Each type of server plugin has its own type-specific
structures and functions. The primary structure is the
type-specific plugin descriptor. This is pointed to by the
info member of the
st_mysql_plugin general plugin descriptor,
but has a structure determined by the requirements of the
plugin type.
For version-control purposes, the first member of the type-specific descriptor for every plugin type is expected to be the interface version for the type. This enables the server to check the type-specific version for every plugin no matter its type. The type-specific descriptor commonly includes callback functions and other information needed by the server to invoke the plugin properly.
The following discussion describes the full-text parser type-specific plugin descriptor.
For a full-text parser plugin, the type-specific descriptor
is an instance of the st_mysql_ftparser
structure in the plugin.h file:
struct st_mysql_ftparser
{
int interface_version;
int (*parse)(MYSQL_FTPARSER_PARAM *param);
int (*init)(MYSQL_FTPARSER_PARAM *param);
int (*deinit)(MYSQL_FTPARSER_PARAM *param);
};
As shown by the structure definition, the descriptor has an
interface version number and contains pointers to three
functions. The version is specified using a symbol of the
form
MYSQL_
(such as
(xxx_INTERFACE_VERSIONMYSQL_FTPARSER_INTERFACE_VERSION for
full-text parser plugins) The init and
deinit members should point to a function
or be set to 0 if the function is not needed. The
parse member must point to the function
that performs the parsing.
A full-text parser plugin is used in two different contexts, indexing and searching. In both contexts, the server calls the initialization and deinitialization functions at the beginning and end of processing each SQL statement that causes the plugin to be invoked. However, during statement processing, the server calls the main parsing function in context-specific fashion:
For indexing, the server calls the parser for each column value to be indexed.
For searching, the server calls the parser to parse the search string. The parser might also be called for rows processed by the statement. In natural language mode, there is no need for the server to call the parser. For boolean mode phrase searches or natural language searches with query expansion, the parser is used to parse column values for information that is not in the index. Also, if a boolean mode search is done for a column that has no
FULLTEXTindex, the built-in parser will be called. (Plugins are associated with specific indexes. If there is no index, no plugin is used.)
The plugin declaration in the general plugin descriptor has
init and deinit
members that point initialization and deinitialization
functions, and so does the type-specific plugin descriptor
to which it points. However, these pairs of functions have
different purposes and are invoked for different reasons:
For the plugin declaration in the general plugin descriptor, the initialization and deinitialization functions are invoked when the plugin is loaded and unloaded.
For the type-specific plugin descriptor, the initialization and deinitialization functions are invoked per SQL statement for which the plugin is used.
Each interface function named in the plugin descriptor
should return zero for success or nonzero for failure, and
each of them receives an argument that points to a
MYSQL_FTPARSER_PARAM structure containing
the parsing context. The structure has this definition:
typedef struct st_mysql_ftparser_param
{
int (*mysql_parse)(struct st_mysql_ftparser_param *,
char *doc, int doc_len);
int (*mysql_add_word)(struct st_mysql_ftparser_param *,
char *word, int word_len,
MYSQL_FTPARSER_BOOLEAN_INFO *boolean_info);
void *ftparser_state;
void *mysql_ftparam;
struct charset_info_st *cs;
char *doc;
int length;
int flags;
enum enum_ftparser_mode mode;
} MYSQL_FTPARSER_PARAM;
The structure members are used as follows:
mysql_parseA pointer to a callback function that invokes the server's built-in parser. Use this callback when the plugin acts as a front end to the built-in parser. That is, when the plugin parsing function is called, it should process the input to extract the text and pass the text to the
mysql_parsecallback.The first parameter for this callback function should be the
paramvalue itself:param->mysql_parse(param, ...);
A front end plugin can extract text and pass it all at once to the built-in parser, or it can extract and pass text to the built-in parser a piece at a time. However, in this case, the built-in parser treats the pieces of text as though there are implicit word breaks between them.
mysql_add_wordA pointer to a callback function that adds a word to a full-text index or to the list of search terms. Use this callback when the parser plugin replaces the built-in parser. That is, when the plugin parsing function is called, it should parse the input into words and invoke the
mysql_add_wordcallback for each word.The first parameter for this callback function should be the
paramvalue itself:param->mysql_add_word(param, ...);
ftparser_stateThis is a generic pointer. The plugin can set it to point to information to be used internally for its own purposes.
mysql_ftparamThis is set by the server. It is passed as the first argument to the
mysql_parseormysql_add_wordcallback.csA pointer to information about the character set of the text, or 0 if no information is available.
docA pointer to the text to be parsed.
lengthThe length of the text to be parsed, in bytes.
flagsParser flags. This is zero if there are no special flags. Currently, the only nonzero flag is
MYSQL_FTFLAGS_NEED_COPY, which means thatmysql_add_word()must save a copy of the word (that is, it cannot use a pointer to the word because the word is in a buffer that will be overwritten.)This flag might be set or reset by MySQL before calling the parser plugin, by the parser plugin itself, or by the
mysql_parse()function.modeThe parsing mode. This value will be one of the folowing constants:
MYSQL_FTPARSER_SIMPLE_MODEParse in fast and simple mode, which is used for indexing and for natural language queries. The parser should pass to the server only those words that should be indexed. If the parser uses length limits or a stopword list to determine which words to ignore, it should not pass such words to the server.
MYSQL_FTPARSER_WITH_STOPWORDSParse in stopword mode. This is used in boolean searches for phrase matching. The parser should pass all words to the server, even stopwords or words that are outside any normal length limits.
MYSQL_FTPARSER_FULL_BOOLEAN_INFOParse in boolean mode. This is used for parsing boolean query strings. The parser should recognize not only words but also boolean-mode operators and pass them to the server as tokens using the
mysql_add_wordcallback. To tell the server what kind of token is being passed, the plugin needs to fill in aMYSQL_FTPARSER_BOOLEAN_INFOstructure and pass a pointer to it.
If the parser is called in boolean mode, the
param->mode value will be
MYSQL_FTPARSER_FULL_BOOLEAN_INFO. The
MYSQL_FTPARSER_BOOLEAN_INFO structure
that the parser uses for passing token information to the
server looks like this:
typedef struct st_mysql_ftparser_boolean_info
{
enum enum_ft_token_type type;
int yesno;
int weight_adjust;
bool wasign;
bool trunc;
/* These are parser state and must be removed. */
byte prev;
byte *quot;
} MYSQL_FTPARSER_BOOLEAN_INFO;
The parser should fill in the structure members as follows:
typeThe token type. The following table shows the permissible types.
yesnoWhether the word must be present for a match to occur. 0 means that the word is optional but increases the match relevance if it is present. Values larger than 0 mean that the word must be present. Values smaller than 0 mean that the word must not be present.
weight_adjustA weighting factor that determines how much a match for the word counts. It can be used to increase or decrease the word's importance in relevance calculations. A value of zero indicates no weight adjustment. Values greater than or less than zero mean higher or lower weight, respectively. The examples at Section 11.9.2, “Boolean Full-Text Searches”, that use the
<and>operators illustrate how weighting works.wasignThe sign of the weighting factor. A negative value acts like the
~boolean-search operator, which causes the word's contribution to the relevance to be negative.truncWhether matching should be done as if the boolean-mode
*truncation operator had been given.
Plugins should not use the prev and
quot members of the
MYSQL_FTPARSER_BOOLEAN_INFO structure.