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Help Center/ GaussDB(DWS)/ SQL Syntax Reference/ SQL Syntax Reference (8.1.3.x)/ Data Types/ Text Search Types

Text Search Types

Updated on 2024-09-03 GMT+08:00

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GaussDB(DWS) offers tsvector and tsquery data types to support full text search. The tsvector type represents a document in a form optimized for text search. The tsquery type similarly represents a text query.

tsvector

The tsvector type represents a retrieval unit, usually a row of text fields in a database table or a combination of these fields.

A tsvector value is a sorted list of distinct lexemes, which are words that have been normalized to merge different variants of the same word. Sorting and duplicate-elimination are done automatically during input.

The to_tsvector function is used to parse and normalize a document string.

Use tsvector to segment a string into lexemes by space. The lexemes are sorted by letter and length. The following is an example:

    
       SELECT 'a fat cat sat on a mat and ate a fat rat'::tsvector;
                      tsvector                      
----------------------------------------------------
 'a' 'and' 'ate' 'cat' 'fat' 'mat' 'on' 'rat' 'sat'
(1 row)

To represent lexemes containing whitespace or punctuation, surround them with quotes:

    
       SELECT $$the lexeme '    ' contains spaces$$::tsvector;
                 tsvector                  
-------------------------------------------
 '    ' 'contains' 'lexeme' 'spaces' 'the'
(1 row)

If a string is enclosed in common single quotation marks, the single quotation marks (') and backslashes (\) embedded in the string must be double-written for escape.

    
       SELECT $$the lexeme 'Joe''s' contains a quote$$::tsvector;
                    tsvector                    
------------------------------------------------
 'Joe''s' 'a' 'contains' 'lexeme' 'quote' 'the'
(1 row)

Optionally, integer positions can be attached to lexemes:

    
       SELECT 'a:1 fat:2 cat:3 sat:4 on:5 a:6 mat:7 and:8 ate:9 a:10 fat:11 rat:12'::tsvector;
                                   tsvector                                    
-------------------------------------------------------------------------------
 'a':1,6,10 'and':8 'ate':9 'cat':3 'fat':2,11 'mat':7 'on':5 'rat':12 'sat':4
(1 row)

A position normally indicates the source word's location in the document. Positional information can be used for proximity ranking. Position values range from 1 to 16383. The default maximum value is 16383. Duplicate positions for the same lexeme are discarded.

Lexemes that have positions can further be labeled with a weight, which can be A, B, C, or D. D is the default weight. It is not displayed in the output:

    
       SELECT 'a:1A fat:2B,4C cat:5D'::tsvector;
          tsvector          
----------------------------
 'a':1A 'cat':5 'fat':2B,4C
(1 row)

Weights usually are used to reflect document structure, for example, by marking title words differently from body words. Text search ranking functions can assign different priorities to the different weight markers.

The following is an example of the standard usage of the tsvector type:

    
       SELECT 'The Fat Rats'::tsvector;
      tsvector      
--------------------
 'Fat' 'Rats' 'The'
(1 row)

For most English-text-searching applications the above words would be considered non-normalized, which should usually be passed through to_tsvector to normalize the words appropriately for searching:

    
       SELECT to_tsvector('english', 'The Fat Rats');
   to_tsvector   
-----------------
 'fat':2 'rat':3
(1 row)

tsquery

The tsquery type represents a retrieval condition. A tsquery value stores lexemes that are to be searched for, and combines them honoring the Boolean operators & (AND), | (OR), and ! (NOT). Parentheses can be used to enforce grouping of the operators. If there is no parenthesis, (NOT) has the highest priority, followed by &(AND), and finally | (OR). The to_tsquery and plainto_tsquery functions will normalize lexemes before the lexemes are converted to the tsquery type.

    
     
       
       SELECT 'fat & rat'::tsquery;
    tsquery    
---------------
 'fat' & 'rat'
(1 row)

SELECT 'fat & (rat | cat)'::tsquery;
          tsquery          
---------------------------
 'fat' & ( 'rat' | 'cat' )
(1 row)

SELECT 'fat & rat & ! cat'::tsquery;
        tsquery         
------------------------
 'fat' & 'rat' & !'cat'
(1 row)

      

    
   

Lexemes in a tsquery can be labeled with one or more weight letters, which match only tsvector lexemes with matching weights.

    
       SELECT 'fat:ab & cat'::tsquery;
     tsquery      
------------------
 'fat':AB & 'cat'
(1 row)

Also, lexemes in a tsquery can be labeled with * to specify prefix matching. The following query will match any word in a tsvector that begins with "super".

    
       SELECT 'super:*'::tsquery;
  tsquery  
-----------
 'super':*
(1 row)

Note that prefix matches are first checked by the text search analyzer. For example, the stem extracted from postgres is postgr, which matches postgraduate. The following result is true:

    
       SELECT to_tsvector( 'postgraduate' ) @@ to_tsquery( 'postgres:*' ) AS RESULT;
  result  
----------
 t
(1 row)

    
       SELECT to_tsquery('postgres:*');
 to_tsquery 
------------
 'postgr':*
(1 row)

The to_tsquery function normalizes words before converting them to the tsquery type. 'Fat:ab & Cats' is normalized to the tsquery type as follows:

    
       SELECT to_tsquery('Fat:ab & Cats');
    to_tsquery    
------------------
 'fat':AB & 'cat'
(1 row)

Parent topic: Data Types

Previous topic: Bit String Types

Next topic: UUID Type

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