**
** xTokenize:
** This function is expected to tokenize the nText byte string indicated
-** by argument pText. pText may not be nul-terminated. The first argument
-** passed to this function is a pointer to an Fts5Tokenizer object returned
-** by an earlier call to xCreate().
+** by argument pText. pText may or may not be nul-terminated. The first
+** argument passed to this function is a pointer to an Fts5Tokenizer object
+** returned by an earlier call to xCreate().
+**
+** The second argument indicates the reason that FTS5 is requesting
+** tokenization of the supplied text. This is always one of the following
+** four values:
+**
+** <ul><li> <b>FTS5_TOKENIZE_DOCUMENT</b> - A document is being inserted into
+** or removed from the FTS table. The tokenizer is being invoked to
+** determine the set of tokens to add to (or delete from) the
+** FTS index.
+**
+** <li> <b>FTS5_TOKENIZE_QUERY</b> - A MATCH query is being executed
+** against the FTS index. The tokenizer is being called to tokenize
+** a bareword or quoted string specified as part of the query.
+**
+** <li> <b>(FTS5_TOKENIZE_QUERY | FTS5_TOKENIZE_PREFIX)</b> - Same as
+** FTS5_TOKENIZE_QUERY, except that the bareword or quoted string is
+** followed by a "*" character, indicating that the last token
+** returned by the tokenizer will be treated as a token prefix.
+**
+** <li> <b>FTS5_TOKENIZE_AUX</b> - The tokenizer is being invoked to
+** satisfy an fts5_api.xTokenize() request made by an auxiliary
+** function. Or an fts5_api.xColumnSize() request made by the same
+** on a columnsize=0 database.
+** </ul>
**
** For each token in the input string, the supplied callback xToken() must
** be invoked. The first argument to it should be a copy of the pointer
-** passed as the second argument to xTokenize(). The next two arguments
-** are a pointer to a buffer containing the token text, and the size of
-** the token in bytes. The 4th and 5th arguments are the byte offsets of
-** the first byte of and first byte immediately following the text from
+** passed as the second argument to xTokenize(). The third and fourth
+** arguments are a pointer to a buffer containing the token text, and the
+** size of the token in bytes. The 4th and 5th arguments are the byte offsets
+** of the first byte of and first byte immediately following the text from
** which the token is derived within the input.
**
+** The second argument passed to the xToken() callback ("tflags") should
+** normally be set to 0. The exception is if the tokenizer supports
+** synonyms. In this case see the discussion below for details.
+**
** FTS5 assumes the xToken() callback is invoked for each token in the
** order that they occur within the input text.
**
** may abandon the tokenization and return any error code other than
** SQLITE_OK or SQLITE_DONE.
**
+** SYNONYM SUPPORT
+**
+** Custom tokenizers may also support synonyms. Consider a case in which a
+** user wishes to query for a phrase such as "first place". Using the
+** built-in tokenizers, the FTS5 query 'first + place' will match instances
+** of "first place" within the document set, but not alternative forms
+** such as "1st place". In some applications, it would be better to match
+** all instances of "first place" or "1st place" regardless of which form
+** the user specified in the MATCH query text.
+**
+** There are several ways to approach this in FTS5:
+**
+** <ol><li> By mapping all synonyms to a single token. In this case, the
+** In the above example, this means that the tokenizer returns the
+** same token for inputs "first" and "1st". Say that token is in
+** fact "first", so that when the user inserts the document "I won
+** 1st place" entries are added to the index for tokens "i", "won",
+** "first" and "place". If the user then queries for '1st + place',
+** the tokenizer substitutes "first" for "1st" and the query works
+** as expected.
+**
+** <li> By adding multiple synonyms for a single term to the FTS index.
+** In this case, when tokenizing query text, the tokenizer may
+** provide multiple synonyms for a single term within the document.
