Plan cache and name space behavior in 9.2
I came across a new behavior in 9.2 with nested plpgsql functions and
temporary table.
create or replace function chck(tname text) returns void as $$
declare
cnt int;
begin
select count(*) from pg_attribute where attrelid = tname::regclass::oid
into cnt;
end;
$$ language plpgsql;
create or replace function train(tname text) returns text as $$
declare
begin
perform chck(tname);
return 'OK';
end;
$$ language plpgsql;
create or replace function test(tname text) returns text as $$
declare
result text;
begin
result = train(tname);
return result;
end;
$$ language plpgsql;
drop table if exists tbl;
create table tbl(a int);
select test('tbl'); -- call 1
drop table if exists tbl;
create temp table tbl(a int);
select test('tbl'); -- call 2
drop table tbl;
I expected success in tname::regclass in the function chck(), but it
actually fails for your first run in the session. The second run of
this script will succeed. I assume it's the cause is the new plan
cache behavior. On the first run of the script, the temporary
namespace was not created when the chck() is called at call 1 , and it
saves namespace, then it searches the old saved namespace for 'tbl' at
call 2, fails to find 'tbl'. Removing the call 1 call it runs
successfully. The second run of this script in the same session
succeeds because the temporary namespace has been created in the first
run. I confirmed it runs fine in 9.1 Is this a bug in 9.2 or an
expected behavior in the new plan cache?
Thanks,
--
Hitoshi Harada
Hitoshi Harada <umi.tanuki@gmail.com> writes:
I expected success in tname::regclass in the function chck(), but it
actually fails for your first run in the session.
Really? Not for me.
In the example as given, I see success for "call 1" and then an error at
"call 2", which is occurring because we're trying to replan the query
with the original search_path, which doesn't include the temp schema
since it didn't exist yet. (The planner is trying to estimate the value
of the expression 'tname::regclass::oid', which is why this happens
with the old search_path setting --- it wouldn't fail at runtime.)
A replan would have failed in previous versions too, but that's much
less likely in previous versions since you'd need to see a relcache
invalidation on one of the referenced tables to make one happen.
There's been periodic discussion of how the plan cache ought to interact
with search_path anyway --- maybe it shouldn't try to restore the
previous search_path setting but rather view current search_path as part
of the cache lookup key. I'd be hesitant to back-patch such a behavior
change, though.
regards, tom lane
On Fri, Sep 14, 2012 at 12:55 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Hitoshi Harada <umi.tanuki@gmail.com> writes:
I expected success in tname::regclass in the function chck(), but it
actually fails for your first run in the session.Really? Not for me.
In the example as given, I see success for "call 1" and then an error at
"call 2", which is occurring because we're trying to replan the query
with the original search_path, which doesn't include the temp schema
since it didn't exist yet.
I'm saying the same thing actually. I see success for call 1 and
error at call 2, which was not observed in 9.1 and older.
A replan would have failed in previous versions too, but that's much
less likely in previous versions since you'd need to see a relcache
invalidation on one of the referenced tables to make one happen.
I don't think so. I tried it in 9.1 and succeeded. I found this
during the test of an external module that has been running back to
8.4. So I wonder if we could say this is a behavior change or a bug.
And I agree the replan failure would be sane if the function was
marked as immutable or stable, but all the functions I defined in the
example is volatile. I'm not sure how others feel, but at least it's
surprising to me that the call 2 keeps the state of call 1 though it
is a volatile function. I have not been tracking the periodic
discussion of plan cache vs search_path, but what is the beneficial
use case of the new behavior?
Thanks,
--
Hitoshi Harada