41.3. Materialized Views
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## 41.3. Materialized Views #

Materialized views in PostgreSQL use the rule system like views do, but persist the results in a table-like form. The main differences between:

    CREATE MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;

and:

    CREATE TABLE mymatview AS SELECT * FROM mytab;

are that the materialized view cannot subsequently be directly updated and that the query used to create the materialized view is stored in exactly the same way that a view's query is stored, so that fresh data can be generated for the materialized view with:

    REFRESH MATERIALIZED VIEW mymatview;

The information about a materialized view in the PostgreSQL system catalogs is exactly the same as it is for a table or view. So for the parser, a materialized view is a relation, just like a table or a view. When a materialized view is referenced in a query, the data is returned directly from the materialized view, like from a table; the rule is only used for populating the materialized view.

While access to the data stored in a materialized view is often much faster than accessing the underlying tables directly or through a view, the data is not always current; yet sometimes current data is not needed. Consider a table which records sales:

    CREATE TABLE invoice (
        invoice_no    integer        PRIMARY KEY,
        seller_no     integer,       -- ID of salesperson
        invoice_date  date,          -- date of sale
        invoice_amt   numeric(13,2)  -- amount of sale
    );

If people want to be able to quickly graph historical sales data, they might want to summarize, and they may not care about the incomplete data for the current date:

    CREATE MATERIALIZED VIEW sales_summary AS
      SELECT
          seller_no,
          invoice_date,
          sum(invoice_amt)::numeric(13,2) as sales_amt
        FROM invoice
        WHERE invoice_date < CURRENT_DATE
        GROUP BY
          seller_no,
          invoice_date;

    CREATE UNIQUE INDEX sales_summary_seller
      ON sales_summary (seller_no, invoice_date);

This materialized view might be useful for displaying a graph in the dashboard created for salespeople. A job could be scheduled to update the statistics each night using this SQL statement:

    REFRESH MATERIALIZED VIEW sales_summary;

Another use for a materialized view is to allow faster access to data brought across from a remote system through a foreign data wrapper. A simple example using `file_fdw` 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 `file_fdw` does not support indexes; this advantage might not apply for other sorts of foreign data access.

Setup:

    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;

Now let's spell-check a word. Using `file_fdw` directly:

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

     count
    -------
         0
    (1 row)

With `EXPLAIN ANALYZE`, we see:

     Aggregate  (cost=21763.99..21764.00 rows=1 width=0) (actual time=188.180..188.181 rows=1 loops=1)
       ->  Foreign Scan on words  (cost=0.00..21761.41 rows=1032 width=0) (actual time=188.177..188.177 rows=0 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

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

     Aggregate  (cost=4.44..4.45 rows=1 width=0) (actual time=0.042..0.042 rows=1 loops=1)
       ->  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 loops=1)
             Index Cond: (word = 'caterpiler'::text)
             Heap Fetches: 0
     Planning time: 0.164 ms
     Execution time: 0.117 ms

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

    SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;

         word
    ---------------
     cater
     caterpillar
     Caterpillar
     caterpillars
     caterpillar's
     Caterpillar's
     caterer
     caterer's
     caters
     catered
    (10 rows)

     Limit  (cost=11583.61..11583.64 rows=10 width=32) (actual time=1431.591..1431.594 rows=10 loops=1)
       ->  Sort  (cost=11583.61..11804.76 rows=88459 width=32) (actual time=1431.589..1431.591 rows=10 loops=1)
             Sort Key: ((word <-> 'caterpiler'::text))
             Sort Method: top-N heapsort  Memory: 25kB
             ->  Foreign Scan on words  (cost=0.00..9672.05 rows=88459 width=32) (actual time=0.057..1286.455 rows=479829 loops=1)
                   Foreign File: /usr/share/dict/words
                   Foreign File Size: 4953699
     Planning time: 0.128 ms
     Execution time: 1431.679 ms

Using the materialized view:

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

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

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