Question

I’m trying to fasten posgresql selection using a function by means of saying it’s immutable or stable, so I have a function

CREATE OR REPLACE FUNCTION get_data(uid uuid)
RETURNS integer AS $$
BEGIN
    RAISE NOTICE 'UUID %', $1;
-- DO SOME STUFF
    RETURN 0;

END;
$$ LANGUAGE plpgsql IMMUTABLE STRICT;

When I call it like:

SELECT get_data('3642e529-b098-4db4-b7e7-6bb62f8dcbba'::uuid)
  FROM table
 WHERE true LIMIT 100;

I have 100 results and only one notice raised

When I call it this way:

SELECT get_data(table.hash)
  FROM table
 WHERE 1 = 1 AND table.hash = '3642e529-b098-4db4-b7e7-6bb62f8dcbba' LIMIT 100;

I have 100 result and 100 notices raised

the condition (table.hash = ‘3642e529-b098-4db4-b7e7-6bb62f8dcbba’) added to make sure that the in param is the same

table.hash is uuid type

The questions is:
So how can force PG to some how cache the result of the function? ( if it’s possibe )
I want to have only one notice ( function call ) be raised in the second case…

Answer 1

In your first example get_data('3642e529-b098-4db4-b7e7-6bb62f8dcbba'::uuid) is a constant, independent of table rows, so it is evaluated once.

In the second example get_data(table.hash) is functionally depending on a column value, therefore it is evaluated once per row.

If you want to evaluate the function once, it cannot depend on a value from a column (when more than one row is processed).

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After discussion in comments, here is an example how to call function only once per hash:

SELECT *, get_data(x.hash) AS some_data_once_per_hash
FROM    (
    SELECT hash, count(*) AS ct
    FROM   table
    WHERE  table.hash = '3642e529-b098-4db4-b7e7-6bb62f8dcbba'
    GROUP  BY 1
    ) x