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145 lines
4.2 KiB
Plaintext
145 lines
4.2 KiB
Plaintext
trgm - Trigram matching for PostgreSQL
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Introduction
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This module is sponsored by Delta-Soft Ltd., Moscow, Russia.
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The pg_trgm contrib module provides functions and index classes
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for determining the similarity of text based on trigram
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matching.
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Definitions
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Trigram (or Trigraph)
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A trigram is a set of three consecutive characters taken
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from a string. A string is considered to have two spaces
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prefixed and one space suffixed when determining the set
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of trigrams that comprise the string.
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eg. The set of trigrams in the word "cat" is " c", " ca",
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"at " and "cat".
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Public Functions
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real similarity(text, text)
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Returns a number that indicates how closely matches the two
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arguments are. A zero result indicates that the two words
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are completely dissimilar, and a result of one indicates that
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the two words are identical.
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real show_limit()
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Returns the current similarity threshold used by the '%'
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operator. This in effect sets the minimum similarity between
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two words in order that they be considered similar enough to
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be misspellings of each other, for example.
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real set_limit(real)
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Sets the current similarity threshold that is used by the '%'
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operator, and is returned by the show_limit() function.
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text[] show_trgm(text)
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Returns an array of all the trigrams of the supplied text
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parameter.
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Public Operators
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text % text (returns boolean)
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The '%' operator returns TRUE if its two arguments have a similarity
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that is greater than the similarity threshold set by set_limit(). It
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will return FALSE if the similarity is less than the current
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threshold.
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Public Index Operator Classes
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gist_trgm_ops
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The pg_trgm module comes with an index operator class that allows a
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developer to create an index over a text column for the purpose
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of very fast similarity searches.
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To use this index, the '%' operator must be used and an appropriate
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similarity threshold for the application must be set.
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eg.
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CREATE TABLE test_trgm (t text);
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CREATE INDEX trgm_idx ON test_trgm USING gist (t gist_trgm_ops);
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At this point, you will have an index on the t text column that you
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can use for similarity searching.
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eg.
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SELECT
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t,
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similarity(t, 'word') AS sml
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FROM
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test_trgm
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WHERE
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t % 'word'
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ORDER BY
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sml DESC, t;
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This will return all values in the text column that are sufficiently
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similar to 'word', sorted from best match to worst. The index will
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be used to make this a fast operation over very large data sets.
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Tsearch2 Integration
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Trigram matching is a very useful tool when used in conjunction
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with a text index created by the Tsearch2 contrib module. (See
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contrib/tsearch2)
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The first step is to generate an auxiliary table containing all
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the unique words in the Tsearch2 index:
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CREATE TABLE words AS SELECT word FROM
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stat('SELECT to_tsvector(''simple'', bodytext) FROM documents');
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Where 'documents' is a table that has a text field 'bodytext'
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that TSearch2 is used to search. The use of the 'simple' dictionary
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with the to_tsvector function, instead of just using the already
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existing vector is to avoid creating a list of already stemmed
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words. This way, only the original, unstemmed words are added
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to the word list.
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Next, create a trigram index on the word column:
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CREATE INDEX words_idx ON words USING gist(word gist_trgm_ops);
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or
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CREATE INDEX words_idx ON words USING gin(word gist_trgm_ops);
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Now, a SELECT query similar to the example above can be used to
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suggest spellings for misspelled words in user search terms. A
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useful extra clause is to ensure that the similar words are also
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of similar length to the misspelled word.
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Note: Since the 'words' table has been generated as a separate,
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static table, it will need to be periodically regenerated so that
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it remains up to date with the word list in the Tsearch2 index.
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Authors
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Oleg Bartunov <oleg@sai.msu.su>, Moscow, Moscow University, Russia
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Teodor Sigaev <teodor@sigaev.ru>, Moscow, Delta-Soft Ltd.,Russia
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Contributors
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Christopher Kings-Lynne wrote this README file
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References
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Tsearch2 Development Site
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http://www.sai.msu.su/~megera/postgres/gist/tsearch/V2/
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GiST Development Site
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http://www.sai.msu.su/~megera/postgres/gist/
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