12.5. 斷詞

Text search parsers are responsible for splitting raw document text into_tokens_and identifying each token’s type, where the set of possible types is defined by the parser itself. Note that a parser does not modify the text at all — it simply identifies plausible word boundaries. Because of this limited scope, there is less need for application-specific custom parsers than there is for custom dictionaries. At presentPostgreSQLprovides just one built-in parser, which has been found to be useful for a wide range of applications.

The built-in parser is namedpg_catalog.default. It recognizes 23 token types, shown inTable 12.1.

Table 12.1. Default Parser’s Token Types

Alias Description Example
asciiword Word, all ASCII letters elephant
word Word, all letters mañana
numword Word, letters and digits beta1
asciihword Hyphenated word, all ASCII up-to-date
hword Hyphenated word, all letters lógico-matemática
numhword Hyphenated word, letters and digits postgresql-beta1
hword_asciipart Hyphenated word part, all ASCII postgresqlin the contextpostgresql-beta1
hword_part Hyphenated word part, all letters lógicoormatemáticain the contextlógico-matemática
hword_numpart Hyphenated word part, letters and digits beta1in the contextpostgresql-beta1
email Email address foo@example.com
protocol Protocol head http://
url URL example.com/stuff/index.html
host Host example.com
url_path URL path /stuff/index.html, in the context of a URL
file File or path name /usr/local/foo.txt, if not within a URL
sfloat Scientific notation -1.234e56
float Decimal notation -1.234
int Signed integer -1234
uint Unsigned integer 1234
version Version number 8.3.0
tag XML tag <a href="dictionaries.html">
entity XML entity &amp;
blank Space symbols (any whitespace or punctuation not otherwise recognized)

Note

The parser’s notion of a“letter”is determined by the database’s locale setting, specificallylc_ctype. Words containing only the basic ASCII letters are reported as a separate token type, since it is sometimes useful to distinguish them. In most European languages, token typeswordandasciiwordshould be treated alike.

emaildoes not support all valid email characters as defined by RFC 5322. Specifically, the only non-alphanumeric characters supported for email user names are period, dash, and underscore.

It is possible for the parser to produce overlapping tokens from the same piece of text. As an example, a hyphenated word will be reported both as the entire word and as each component:

  1. SELECT alias, description, token FROM ts_debug('foo-bar-beta1');
  2. alias | description | token
  3. -----------------+------------------------------------------+---------------
  4. numhword | Hyphenated word, letters and digits | foo-bar-beta1
  5. hword_asciipart | Hyphenated word part, all ASCII | foo
  6. blank | Space symbols | -
  7. hword_asciipart | Hyphenated word part, all ASCII | bar
  8. blank | Space symbols | -
  9. hword_numpart | Hyphenated word part, letters and digits | beta1

This behavior is desirable since it allows searches to work for both the whole compound word and for components. Here is another instructive example:

  1. SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.html');
  2. alias | description | token
  3. ----------+---------------+------------------------------
  4. protocol | Protocol head | http://
  5. url | URL | example.com/stuff/index.html
  6. host | Host | example.com
  7. url_path | URL path | /stuff/index.html