RE: Function search_path

>> Here is an example where I can show significant extra memory consumption when setting search_path on a function:
>
>I got around to testing this example today, and I don't see what you're seeing --- the memory consumption seems
stable,and about the same with or without the "SET search_path" clause. 
>
>You didn't specify exactly how to do this bit:
>
>--populate 1,000,000 rows with random values from 1,000 surnames for 'SURNAME'
>
>so I did it like this:
>
>INSERT INTO test_search_path(date_last_modified, last_name) select clock_timestamp(), (random()*1000)::int::text from
generate_series(1,1000000);
>
>The example doesn't seem like it'd be terribly data-dependent, but maybe that's wrong?
>
>Also, I assume you're running a reasonably up-to-date PG release, otherwise I'd be wondering about leaks in the GIN
indexAM; we've fixed some issues of that sort in GIN bulk updates in the past. 
>But again, it's not clear what the connection to a function SET clause would be.  So I lack a plausible theory at the
moment.

Tom,
Thanks for your looking at this.

I tested this well on 9.6.7 but I saw the same issue on 10.3.
You are correct the issue is not data dependant and I saw the same issues with the random numeric inserts you used.

On a server with a lot of memory(100GB) and using the postgres default memory settings, the update was able to complete
butwhen watching top there was a clear spike in %MEM for the update when the function search_path was set. 
The issue becomes more noticeable as you add more rows, for example at 5,000,000 %MEM went over 35. Without the
search_pathset %MEM stays real low and the update was a lot quicker. 

Here is the more complete test case:

CREATE SCHEMA test_search_path AUTHORIZATION test_search_path;

--then logged in as the test_search_path user

CREATE TABLE test_search_path(date_last_modified timestamp, last_name varchar);

INSERT INTO test_search_path(date_last_modified, last_name) select clock_timestamp(), (random()*1000)::int::text from
generate_series(1,1000000);

CREATE OR REPLACE FUNCTION clean_name_upper(original_name varchar)
  RETURNS varchar
  LANGUAGE plpgsql
AS $$
BEGIN
  RETURN trim(upper(original_name));
END;
$$ IMMUTABLE STRICT;

CREATE OR REPLACE FUNCTION split_name_on_hyphen(original_name character varying)
  RETURNS text[]
  LANGUAGE plpgsql
AS $$
BEGIN
    RETURN string_to_array(clean_name_upper(original_name), '-');
END;
$$ IMMUTABLE STRICT SET search_path = '$user';

CREATE INDEX idx_test_search_path_clean_name_upper_last_name ON test_search_path (clean_name_upper(last_name));
CREATE INDEX idx_test_search_path_split_name_on_hyphen_last_name ON test_search_path USING
gin(split_name_on_hyphen(last_name));

UPDATE test_search_path SET date_last_modified = (date_last_modified - interval '7 days');

There seems to be something about the inner function (clean_name_upper) being used in an index of its own on the same
fieldas the GIN functional index (split_name_on_hyphen) that is causing this. 

Thanks,
Manfred

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