diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/pgtrgm.sgml | 22 |
1 files changed, 21 insertions, 1 deletions
diff --git a/doc/src/sgml/pgtrgm.sgml b/doc/src/sgml/pgtrgm.sgml index 376ab85823b..1b75f0292b0 100644 --- a/doc/src/sgml/pgtrgm.sgml +++ b/doc/src/sgml/pgtrgm.sgml @@ -117,6 +117,14 @@ <function>set_limit</>. </entry> </row> + <row> + <entry><type>text</> <literal><-></literal> <type>text</></entry> + <entry><type>real</type></entry> + <entry> + Returns the <quote>distance</> between the arguments, that is + one minus the <function>similarity()</> value. + </entry> + </row> </tbody> </tgroup> </table> @@ -129,7 +137,7 @@ The <filename>pg_trgm</filename> module provides GiST and GIN index operator classes that allow you to create an index over a text column for the purpose of very fast similarity searches. These index types support - the <literal>%</> similarity operator (and no other operators, so you may + the above-described similarity operators (and no other operators, so you may want a regular B-tree index too). </para> @@ -162,6 +170,18 @@ SELECT t, similarity(t, '<replaceable>word</>') AS sml </para> <para> + A variant of the above query is +<programlisting> +SELECT t, t <-> '<replaceable>word</>' AS dist + FROM test_trgm + ORDER BY dist LIMIT 10; +</programlisting> + This can be implemented quite efficiently by GiST indexes, but not + by GIN indexes. It will usually beat the first formulation when only + a small number of the closest matches is wanted. + </para> + + <para> The choice between GiST and GIN indexing depends on the relative performance characteristics of GiST and GIN, which are discussed elsewhere. As a rule of thumb, a GIN index is faster to search than a GiST index, but |