using <literal>file_fdw</literal> is below, with timings,
    but since this is using cache on the local system the performance
    difference compared to access to a remote system would usually be greater
    than shown here.  Notice we are also exploiting the ability to put an
    index on the materialized view, whereas <literal>file_fdw</literal> does
    not support indexes; this advantage might not apply for other sorts of
    foreign data access.
</para>

<para>
    Setup:

<programlisting>
CREATE EXTENSION file_fdw;
CREATE SERVER local_file FOREIGN DATA WRAPPER file_fdw;
CREATE FOREIGN TABLE words (word text NOT NULL)
  SERVER local_file
  OPTIONS (filename '/usr/share/dict/words');
CREATE MATERIALIZED VIEW wrd AS SELECT * FROM words;
CREATE UNIQUE INDEX wrd_word ON wrd (word);
CREATE EXTENSION pg_trgm;
CREATE INDEX wrd_trgm ON wrd USING gist (word gist_trgm_ops);
VACUUM ANALYZE wrd;
</programlisting>

    Now let's spell-check a word.  Using <literal>file_fdw</literal> directly:

<programlisting>
SELECT count(*) FROM words WHERE word = 'caterpiler';

 count
-------
     0
(1 row)
</programlisting>

    With <command>EXPLAIN ANALYZE</command>, we see:

<programlisting>
 Aggregate  (cost=21763.99..21764.00 rows=1 width=0) (actual time=188.180..188.181 rows=1.00 loops=1)
   -&gt;  Foreign Scan on words  (cost=0.00..21761.41 rows=1032 width=0) (actual time=188.177..188.177 rows=0.00 loops=1)
         Filter: (word = 'caterpiler'::text)
         Rows Removed by Filter: 479829
         Foreign File: /usr/share/dict/words
         Foreign File Size: 4953699
 Planning time: 0.118 ms
 Execution time: 188.273 ms
</programlisting>

    If the materialized view is used instead, the query is much faster:

<programlisting>
 Aggregate  (cost=4.44..4.45 rows=1 width=0) (actual time=0.042..0.042 rows=1.00 loops=1)
   -&gt;  Index Only Scan using wrd_word on wrd  (cost=0.42..4.44 rows=1 width=0) (actual time=0.039..0.039 rows=0.00 loops=1)
         Index Cond: (word = 'caterpiler'::text)
         Heap Fetches: 0
         Index Searches: 1
 Planning time: 0.164 ms
 Execution time: 0.117 ms
</programlisting>

    Either way, the word is spelled wrong, so let's look for what we might
    have wanted.  Again using <literal>file_fdw</literal> and
    <literal>pg_trgm</literal>:

<programlisting>
SELECT word FROM words ORDER BY word &lt;-&gt; 'caterpiler' LIMIT 10;

     word
---------------
 cater
 caterpillar
 Caterpillar
 caterpillars
 caterpillar's
 Caterpillar's
 caterer
 caterer's
 caters
 catered
(10 rows)
</programlisting>

<programlisting>
 Limit  (cost=11583.61..11583.64 rows=10 width=32) (actual time=1431.591..1431.594 rows=10.00 loops=1)
   -&gt;  Sort  (cost=11583.61..11804.76 rows=88459 width=32) (actual time=1431.589..1431.591 rows=10.00 loops=1)
         Sort Key: ((word &lt;-&gt; 'caterpiler'::text))
         Sort Method: top-N heapsort  Memory: 25kB
         -&gt;  Foreign Scan on words  (cost=0.00..9672.05 rows=88459 width=32) (actual time=0.057..1286.455 rows=479829.00 loops=1)
               Foreign File: /usr/share/dict/words
               Foreign File Size: 4953699
 Planning time: 0.128 ms
 Execution time: 1431.679 ms
</programlisting>

    Using the materialized view:

<programlisting>
 Limit  (cost=0.29..1.06 rows=10 width=10) (actual time=187.222..188.257 rows=10.00 loops=1)
   -&gt;  Index Scan using wrd_trgm on wrd  (cost=0.29..37020.87 rows=479829 width=10) (actual time=187.219..188.252 rows=10.00 loops=1)
         Order By: (word &lt;-&gt; 'caterpiler'::text)
         Index Searches: 1
 Planning time: 0.196 ms
 Execution time: 198.640 ms
</programlisting>

    If you can tolerate periodic update of the remote data to the local
    database, the performance benefit can be substantial.
</para>

</sect1>

<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>

<indexterm zone="rules-update">
 <primary>rule</primary>
 <secondary sortas="INSERT">for