Planner performance extremely affected by an hanging transaction (20-30 times)?

Hi all

We're experiencing a very strange performance issue. Our setup is a bit
more complicated, but we've managed to isolate and replicate the core
problem. Here's what we observe:

We took a strong machine (128 GB RAM, 8-core CPU, SSD drives...) and
installed a fresh copy of PostgreSQL 9.2 (Ubuntu Server 12.04 LTS, default
configuration).

Then, we created a test database with the following schema:

CREATE TABLE offers
(
id bigserial NOT NULL,
name character varying NOT NULL,
category_id bigint NOT NULL,
CONSTRAINT offers_pkey PRIMARY KEY (id)
);

CREATE TABLE categories
(
id bigserial NOT NULL,
name character varying NOT NULL,
CONSTRAINT categories_pkey PRIMARY KEY (id)
);

and populated it with in the following way:

insert into categories (name) select 'category_' || x from
generate_series(1,1000) as x;
insert into offers (name, category_id) select 'offer_' || x, floor(random()
* 1000) + 1 from generate_series(1,1000*1000) as x;

Finally, we created a python script to make simple queries in a loop:

while True:
id = random.randrange(1, 1000 * 1000)
db.execute('select offers.id, offers.name, categories.id,
categories.name from offers left join categories on categories.id =
offers.category_id where offers.id = %s', (id,))
print db.fetchall()

We start 20 instances simultaneously and measure performance:

parallel -j 20 ./test.py -- $(seq 1 20) | pv -i1 -l > /dev/null

Normally we observe about 30k QPS what's a satisfying result (without any
tuning at all).

The problem occurs when we open a second console, start psql and type:

pgtest=> begin; insert into categories (name) select 'category_' || x from
generate_series(1,1000) as x;

We start a transaction and insert 1k records to the 'categories' table.
After that performance of the benchmark described above immediately drops
to about 1-2k QPS. That's 20-30 times! After closing the transaction
(committing or aborting - doesn't matter) it immediately jumps back to 30k
QPS.

Restarting the running script and other simple tricks do not change
anything. The hanging, open transaction is causing a huge slowdown. What's
more when doing similar insert (even much larger) to the 'offers' table we
do not observe this effect.

We analyzed the problem a bit deeper and find out that the actual query
execution times are not affected that much. They are constantly close to
0.5 ms. This can be observed in a server log (after enabling appropriate
option) and this can be found in 'explain analyze...' result. Also the
query plan returned do not change and looks optimal (pkey scan for 'offers'
+ pkey scan for 'categories').

After a few random thought we've finally turned the 'log_planner_stats'
option and found out that the planner executions times are highly affected
by the hanging transaction. Here's the typical output in the initial
situation:

2013-09-17 21:54:59 UTC LOG: PLANNER STATISTICS
2013-09-17 21:54:59 UTC DETAIL: ! system usage stats:
! 0.000137 elapsed 0.000000 user 0.000000 system sec
! [2.169670 user 0.383941 sys total]
! 0/0 [0/11520] filesystem blocks in/out
! 0/0 [0/7408] page faults/reclaims, 0 [0] swaps
! 0 [0] signals rcvd, 0/0 [0/0] messages rcvd/sent
! 0/0 [1362/7648] voluntary/involuntary context switches

And here's a typical output with a hanging transaction:

2013-09-17 21:56:12 UTC LOG: PLANNER STATISTICS
2013-09-17 21:56:12 UTC DETAIL: ! system usage stats:
! 0.027251 elapsed 0.008999 user 0.001000 system sec
! [32.722025 user 3.550460 sys total]
! 0/0 [0/115128] filesystem blocks in/out
! 0/0 [0/7482] page faults/reclaims, 0 [0] swaps
! 0 [0] signals rcvd, 0/0 [0/0] messages rcvd/sent
! 1/6 [12817/80202] voluntary/involuntary context switches

As you can see it takes over 100 times more time when the extra transaction
is open!

Any ideas why's that? It extremely affects total query time.

I know that using prepared statements to solves the problem completely, so
we can deal with it, but we've already spend a lot of time to go that far
and I'm personally a bit curious what's the fundamental reason for such a
behavior.

I'll be very thankful for any explanation what's going on here!

Thanks,
BR