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v3.1

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Parser

Text search parsers are responsible for splitting raw document text into tokens and identifying each token's type, where the type set is defined by the parser itself. Note that a parser does not modify the text; it simply identifies plausible word boundaries. Because of this limit, there is less need for application-specific custom parsers than there is for custom dictionaries.

Currently, MogDB provides the following built-in parsers: pg_catalog.default for English configuration, and pg_catalog.ngram and pg_catalog.pound for full text search in texts containing Chinese, or both Chinese and English.

The built-in parser is named pg_catalog.default. It recognizes 23 token types, shown in Table 1.

Table 1 Token types of the default parser

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 mogdb-beta1
hword_asciipart Hyphenated word part, all ASCII mogdb in the context mogdb-beta1
hword_part Hyphenated word part, all letters lógico or matemática in the context lógico-matemática
hword_numpart Hyphenated word part, letters and digits beta1 in the context mogdb-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 -1.23E+56
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 &
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, specifically lc_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 types word and asciiword should be treated alike.

email does 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 identify overlapping tokens in the same piece of text. For example, a hyphenated word will be reported both as the entire word and as each component.

mogdb=# SELECT alias, description, token FROM ts_debug('english','foo-bar-beta1');
      alias      |               description                |     token
-----------------+------------------------------------------+---------------
 numhword        | Hyphenated word, letters and digits      | foo-bar-beta1
 hword_asciipart | Hyphenated word part, all ASCII          | foo
 blank           | Space symbols                            | -
 hword_asciipart | Hyphenated word part, all ASCII          | bar
 blank           | Space symbols                            | -
 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:

mogdb=# SELECT alias, description, token FROM ts_debug('english','http://example.com/stuff/index.html');
  alias   |  description  |            token
----------+---------------+------------------------------
 protocol | Protocol head | http://
 url      | URL           | example.com/stuff/index.html
 host     | Host          | example.com
 url_path | URL path      | /stuff/index.html

N-gram is a mechanical word segmentation method, and applies to no semantic Chinese segmentation scenarios. The N-gram segmentation method ensures the completeness of the segmentation. However, to cover all the possibilities, it adds unnecessary words to the index, resulting in a large number of index items. N-gram supports Chinese coding, including GBK and UTF-8, and has six built-in token types, as shown in Table 2.

Table 2 Token types

Alias Description
zh_words chinese words
en_word english word
numeric numeric data
alnum alnum string
grapsymbol graphic symbol
multisymbol multiple symbol

Pound segments words in a fixed format. It is used to segment to-be-parsed nonsense Chinese and English words that are separated by fixed separators. It supports Chinese encoding (including GBK and UTF8) and English encoding (including ASCII). Six built-in token types are available, as listed in Table 3. Five types of delimiters are supported, as shown in Table 4, and the default delimiter is #. The maximum length of a token is 256 characters.

Table 3 Token types

Alias Description
zh_words chinese words
en_word english word
numeric numeric data
alnum alnum string
grapsymbol graphic symbol
multisymbol multiple symbol

Table 4 Separator types

Separator Description
@ Special character
# Special character
$ Special character
% Special character
/ Special character
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