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Diffstat (limited to 'doc/src/sgml/textsearch.sgml')
| -rw-r--r-- | doc/src/sgml/textsearch.sgml | 26 |
1 files changed, 25 insertions, 1 deletions
diff --git a/doc/src/sgml/textsearch.sgml b/doc/src/sgml/textsearch.sgml index 2da75955d0b..67e4901c921 100644 --- a/doc/src/sgml/textsearch.sgml +++ b/doc/src/sgml/textsearch.sgml @@ -264,7 +264,7 @@ SELECT 'fat & cow'::tsquery @@ 'a fat cat sat on a mat and ate a fat rat'::t text, any more than a <type>tsvector</type> is. A <type>tsquery</type> contains search terms, which must be already-normalized lexemes, and may combine multiple terms using AND, OR, NOT, and FOLLOWED BY operators. - (For details see <xref linkend="datatype-tsquery">.) There are + (For syntax details see <xref linkend="datatype-tsquery">.) There are functions <function>to_tsquery</>, <function>plainto_tsquery</>, and <function>phraseto_tsquery</> that are helpful in converting user-written text into a proper @@ -323,6 +323,8 @@ text @@ text at least one of its arguments must appear, while the <literal>!</> (NOT) operator specifies that its argument must <emphasis>not</> appear in order to have a match. + For example, the query <literal>fat & ! rat</> matches documents that + contain <literal>fat</> but not <literal>rat</>. </para> <para> @@ -377,6 +379,28 @@ SELECT phraseto_tsquery('the cats ate the rats'); then <literal>&</literal>, then <literal><-></literal>, and <literal>!</literal> most tightly. </para> + + <para> + It's worth noticing that the AND/OR/NOT operators mean something subtly + different when they are within the arguments of a FOLLOWED BY operator + than when they are not, because within FOLLOWED BY the exact position of + the match is significant. For example, normally <literal>!x</> matches + only documents that do not contain <literal>x</> anywhere. + But <literal>!x <-> y</> matches <literal>y</> if it is not + immediately after an <literal>x</>; an occurrence of <literal>x</> + elsewhere in the document does not prevent a match. Another example is + that <literal>x & y</> normally only requires that <literal>x</> + and <literal>y</> both appear somewhere in the document, but + <literal>(x & y) <-> z</> requires <literal>x</> + and <literal>y</> to match at the same place, immediately before + a <literal>z</>. Thus this query behaves differently from + <literal>x <-> z & y <-> z</>, which will match a + document containing two separate sequences <literal>x z</> and + <literal>y z</>. (This specific query is useless as written, + since <literal>x</> and <literal>y</> could not match at the same place; + but with more complex situations such as prefix-match patterns, a query + of this form could be useful.) + </para> </sect2> <sect2 id="textsearch-intro-configurations"> |