Refresh a materialized view automatically using a

Question

I have a materialized view on a PostgreSQL 9.3 database which seldom changes (about twice a day). But when it does, I'd like to update its data promptly.

Here is what I was thinking about so far:

There is a materialized view mat_view which gets its data from tables table1 and table2 using some join statement.

Whenever something in table1 or table2 changes, I already have a trigger wich updates a little configuration table config consisting of

table_name | mat_view_name | need_update
-----------+---------------+------------
table1     | mat_view      | TRUE/FALSE
table2     | mat_view      | TRUE/FALSE

So if anything in table1 changes (there's a trigger on UPDATE and on DELETE for every statement), the field need_update in the first row is set to TRUE. The same goes for table2 and the second row.

Obviously, if need_update is TRUE, then the materialized view must be refreshed.

UPDATE: Since materialized views do not support rules (as @pozs mentioned in a comment below), I would go one step further. I'd create a dummy view v_mat_view with the definition "SELECT * FROM mat_view". When the user does a SELECT on this view, I need to create a rule ON SELECT which does the following:

• check whether mat_view should be updated (SELECT 1 FROM config WHERE mat_view_name='mat_view' AND need_update=TRUE)
• reset the need_update flag with UPDATE config SET need_update=FALSE where mat_view_name='mat_view'
• REFRESH MATERIALIZED VIEW mat_view
• and at last do the original SELECT statement but with mat_view as the target.

UPDATE2: I tried creating the steps above:

Create a function that handles the four points mentioned above:

CREATE OR REPLACE FUNCTION mat_view_selector()
RETURNS SETOF mat_view AS $body$
BEGIN
  -- here is checking whether to refresh the mat_view
  -- then return the select:
  RETURN QUERY SELECT * FROM mat_view;
END;
$body$ LANGUAGE plpgsql;

Create the view v_mat_view which really selects from the function mat_view_selector:

CREATE TABLE v_mat_view AS SELECT * from mat_view LIMIT 1;
DELETE FROM v_mat_view;

CREATE RULE "_RETURN" AS
    ON SELECT TO v_mat_view
    DO INSTEAD 
        SELECT * FROM mat_view_selector();
    -- this also converts the empty table 'v_mat_view' into a view.

The result is unsatisfying:

# explain analyze select field1 from v_mat_view where field2 = 44;
QUERY PLAN
Function Scan on mat_view_selector (cost=0.25..12.75 rows=5 width=4)
(actual time=15.457..18.048 rows=1 loops=1)
Filter: (field2 = 44)
Rows Removed by Filter: 20021
Total runtime: 31.753 ms

in comparison to selecting from the mat_view itself:

# explain analyze select field1 from mat_view where field2 = 44;
QUERY PLAN
Index Scan using mat_view_field2 on mat_view (cost=0.29..8.30 rows=1 width=4)
  (actual time=0.015..0.016 rows=1 loops=1)
Index Cond: (field2 = 44)
Total runtime: 0.036 ms

So essentially it DOES work, but performance might be an issue.

Anyone have better ideas? If not, then I would have to implement it somehow in the application logic or worse: run a simple cronjob that runs every minute or so. :-(

Answer 1

You should refresh the view in triggers after insert/update/delete/truncate for each statement on table1 and table2.

create or replace function refresh_mat_view()
returns trigger language plpgsql
as $$
begin
    refresh materialized view mat_view;
    return null;
end $$;

create trigger refresh_mat_view
after insert or update or delete or truncate
on table1 for each statement 
execute procedure refresh_mat_view();

create trigger refresh_mat_view
after insert or update or delete or truncate
on table2 for each statement 
execute procedure refresh_mat_view();

In this way your materialized view is always up to date. This simple solution might be hard to accept with frequent inserts/updates and sporadic selects. In your case (seldom changes about twice a day) it ideally fits your needs.

---

To realize deferred refresh of a materialized view you need one of the following features:

• asynchronous trigger
• trigger before select
• rule on select before

Postgres has none of them, so it seems that there is no clear postgres solution.

Taking this into account I would consider a wrapper function for selects on mat_view, e.g.

CREATE OR REPLACE FUNCTION select_from_mat_view(where_clause text)
RETURNS SETOF mat_view AS $body$
BEGIN
  -- here is checking whether to refresh the mat_view
  -- then return the select:
  RETURN QUERY EXECUTE FORMAT ('SELECT * FROM mat_view %s', where_clause);
END;
$body$ LANGUAGE plpgsql;

If it is acceptable in practice depends on particulars I do not know about.

Answer 2

PostgreSQL 9.4 added REFRESH CONCURRENTLY to Materialized Views.

This may be what you're looking for when you describe trying to setup an asynchronous update of the materialized view.

Users selecting from the materialized view will see incorrect data until the refresh finishes, but in many scenarios that use a materialized view, this is an acceptable tradeoff.

Use a statement level trigger that watches the underlying tables for any changes and then refreshes the materialized view concurrently.