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38.3. 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
    ORDER 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 on a foreign data wrapper to a remote system could be greater. 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 '/etc/dictionaries-common/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)

The plan is:

 Aggregate  (cost=4125.19..4125.20 rows=1 width=0) (actual time=26.013..26.014 rows=1 loops=1)
   ->  Foreign Scan on words  (cost=0.00..4124.70 rows=196 width=0) (actual time=26.011..26.011 rows=0 loops=1)
         Filter: (word = 'caterpiler'::text)
         Rows Removed by Filter: 99171
         Foreign File: /etc/dictionaries-common/words
         Foreign File Size: 938848
 Total runtime: 26.081 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=0.074..0.074 rows=1 loops=1)
   ->  Index Only Scan using wrd_word on wrd  (cost=0.42..4.44 rows=1 width=0) (actual time=0.071..0.071 rows=0 loops=1)
         Index Cond: (word = 'caterpiler'::text)
         Heap Fetches: 0
 Total runtime: 0.119 ms

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

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=2195.70..2195.72 rows=10 width=32) (actual time=218.904..218.906 rows=10 loops=1)
   ->  Sort  (cost=2195.70..2237.61 rows=16765 width=32) (actual time=218.902..218.904 rows=10 loops=1)
         Sort Key: ((word <-> 'caterpiler'::text))
         Sort Method: top-N heapsort  Memory: 25kB
         ->  Foreign Scan on words  (cost=0.00..1833.41 rows=16765 width=32) (actual time=0.046..200.965 rows=99171 loops=1)
               Foreign File: /etc/dictionaries-common/words
               Foreign File Size: 938848
 Total runtime: 218.966 ms

Using the materialized view:

 Limit  (cost=0.28..1.02 rows=10 width=9) (actual time=24.916..25.079 rows=10 loops=1)
   ->  Index Scan using wrd_trgm on wrd  (cost=0.28..7383.70 rows=99171 width=9) (actual time=24.914..25.076 rows=10 loops=1)
         Order By: (word <-> 'caterpiler'::text)
 Total runtime: 25.884 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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