diff options
| -rw-r--r-- | contrib/pg_trgm/trgm_op.c | 4 | ||||
| -rw-r--r-- | doc/src/sgml/pgtrgm.sgml | 56 |
2 files changed, 44 insertions, 16 deletions
diff --git a/contrib/pg_trgm/trgm_op.c b/contrib/pg_trgm/trgm_op.c index f7e96acc53c..306d60bd3bb 100644 --- a/contrib/pg_trgm/trgm_op.c +++ b/contrib/pg_trgm/trgm_op.c @@ -456,7 +456,7 @@ iterate_word_similarity(int *trg2indexes, lastpos[trgindex] = i; } - /* Adjust lower bound if this trigram is present in required substring */ + /* Adjust upper bound if this trigram is present in required substring */ if (found[trgindex]) { int prev_lower, @@ -473,7 +473,7 @@ iterate_word_similarity(int *trg2indexes, smlr_cur = CALCSML(count, ulen1, ulen2); - /* Also try to adjust upper bound for greater similarity */ + /* Also try to adjust lower bound for greater similarity */ tmp_count = count; tmp_ulen2 = ulen2; prev_lower = lower; diff --git a/doc/src/sgml/pgtrgm.sgml b/doc/src/sgml/pgtrgm.sgml index 338ef30fbcd..b5d893c9fbb 100644 --- a/doc/src/sgml/pgtrgm.sgml +++ b/doc/src/sgml/pgtrgm.sgml @@ -99,12 +99,10 @@ </entry> <entry><type>real</type></entry> <entry> - Returns a number that indicates how similar the first string - to the most similar word of the second string. The function searches in - the second string a most similar word not a most similar substring. The - range of the result is zero (indicating that the two strings are - completely dissimilar) to one (indicating that the first string is - identical to one of the words of the second string). + Returns a number that indicates the greatest similarity between + the set of trigrams in the first string and any continuous extent + of an ordered set of trigrams in the second string. For details, see + the explanation below. </entry> </row> <row> @@ -131,6 +129,34 @@ </tgroup> </table> + <para> + Consider the following example: + +<programlisting> +# SELECT word_similarity('word', 'two words'); + word_similarity +----------------- + 0.8 +(1 row) +</programlisting> + + In the first string, the set of trigrams is + <literal>{" w"," wo","ord","wor","rd "}</literal>. + In the second string, the ordered set of trigrams is + <literal>{" t"," tw",two,"wo "," w"," wo","wor","ord","rds", ds "}</literal>. + The most similar extent of an ordered set of trigrams in the second string + is <literal>{" w"," wo","wor","ord"}</literal>, and the similarity is + <literal>0.8</literal>. + </para> + + <para> + This function returns a value that can be approximately understood as the + greatest similarity between the first string and any substring of the second + string. However, this function does not add padding to the boundaries of + the extent. Thus, a whole word match gets a higher score than a match with + a part of the word. + </para> + <table id="pgtrgm-op-table"> <title><filename>pg_trgm</filename> Operators</title> <tgroup cols="3"> @@ -156,10 +182,11 @@ <entry><type>text</type> <literal><%</literal> <type>text</type></entry> <entry><type>boolean</type></entry> <entry> - Returns <literal>true</literal> if its first argument has the similar word in - the second argument and they have a similarity that is greater than the - current word similarity threshold set by - <varname>pg_trgm.word_similarity_threshold</varname> parameter. + Returns <literal>true</literal> if the similarity between the trigram + set in the first argument and a continuous extent of an ordered trigram + set in the second argument is greater than the current word similarity + threshold set by <varname>pg_trgm.word_similarity_threshold</varname> + parameter. </entry> </row> <row> @@ -302,10 +329,11 @@ SELECT t, word_similarity('<replaceable>word</replaceable>', t) AS sml WHERE '<replaceable>word</replaceable>' <% t ORDER BY sml DESC, t; </programlisting> - This will return all values in the text column that have a word - which sufficiently similar to <replaceable>word</replaceable>, sorted from best - match to worst. The index will be used to make this a fast operation - even over very large data sets. + This will return all values in the text column for which there is a + continuous extent in the corresponding ordered trigram set that is + sufficiently similar to the trigram set of <replaceable>word</replaceable>, + sorted from best match to worst. The index will be used to make this + a fast operation even over very large data sets. </para> <para> |