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Commitaea7c17

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Rework word_similarity documentation, make it close to actual algorithm.
word_similarity before claimed as returning similarity of closest word instring, but, actually it returns similarity of substring. Also fix mistypedcomments.Author: Alexander KorotkovReview by: David Steele, Liudmila MantrovaDiscussionis:https://www.postgresql.org/message-id/flat/CY4PR17MB13207ED8310F847CF117EED0D85A0@CY4PR17MB1320.namprd17.prod.outlook.comhttps://www.postgresql.org/message-id/flat/f43b242d-000c-f4c8-cb8b-d37e9752cd93%40postgrespro.ru
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‎contrib/pg_trgm/trgm_op.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -456,7 +456,7 @@ iterate_word_similarity(int *trg2indexes,
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lastpos[trgindex]=i;
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}
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/* Adjustlower bound if this trigram is present in required substring */
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/* Adjustupper bound if this trigram is present in required substring */
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if (found[trgindex])
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{
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intprev_lower,
@@ -473,7 +473,7 @@ iterate_word_similarity(int *trg2indexes,
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smlr_cur=CALCSML(count,ulen1,ulen2);
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/* Also try to adjustupper bound for greater similarity */
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/* Also try to adjustlower bound for greater similarity */
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tmp_count=count;
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tmp_ulen2=ulen2;
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prev_lower=lower;

‎doc/src/sgml/pgtrgm.sgml

Lines changed: 42 additions & 14 deletions
Original file line numberDiff line numberDiff line change
@@ -99,12 +99,10 @@
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</entry>
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<entry><type>real</type></entry>
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<entry>
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Returns a number that indicates how similar the first string
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to the most similar word of the second string. The function searches in
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the second string a most similar word not a most similar substring. The
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range of the result is zero (indicating that the two strings are
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completely dissimilar) to one (indicating that the first string is
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identical to one of the words of the second string).
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Returns a number that indicates the greatest similarity between
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the set of trigrams in the first string and any continuous extent
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of an ordered set of trigrams in the second string. For details, see
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the explanation below.
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</entry>
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</row>
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<row>
@@ -131,6 +129,34 @@
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</tgroup>
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</table>
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<para>
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Consider the following example:
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<programlisting>
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# SELECT word_similarity('word', 'two words');
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word_similarity
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-----------------
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0.8
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(1 row)
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</programlisting>
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In the first string, the set of trigrams is
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<literal>{" w"," wo","ord","wor","rd "}</literal>.
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In the second string, the ordered set of trigrams is
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<literal>{" t"," tw",two,"wo "," w"," wo","wor","ord","rds", ds "}</literal>.
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The most similar extent of an ordered set of trigrams in the second string
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is <literal>{" w"," wo","wor","ord"}</literal>, and the similarity is
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<literal>0.8</literal>.
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</para>
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<para>
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This function returns a value that can be approximately understood as the
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greatest similarity between the first string and any substring of the second
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string. However, this function does not add padding to the boundaries of
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the extent. Thus, a whole word match gets a higher score than a match with
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a part of the word.
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</para>
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<table id="pgtrgm-op-table">
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<title><filename>pg_trgm</filename> Operators</title>
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<tgroup cols="3">
@@ -156,10 +182,11 @@
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<entry><type>text</type> <literal>&lt;%</literal> <type>text</type></entry>
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<entry><type>boolean</type></entry>
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<entry>
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Returns <literal>true</literal> if its first argument has the similar word in
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the second argument and they have a similarity that is greater than the
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current word similarity threshold set by
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<varname>pg_trgm.word_similarity_threshold</varname> parameter.
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Returns <literal>true</literal> if the similarity between the trigram
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set in the first argument and a continuous extent of an ordered trigram
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set in the second argument is greater than the current word similarity
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threshold set by <varname>pg_trgm.word_similarity_threshold</varname>
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parameter.
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</entry>
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</row>
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<row>
@@ -302,10 +329,11 @@ SELECT t, word_similarity('<replaceable>word</replaceable>', t) AS sml
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WHERE '<replaceable>word</replaceable>' &lt;% t
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ORDER BY sml DESC, t;
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</programlisting>
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This will return all values in the text column that have a word
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which sufficiently similar to <replaceable>word</replaceable>, sorted from best
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match to worst. The index will be used to make this a fast operation
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even over very large data sets.
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This will return all values in the text column for which there is a
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continuous extent in the corresponding ordered trigram set that is
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sufficiently similar to the trigram set of <replaceable>word</replaceable>,
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sorted from best match to worst. The index will be used to make this
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a fast operation even over very large data sets.
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</para>
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<para>

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