This section describes additional functions and operators that are useful in connection with text search.
Section 12.3.1 showed how raw textual documents can be converted into tsvector values. PostgreSQL also provides functions and operators that can be used to manipulate documents that are already in tsvector form.
The tsvector concatenation
operator returns a vector which combines the lexemes and
positional information of the two vectors given as
arguments. Positions and weight labels are retained
during the concatenation. Positions appearing in the
right-hand vector are offset by the largest position
mentioned in the left-hand vector, so that the result is
nearly equivalent to the result of performing
to_tsvector on the
concatenation of the two original document strings. (The
equivalence is not exact, because any stop-words removed
from the end of the left-hand argument will not affect
the result, whereas they would have affected the
positions of the lexemes in the right-hand argument if
textual concatenation were used.)
One advantage of using concatenation in the vector
form, rather than concatenating text before applying
to_tsvector, is that you
can use different configurations to parse different
sections of the document. Also, because the
setweight function marks all lexemes of
the given vector the same way, it is necessary to parse
the text and do
before concatenating if you want to label different parts
of the document with different weights.
setweight returns a copy
of the input vector in which every position has been
labeled with the given weight, either A, B, C, or D.
(D is the default for new
vectors and as such is not displayed on output.) These
labels are retained when vectors are concatenated,
allowing words from different parts of a document to be
weighted differently by ranking functions.
Note that weight labels apply to positions, not lexemes. If the input
vector has been stripped of positions then
setweight does nothing.
Returns the number of lexemes stored in the vector.
Returns a vector which lists the same lexemes as the given vector, but which lacks any position or weight information. While the returned vector is much less useful than an unstripped vector for relevance ranking, it will usually be much smaller.
Section 12.3.2 showed how raw textual queries can be converted into tsquery values. PostgreSQL also provides functions and operators that can be used to manipulate queries that are already in tsquery form.
Returns the AND-combination of the two given queries.
Returns the OR-combination of the two given queries.
Returns the negation (NOT) of the given query.
Returns the number of nodes (lexemes plus operators) in a tsquery. This function is useful to determine if the query is meaningful (returns > 0), or contains only stop words (returns 0). Examples:
SELECT numnode(plainto_tsquery('the any')); NOTICE: query contains only stopword(s) or doesn't contain lexeme(s), ignored numnode --------- 0 SELECT numnode('foo & bar'::tsquery); numnode --------- 3
Returns the portion of a tsquery that can be used for searching an index. This function is useful for detecting unindexable queries, for example those containing only stop words or only negated terms. For example:
SELECT querytree(to_tsquery('!defined')); querytree -----------
ts_rewrite family of
functions search a given tsquery for
occurrences of a target subquery, and replace each occurrence
with a substitute subquery. In essence this operation is a
tsquery-specific version of substring
replacement. A target and substitute combination can be
thought of as a query rewrite rule.
A collection of such rewrite rules can be a powerful search
aid. For example, you can expand the search using synonyms
(e.g., new york, big apple, nyc,
gotham) or narrow the search to
direct the user to some hot topic. There is some overlap in
functionality between this feature and thesaurus dictionaries
12.6.4). However, you can modify a set of rewrite rules
on-the-fly without reindexing, whereas updating a thesaurus
requires reindexing to be effective.
This form of
ts_rewrite simply applies a single
rewrite rule: target is
replaced by substitute
wherever it appears in query. For example:
SELECT ts_rewrite('a & b'::tsquery, 'a'::tsquery, 'c'::tsquery); ts_rewrite ------------ 'b' & 'c'
This form of
ts_rewrite accepts a starting
query and a SQL
select command, which
is given as a text string. The select must yield two columns of
tsquery type. For each row of the
occurrences of the first column value (the target) are
replaced by the second column value (the substitute)
within the current query value. For example:
CREATE TABLE aliases (t tsquery PRIMARY KEY, s tsquery); INSERT INTO aliases VALUES('a', 'c'); SELECT ts_rewrite('a & b'::tsquery, 'SELECT t,s FROM aliases'); ts_rewrite ------------ 'b' & 'c'
Note that when multiple rewrite rules are applied in this way, the order of application can be important; so in practice you will want the source query to ORDER BY some ordering key.
