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, openGauss 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

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 usernames 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.

openGauss=# 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:

openGauss=# 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

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

Table 4 Separator types