Problem description

PL/pgSQL scans all partitions at first time per each new session.

prepare enviroment

CREATE TABLE journal (
    id      SERIAL,
    dt      TIMESTAMP NOT NULL,
    level   INTEGER,
    msg     TEXT);

INSERT INTO journal (dt, level, msg)
SELECT g, random() * 6, md5(g::text)
FROM generate_series('2015-01-01'::date, '2015-01-05'::date, '1 hour') as g;

SELECT create_range_partitions('journal', 'dt', '2015-01-01'::date, '1 day'::interval);

Tests

Make ACCESS EXCLUSIVE for one table in separate transaction:

begin;
lock table journal_2 in ACCESS EXCLUSIVE mode;

SQL-request is good:

SELECT * FROM journal WHERE dt >= '2015-01-05';

"Append (cost=0.00..20.70 rows=1070 width=49)"
" -> Seq Scan on journal_5 (cost=0.00..20.70 rows=1070 width=48)"

PL/pgSQL request is strange:

create or replace function test()
returns void
language plpgsql
as $function$
begin
	perform * FROM journal WHERE dt >= '2015-01-05';
end;
$function$;

select test();

It'll be wait until the lock for journal_2 is released
Note: if release the lock and lock again, then execute function test() again in the same session, then everything will be good. The request won't be wait the lock release.

Environment

  extname   | extowner | extnamespace | extrelocatable | extversion |   extconfig   | extcondition 
------------+----------+--------------+----------------+------------+---------------+--------------
 plpgsql    |       10 |           11 | f              | 1.0        |               | 
 pg_pathman |       10 |         2200 | f              | 1.5        | {58332,58343} | {"",""}

<!-- Put the result of (SELECT version()) below -->
                                             version                                              
--------------------------------------------------------------------------------------------------
 PostgreSQL 9.6.11 on x86_64-pc-linux-gnu, compiled by gcc (Debian 4.9.2-10+deb8u1) 4.9.2, 64-bit

<!-- For Postgres Pro: put the result of (SELECT pgpro_version()) below -->
                                            pgpro_version                                            
-----------------------------------------------------------------------------------------------------
 PostgresPro 9.6.11.1 on x86_64-pc-linux-gnu, compiled by gcc (Debian 4.9.2-10+deb8u1) 4.9.2, 64-bit

<!-- For Postgres Pro: put the result of (SELECT pgpro_edition()) below -->
 pgpro_edition 
---------------
 standard

<!-- SELECT pathman_version() -->
 pathman_version 
-----------------
 1.5.10