Let's consider a real-life astronomical example. We'll expand query supernovae using table-driven rewriting rules:
CREATE TABLE aliases (t tsquery primary key, s tsquery); INSERT INTO aliases VALUES(to_tsquery('supernovae'), to_tsquery('supernovae|sn')); SELECT ts_rewrite(to_tsquery('supernovae & crab'), 'SELECT * FROM aliases'); ts_rewrite --------------------------------- 'crab' & ( 'supernova' | 'sn' )
We can change the rewriting rules just by updating the table:
UPDATE aliases SET s = to_tsquery('supernovae|sn & !nebulae') WHERE t = to_tsquery('supernovae'); SELECT ts_rewrite(to_tsquery('supernovae & crab'), 'SELECT * FROM aliases'); ts_rewrite --------------------------------------------- 'crab' & ( 'supernova' | 'sn' & !'nebula' )
Rewriting can be slow when there are many rewriting rules, since it checks every rule for a possible match. To filter out obvious non-candidate rules we can use the containment operators for the tsquery type. In the example below, we select only those rules which might match the original query:
SELECT ts_rewrite('a & b'::tsquery, 'SELECT t,s FROM aliases WHERE ''a & b''::tsquery @> t'); ts_rewrite ------------ 'b' & 'c'
When using a separate column to store the tsvector representation of your documents, it is necessary to create a trigger to update the tsvector column when the document content columns change. Two built-in trigger functions are available for this, or you can write your own.
tsvector_update_trigger(tsvector_column_name, config_name, text_column_name [, ... ]) tsvector_update_trigger_column(tsvector_column_name, config_column_name, text_column_name [, ... ])
These trigger functions automatically compute a tsvector column from one or more textual columns, under the control of parameters specified in the CREATE TRIGGER command. An example of their use is:
CREATE TABLE messages ( title text, body text, tsv tsvector ); CREATE TRIGGER tsvectorupdate BEFORE INSERT OR UPDATE ON messages FOR EACH ROW EXECUTE PROCEDURE tsvector_update_trigger(tsv, 'pg_catalog.english', title, body); INSERT INTO messages VALUES('title here', 'the body text is here'); SELECT * FROM messages; title | body | tsv ------------+-----------------------+---------------------------- title here | the body text is here | 'bodi':4 'text':5 'titl':1 SELECT title, body FROM messages WHERE tsv @@ to_tsquery('title & body'); title | body ------------+----------------------- title here | the body text is here
Having created this trigger, any change in title or body will automatically be reflected into tsv, without the application having to worry about it.
The first trigger argument must be the name of the
tsvector column to be updated. The second
argument specifies the text search configuration to be used to
perform the conversion. For
tsvector_update_trigger, the configuration
name is simply given as the second trigger argument. It must be
schema-qualified as shown above, so that the trigger behavior
will not change with changes in search_path. For
tsvector_update_trigger_column, the second
trigger argument is the name of another table column, which
must be of type regconfig. This allows a
per-row selection of configuration to be made. The remaining
argument(s) are the names of textual columns (of type
text, varchar, or
char). These will be included in the
document in the order given. NULL values will be skipped (but
the other columns will still be indexed).
A limitation of these built-in triggers is that they treat all the input columns alike. To process columns differently — for example, to weight title differently from body — it is necessary to write a custom trigger. Here is an example using PL/pgSQL as the trigger language:
CREATE FUNCTION messages_trigger() RETURNS trigger AS $$ begin new.tsv := setweight(to_tsvector('pg_catalog.english', coalesce(new.title,'')), 'A') || setweight(to_tsvector('pg_catalog.english', coalesce(new.body,'')), 'D'); return new; end $$ LANGUAGE plpgsql; CREATE TRIGGER tsvectorupdate BEFORE INSERT OR UPDATE ON messages FOR EACH ROW EXECUTE PROCEDURE messages_trigger();
Keep in mind that it is important to specify the configuration name explicitly when creating tsvector values inside triggers, so that the column's contents will not be affected by changes to default_text_search_config. Failure to do this is likely to lead to problems such as search results changing after a dump and reload.
ts_stat is useful
for checking your configuration and for finding stop-word
ts_stat(sqlquery text, [ weights text, ] OUT word text, OUT ndoc integer, OUT nentry integer) returns setof record
sqlquery is a text value
containing an SQL query which must return a single tsvector column.
ts_stat executes the query and returns
statistics about each distinct lexeme (word) contained in the
tsvector data. The columns returned
word text — the value of a lexeme
ndoc integer — number of documents (tsvectors) the word occurred in
nentry integer — total number of occurrences of the word
If weights is supplied, only occurrences having one of those weights are counted.
For example, to find the ten most frequent words in a document collection:
SELECT * FROM ts_stat('SELECT vector FROM apod') ORDER BY nentry DESC, ndoc DESC, word LIMIT 10;
The same, but counting only word occurrences with weight A or B:
SELECT * FROM ts_stat('SELECT vector FROM apod', 'ab') ORDER BY nentry DESC, ndoc DESC, word LIMIT 10;
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