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>Chapter 12. Full Text Search</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="TEXTSEARCH-TABLES"
>12.2. Tables and Indexes</A
></H1
><P
> The examples in the previous section illustrated full text matching using
simple constant strings. This section shows how to search table data,
optionally using indexes.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="TEXTSEARCH-TABLES-SEARCH"
>12.2.1. Searching a Table</A
></H2
><P
> It is possible to do a full text search without an index. A simple query
to print the <TT
CLASS="STRUCTNAME"
>title</TT
> of each row that contains the word
<TT
CLASS="LITERAL"
>friend</TT
> in its <TT
CLASS="STRUCTFIELD"
>body</TT
> field is:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT title
FROM pgweb
WHERE to_tsvector('english', body) @@ to_tsquery('english', 'friend');</PRE
><P>
This will also find related words such as <TT
CLASS="LITERAL"
>friends</TT
>
and <TT
CLASS="LITERAL"
>friendly</TT
>, since all these are reduced to the same
normalized lexeme.
</P
><P
> The query above specifies that the <TT
CLASS="LITERAL"
>english</TT
> configuration
is to be used to parse and normalize the strings. Alternatively we
could omit the configuration parameters:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT title
FROM pgweb
WHERE to_tsvector(body) @@ to_tsquery('friend');</PRE
><P>
This query will use the configuration set by <A
HREF="runtime-config-client.html#GUC-DEFAULT-TEXT-SEARCH-CONFIG"
>default_text_search_config</A
>.
</P
><P
> A more complex example is to
select the ten most recent documents that contain <TT
CLASS="LITERAL"
>create</TT
> and
<TT
CLASS="LITERAL"
>table</TT
> in the <TT
CLASS="STRUCTNAME"
>title</TT
> or <TT
CLASS="STRUCTNAME"
>body</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT title
FROM pgweb
WHERE to_tsvector(title || ' ' || body) @@ to_tsquery('create & table')
ORDER BY last_mod_date DESC
LIMIT 10;</PRE
><P>
For clarity we omitted the <CODE
CLASS="FUNCTION"
>coalesce</CODE
> function calls
which would be needed to find rows that contain <TT
CLASS="LITERAL"
>NULL</TT
>
in one of the two fields.
</P
><P
> Although these queries will work without an index, most applications
will find this approach too slow, except perhaps for occasional ad-hoc
searches. Practical use of text searching usually requires creating
an index.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="TEXTSEARCH-TABLES-INDEX"
>12.2.2. Creating Indexes</A
></H2
><P
> We can create a <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> index (<A
HREF="textsearch-indexes.html"
>Section 12.9</A
>) to speed up text searches:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX pgweb_idx ON pgweb USING gin(to_tsvector('english', body));</PRE
><P>
Notice that the 2-argument version of <CODE
CLASS="FUNCTION"
>to_tsvector</CODE
> is
used. Only text search functions that specify a configuration name can
be used in expression indexes (<A
HREF="indexes-expressional.html"
>Section 11.7</A
>).
This is because the index contents must be unaffected by <A
HREF="runtime-config-client.html#GUC-DEFAULT-TEXT-SEARCH-CONFIG"
>default_text_search_config</A
>. If they were affected, the
index contents might be inconsistent because different entries could
contain <TT
CLASS="TYPE"
>tsvector</TT
>s that were created with different text search
configurations, and there would be no way to guess which was which. It
would be impossible to dump and restore such an index correctly.
</P
><P
> Because the two-argument version of <CODE
CLASS="FUNCTION"
>to_tsvector</CODE
> was
used in the index above, only a query reference that uses the 2-argument
version of <CODE
CLASS="FUNCTION"
>to_tsvector</CODE
> with the same configuration
name will use that index. That is, <TT
CLASS="LITERAL"
>WHERE
to_tsvector('english', body) @@ 'a & b'</TT
> can use the index,
but <TT
CLASS="LITERAL"
>WHERE to_tsvector(body) @@ 'a & b'</TT
> cannot.
This ensures that an index will be used only with the same configuration
used to create the index entries.
</P
><P
> It is possible to set up more complex expression indexes wherein the
configuration name is specified by another column, e.g.:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX pgweb_idx ON pgweb USING gin(to_tsvector(config_name, body));</PRE
><P>
where <TT
CLASS="LITERAL"
>config_name</TT
> is a column in the <TT
CLASS="LITERAL"
>pgweb</TT
>
table. This allows mixed configurations in the same index while
recording which configuration was used for each index entry. This
would be useful, for example, if the document collection contained
documents in different languages. Again,
queries that are meant to use the index must be phrased to match, e.g.,
<TT
CLASS="LITERAL"
>WHERE to_tsvector(config_name, body) @@ 'a & b'</TT
>.
</P
><P
> Indexes can even concatenate columns:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX pgweb_idx ON pgweb USING gin(to_tsvector('english', title || ' ' || body));</PRE
><P>
</P
><P
> Another approach is to create a separate <TT
CLASS="TYPE"
>tsvector</TT
> column
to hold the output of <CODE
CLASS="FUNCTION"
>to_tsvector</CODE
>. This example is a
concatenation of <TT
CLASS="LITERAL"
>title</TT
> and <TT
CLASS="LITERAL"
>body</TT
>,
using <CODE
CLASS="FUNCTION"
>coalesce</CODE
> to ensure that one field will still be
indexed when the other is <TT
CLASS="LITERAL"
>NULL</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE pgweb ADD COLUMN textsearchable_index_col tsvector;
UPDATE pgweb SET textsearchable_index_col =
to_tsvector('english', coalesce(title,'') || ' ' || coalesce(body,''));</PRE
><P>
Then we create a <ACRONYM
CLASS="ACRONYM"
>GIN</ACRONYM
> index to speed up the search:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX textsearch_idx ON pgweb USING gin(textsearchable_index_col);</PRE
><P>
Now we are ready to perform a fast full text search:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT title
FROM pgweb
WHERE textsearchable_index_col @@ to_tsquery('create & table')
ORDER BY last_mod_date DESC
LIMIT 10;</PRE
><P>
</P
><P
> When using a separate column to store the <TT
CLASS="TYPE"
>tsvector</TT
>
representation,
it is necessary to create a trigger to keep the <TT
CLASS="TYPE"
>tsvector</TT
>
column current anytime <TT
CLASS="LITERAL"
>title</TT
> or <TT
CLASS="LITERAL"
>body</TT
> changes.
<A
HREF="textsearch-features.html#TEXTSEARCH-UPDATE-TRIGGERS"
>Section 12.4.3</A
> explains how to do that.
</P
><P
> One advantage of the separate-column approach over an expression index
is that it is not necessary to explicitly specify the text search
configuration in queries in order to make use of the index. As shown
in the example above, the query can depend on
<TT
CLASS="VARNAME"
>default_text_search_config</TT
>. Another advantage is that
searches will be faster, since it will not be necessary to redo the
<CODE
CLASS="FUNCTION"
>to_tsvector</CODE
> calls to verify index matches. (This is more
important when using a GiST index than a GIN index; see <A
HREF="textsearch-indexes.html"
>Section 12.9</A
>.) The expression-index approach is
simpler to set up, however, and it requires less disk space since the
<TT
CLASS="TYPE"
>tsvector</TT
> representation is not stored explicitly.
</P
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