{sql/net_pkg.cc::send_ok()}.

@example
+-----------------------------------------------+
| Header        | No of Rows    | Affected Rows |
|               | 1 Byte        | 1-8 Byte      |
|-----------------------------------------------|
| ID (last_insert_id)   | Status | Length       |
| 1-8 Byte              | 2 Byte | 1-8 Byte     |
|-----------------------------------------------|
| Messagetext                                   |
| n Byte                                        |
+-----------------------------------------------+
@end example

@table @asis
@item Header
@item 1 byte number of rows ? (always 0 ?)
@item 1-8 bytes affected rows
@item 1-8 byte id (last_insert_id) 
@item 2 byte Status (usually 0)
@item If the OK-packege includes a message:
@item 1-8 bytes length of message
@item n bytes messagetext
@end table


@node error packet,  , ok packet, basic packets
@subsection Error Packet	

@example
+-----------------------------------------------+
| Header        | Status code   | Error no      |
|               | 1 Byte        | 2 Byte        |
|-----------------------------------------------|
| Messagetext                          | 0x00   |
| n Byte                               | 1 Byte |
+-----------------------------------------------+
@end example

@table @asis
@item Header
@item 1 byte status code (0xFF = ERROR)
@item 2 byte error number (is only sent to new 3.23 clients.
@item n byte errortext
@item 1 byte 0x00
@end table


@node communication, fieldtype codes, basic packets, protocol
@section Communication

> Packet from server to client
< Paket from client tor server

	Login
	------
		> 1. packet	
		Header
		1 byte protocolversion
		n byte serverversion
		1 byte 0x00
		4 byte threadnumber 
		8 byte crypt seed 
		1 byte 0x00
		2 byte CLIENT_xxx options (see include/mysql_com.h
			that is supported by the server
		1 byte number of current server charset
		2 byte server status variables (SERVER_STATUS_xxx flags)
		13 byte 0x00 (not used yet).

		< 2. packet	
		Header
		2 byte CLIENT_xxx options
		3 byte max_allowed_packet for the client
		n byte username
		1 byte 0x00
		8 byte crypted password
		1 byte 0x00
		n byte databasename
		1 byte 0x00 

		> 3. packet	
		OK-packet


	Command
	--------
		< 1. packet	
		Header
		1 byte command type (e.g.0x03 = query)
		n byte query

	Result set (after command)
	--------------------------
		> 2. packet	
		Header
		1-8 byte field_count (packed with net_store_length())
		
		If field_count == 0 (command): 
		1-8 byte affected rows
		1-8 byte insert id
		2 bytes server_status (SERVER_STATUS_xx) 		
		
		If field_count == NULL_LENGTH (251)
		LOAD DATA LOCAL INFILE

		If field_count > 0 Result Set:

		> n packets	
		Header Info
		Column description: 5 data object /column
		(See code in unpack_fields())
		
		Columninfo for each column:
			1 data block table_name
			    1 byte length of block
			    n byte data
			1 data block field_name
			    1 byte length of block...
			    n byte data
			1 data block display length of field
			    1 byte length of block
			    3 bytes display length of filed
			1 data block type field of type (enum_field_types)
			    1 byte length of block
			    1 bytexs field of type
			1 data block flags
			    1 byte length of block
			    2 byte flags for the columns (NOT_NULL_FLAG, ZEROFILL_FLAG....)
			    1 byte decimals

		if table definition:
			1 data block default value

		Actual result (one packet per row):
		4 byte header
		1-8 byte length of data
		n data
		
 
@node fieldtype codes,  , communication, protocol
@section Fieldtype Codes

@example
                display_length  |enum_field_type        |flags
                ----------------------------------------------------
Blob            03 FF FF 00     |01 FC                  |03 90 00 00
Mediumblob      03 FF FF FF     |01 FC                  |03 90 00 00
Tinyblob        03 FF 00 00     |01 FC                  |03 90 00 00
Text            03 FF FF 00     |01 FC                  |03 10 00 00
Mediumtext      03 FF FF FF     |01 FC                  |03 10 00 00
Tinytext        03 FF 00 00     |01 FC                  |03 10 00 00
Integer         03 0B 00 00     |01 03                  |03 03 42 00
Mediumint       03 09 00 00     |01 09                  |03 00 00 00
Smallint        03 06 00 00     |01 02                  |03 00 00 00
Tinyint         03 04 00 00     |01 01                  |03 00 00 00
Varchar         03 XX 00 00     |01 FD                  |03 00 00 00
Enum            03 05 00 00     |01 FE                  |03 00 01 00
Datetime        03 13 00 00     |01 0C                  |03 00 00 00
Timestamp       03 0E 00 00     |01 07                  |03 61 04 00
Time            03 08 00 00     |01 0B                  |03 00 00 00
Date            03 0A 00 00     |01 0A                  |03 00 00 00
@end example

@c The Index was empty, and ugly, so I removed it. (jcole, Sep 7, 2000)

@c @node Index
@c @unnumbered Index

@c @printindex fn

@node Fulltext Search,  , protocol, Top
@chapter Fulltext Search in MySQL

Hopefully, sometime there will be complete description of
fulltext search algorithms.
Now it's just unsorted notes.

@menu
* Weighting in boolean mode::  
@end menu

@node Weighting in boolean mode, , , Fulltext Search
@section Weighting in boolean mode

The basic idea is as follows: in expression
@code{A or B or (C and D and E)}, either @code{A} or @code{B} alone
is enough to match the whole expression. While @code{C},
@code{D}, and @code{E} should @strong{all} match. So it's
reasonable to assign weight 1 to @code{A}, @code{B}, and
@code{(C and D and E)}. And @code{C}, @code{D}, and @code{E}
should get a weight of 1/3.

Things become more complicated when considering boolean
operators, as used in MySQL FTB. Obvioulsy, @code{+A +B}
should be treated as @code{A and B}, and @code{A B} -
as @code{A or B}. The problem is, that @code{+A B} can @strong{not}
be rewritten in and/or terms (that's the reason why this - extended -
set of operators was chosen). Still, aproximations can be used.
@code{+A B C} can be approximated as @code{A or (A and (B or C))}
or as @code{A or (A and B) or (A and C) or (A and B and C)}.
Applying the above logic (and omitting mathematical
transformations and normalization) one gets that for
@code{+A_1 +A_2 ... +A_N B_1 B_2 ... B_M} the weights
should be: @code{A_i = 1/N}, @code{B_j=1} if @code{N==0}, and,
otherwise, in the first rewritting approach @code{B_j = 1/3},
and in the second one - @code{B_j = (1+(M-1)*2^M)/(M*(2^(M+1)-1))}.

The second expression gives somewhat steeper increase in total
weight as number of matched B's increases, because it assigns
higher weights to individual B's. Also the first expression in
much simplier. So it is the first one, that is implemented in MySQL.

@summarycontents
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@bye