+** FTS5 then queries the index for each synonym individually. For
+** example, faced with the query:
+**
+** <codeblock>
+** ... MATCH 'first place'</codeblock>
+**
+** the tokenizer offers both "1st" and "first" as synonyms for the
+** first token in the MATCH query and FTS5 effectively runs a query
+** similar to:
+**
+** <codeblock>
+** ... MATCH '(first OR 1st) place'</codeblock>
+**
+** except that, for the purposes of auxiliary functions, the query
+** still appears to contain just two phrases - "(first OR 1st)"
+** being treated as a single phrase.
+**
+** <li> By adding multiple synonyms for a single term to the FTS index.
+** Using this method, when tokenizing document text, the tokenizer
+** provides multiple synonyms for each token. So that when a
+** document such as "I won first place" is tokenized, entries are
+** added to the FTS index for "i", "won", "first", "1st" and
+** "place".
+**
+** This way, even if the tokenizer does not provide synonyms
+** when tokenizing query text (it should not - to do would be
+** inefficient), it doesn't matter if the user queries for
+** 'first + place' or '1st + place', as there are entires in the
+** FTS index corresponding to both forms of the first token.
+** </ol>
+**
+** Whether is is parsing document or query text, any call to xToken that
+** specifies a <i>tflags</i> argument with the FTS5_TOKEN_COLOCATED bit
+** is considered to supply a synonym for the previous token. For example,
+** when parsing the document "I won first place", a tokenizer that supports
+** synonyms would call xToken() 5 times, as follows:
+**
+** <codeblock>
+** xToken(pCtx, 0, "i", 1, 0, 1);
+** xToken(pCtx, 0, "won", 3, 2, 5);
+** xToken(pCtx, 0, "first", 5, 6, 11);
+** xToken(pCtx, FTS5_TOKEN_COLOCATED, "1st", 3, 6, 11);
+** xToken(pCtx, 0, "place", 5, 12, 17);
+**</codeblock>
+**
+** It is an error to specify the FTS5_TOKEN_COLOCATED flag the first time
+** xToken() is called. Multiple synonyms may be specified for a single token
+** by making multiple calls to xToken(FTS5_TOKEN_COLOCATED) in sequence.
+** There is no limit to the number of synonyms that may be provided for a
+** single token.
+**
+** In many cases, method (1) above is the best approach. It does not add
+** extra data to the FTS index or require FTS5 to query for multiple terms,
+** so it is efficient in terms of disk space and query speed. However, it
+** does not support prefix queries very well. If, as suggested above, the
+** token "first" is subsituted for "1st" by the tokenizer, then the query:
+**
+** <codeblock>
+** ... MATCH '1s*'</codeblock>
+**
+** will not match documents that contain the token "1st" (as the tokenizer
+** will probably not map "1s" to any prefix of "first").
+**
+** For full prefix support, method (3) may be preferred. In this case,
+** because the index contains entries for both "first" and "1st", prefix
+** queries such as 'fi*' or '1s*' will match correctly. However, because
+** extra entries are added to the FTS index, this method uses more space
+** within the database.
+**
+** Method (2) offers a midpoint between (1) and (3). Using this method,
+** a query such as '1s*' will match documents that contain the literal
+** token "1st", but not "first" (assuming the tokenizer is not able to
+** provide synonyms for prefixes). However, a non-prefix query like '1st'
+** will match against "1st" and "first". This method does not require
+** extra disk space, as no extra entries are added to the FTS index.
+** On the other hand, it may require more CPU cycles to run MATCH queries,
+** as separate queries of the FTS index are required for each synonym.
+**
+** When using methods (2) or (3), it is important that the tokenizer only
+** provide synonyms when tokenizing document text (method (2)) or query
+** text (method (3)), not both. Doing so will not cause any errors, but is
+** inefficient.
*/
typedef struct Fts5Tokenizer Fts5Tokenizer;
typedef struct fts5_tokenizer fts5_tokenizer;
void (*xDelete)(Fts5Tokenizer*);
int (*xTokenize)(Fts5Tokenizer*,
void *pCtx,
- int flags,
+ int flags, /* Mask of FTS5_TOKENIZE_* flags */
const char *pText, int nText,
int (*xToken)(
void *pCtx, /* Copy of 2nd argument to xTokenize() */
projects / thirdparty / sqlite.git / commitdiff
