Question

I am running into a wierd issue with the execution (or non-execution) of certain queries within postgresql RULES. This is a companion (but different) question to Postgresql subquery execution within ON UPDATE rule (maybe bug in postgresql?) – but as it turns out, there is a related issue of certain queries not executing at all within postgresql RULEs, versus just the issue of query execution order.

Consider the following code block:

DROP TABLE foo_table;
DROP TABLE bar_table;

--------------

CREATE TABLE foo_table (c1 text, c2 int);
CREATE TABLE bar_table (c1 text, c2 int);

--------------

CREATE OR REPLACE FUNCTION debug_immutable(anyelement) RETURNS integer AS $$

pg_raise('notice', 'debug(): ' . json_encode($args[0]));

return rand(1, 10000);

$$ LANGUAGE PLPHP IMMUTABLE;

CREATE OR REPLACE FUNCTION debug_volatile(anyelement) RETURNS integer AS $$

pg_raise('notice', 'debug(): ' . json_encode($args[0]));

return rand(1, 10000);

$$ LANGUAGE PLPHP VOLATILE;

-------------

CREATE OR REPLACE RULE foo_update_rule AS ON UPDATE TO foo_table DO INSTEAD
(

SELECT debug_immutable('debug_immutable call 1'::text); -- Query #1

SELECT debug_volatile('debug_volatile call 1'::text);   -- Query #2

INSERT INTO foo_table (c1, c2) values ('foo', 123456);  -- Query #3

INSERT INTO bar_table (c1, c2) values ('bar', 123456);  -- Query #4

SELECT debug_immutable('debug_immutable call 2'::text); -- Query #5

SELECT debug_volatile('debug_volatile call 2'::text);   -- Query #6
);

-----------------------------------------------

UPDATE foo_table SET c1 = NULL where c1 = 'aaa';

SELECT * FROM foo_table;

SELECT * FROM bar_table;

The output only displays the following:

NOTICE:  plphp: debug(): "debug_immutable call 1"
NOTICE:  plphp: debug(): "debug_immutable call 2"
Total query runtime: 46 ms.
0 rows retrieved.

And both foo_table and bar_table shows to be empty after the code block is executed:

# select * from foo_table;
 c1 | c2 
----+----
(0 rows)

# select * from bar_table;
 c1 | c2 
----+----
(0 rows)

The output implies that only queries #1 and #5 are executed, and queries #2, 3, 4 and 6 are never executed. Why is this? How can the above code block be changed so that queries #2, 3, 4 and 6 are executed?

Answer 1

The immutable functions are evaluated at the planning stage, before the query is executed. The volatile ones are evaluated on a per need basis. You’ve zero rows returned, so they’re never called.