12.4. 延伸功能
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This section describes additional functions and operators that are useful in connection with text search.
showed how raw textual documents can be converted intotsvectorvalues.PostgreSQLalso provides functions and operators that can be used to manipulate documents that are already intsvectorform.
tsvector
||
tsvector
Thetsvectorconcatenation 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 performingto_tsvectoron 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 applyingto_tsvector, is that you can use different configurations to parse different sections of the document. Also, because thesetweightfunction marks all lexemes of the given vector the same way, it is necessary to parse the text and dosetweightbefore concatenating if you want to label different parts of the document with different weights.
setweight(
vector
tsvector
,
weight
"char"
) returns
tsvector
setweightreturns a copy of the input vector in which every position has been labeled with the givenweight, eitherA,B,C, orD. (Dis 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 thensetweightdoes nothing.
length(
vector
tsvector
) returns
integer
Returns the number of lexemes stored in the vector.
strip(
vector
tsvector
) returns
tsvector
Returns a vector that lists the same lexemes as the given vector, but lacks any position or weight information. The result is usually much smaller than an unstripped vector, but it is also less useful. Relevance ranking does not work as well on stripped vectors as unstripped ones. Also, the<->(FOLLOWED BY)tsqueryoperator will never match stripped input, since it cannot determine the distance between lexeme occurrences.
tsquery
&
&
tsquery
Returns the AND-combination of the two given queries.
tsquery
||
tsquery
Returns the OR-combination of the two given queries.
!!
tsquery
Returns the negation (NOT) of the given query.
tsquery
<
-
>
tsquery
Returns a query that searches for a match to the first given query immediately followed by a match to the second given query, using the<->(FOLLOWED BY)tsqueryoperator. For example:
tsquery_phrase(
query1
tsquery
,
query2
tsquery
[,
distance
integer
]) returns
tsquery
Returns a query that searches for a match to the first given query followed by a match to the second given query at a distance of atdistance_lexemes, using the<N_>tsqueryoperator. For example:
numnode(
query
tsquery
) returns
integer
Returns the number of nodes (lexemes plus operators) in atsquery. This function is useful to determine if the_query_is meaningful (returns > 0), or contains only stop words (returns 0). Examples:
querytree(
query
tsquery
) returns
text
Returns the portion of atsquerythat 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:
ts_rewrite (
query
tsquery
,
target
tsquery
,
substitute
tsquery
) returns
tsquery
This form ofts_rewritesimply applies a single rewrite rule:target_is replaced bysubstitutewherever it appears inquery_. For example:
ts_rewrite (
query
tsquery
,
select
text
) returns
tsquery
This form ofts_rewriteaccepts a startingquery_and a SQLselectcommand, which is given as a text string. Theselectmust yield two columns oftsquerytype. For each row of theselectresult, occurrences of the first column value (the target) are replaced by the second column value (the substitute) within the currentquery_value. For example:
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 toORDER BYsome ordering key.
Let's consider a real-life astronomical example. We'll expand querysupernovaeusing table-driven rewriting rules:
We can change the rewriting rules just by updating the table:
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 thetsquerytype. In the example below, we select only those rules which might match the original query:
When using a separate column to store thetsvectorrepresentation of your documents, it is necessary to create a trigger to update thetsvectorcolumn when the document content columns change. Two built-in trigger functions are available for this, or you can write your own.
These trigger functions automatically compute atsvectorcolumn from one or more textual columns, under the control of parameters specified in theCREATE TRIGGERcommand. An example of their use is:
Having created this trigger, any change intitleorbodywill automatically be reflected intotsv, without the application having to worry about it.
The first trigger argument must be the name of thetsvectorcolumn to be updated. The second argument specifies the text search configuration to be used to perform the conversion. Fortsvector_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 insearch_path. Fortsvector_update_trigger_column, the second trigger argument is the name of another table column, which must be of typeregconfig. This allows a per-row selection of configuration to be made. The remaining argument(s) are the names of textual columns (of typetext,varchar, orchar). 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 usingPL/pgSQLas the trigger language:
Keep in mind that it is important to specify the configuration name explicitly when creatingtsvectorvalues inside triggers, so that the column's contents will not be affected by changes todefault_text_search_config. Failure to do this is likely to lead to problems such as search results changing after a dump and reload.
The functionts_statis useful for checking your configuration and for finding stop-word candidates.
_sqlquery_is a text value containing an SQL query which must return a singletsvectorcolumn.ts_statexecutes the query and returns statistics about each distinct lexeme (word) contained in thetsvectordata. The columns returned are
wordtext— the value of a lexeme
ndocinteger— number of documents (tsvectors) the word occurred in
nentryinteger— 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:
The same, but counting only word occurrences with weightAorB:
A full list oftsvector-related functions is available in.
showed how raw textual queries can be converted intotsqueryvalues.PostgreSQLalso provides functions and operators that can be used to manipulate queries that are already intsqueryform.
Thets_rewritefamily of functions search a giventsqueryfor occurrences of a target subquery, and replace each occurrence with a substitute subquery. In essence this operation is atsquery-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 (). However, you can modify a set of rewrite rules on-the-fly without reindexing, whereas updating a thesaurus requires reindexing to be effective.