Postgres client locking up when creating new table

Question

I'm trying to create a new table in postgres but when I do it just hangs after the CREATE TABLE call.

$ sudo usermod -s /bin/bash postgres
$ sudo su - postgres
postgres@host:~$ psql ranking_analytics
psql (8.4.8)
Type "help" for help.

ranking_analytics=# BEGIN;
BEGIN
ranking_analytics=# CREATE TABLE "about_contactmessage" (
ranking_analytics(#     "id" serial NOT NULL PRIMARY KEY,
ranking_analytics(#     "user_id" integer NOT NULL REFERENCES "auth_user" ("id") DEFERRABLE INITIALLY DEFERRED,
ranking_analytics(#     "subject" text NOT NULL,
ranking_analytics(#     "message" text NOT NULL,
ranking_analytics(#     "recorded_time" timestamp with time zone NOT NULL
ranking_analytics(# )
ranking_analytics-# ;
NOTICE:  CREATE TABLE will create implicit sequence "about_contactmessage_id_seq" for serial column "about_contactmessage.id"

It will then just sit here indefinately until I CTRL-C it.

There are other tables in the database and this one doesn't already exist:

ranking_analytics=# \d about_contactmessage
Did not find any relation named "about_contactmessage".

I'm able to do insert and delete queries on other tables in the database without a problem:

ranking_analytics=# insert into locations_continent (continent_name) VALUES ('testing');
INSERT 0 1
ranking_analytics=# delete from locations_continent where continent_name = 'testing';
DELETE 1

There is plenty of drive space on the machine:

$ df -H
Filesystem             Size   Used  Avail Use% Mounted on
/dev/xvda               21G   2.3G    18G  12% /
devtmpfs               255M   132k   255M   1% /dev
none                   257M   476k   256M   1% /dev/shm
none                   257M    54k   257M   1% /var/run
none                   257M      0   257M   0% /var/lock

Any ideas what could be wrong?

Answer 1

If restarting postgres is an option, then that will most likely solve the issue and will save you from spending time reading the rest of this answer :-)

Check the pg_stat_activity view, there is probably some other transaction blocking the schema change.

select * from pg_stat_activity 
where 
wait_event_type is NULL and xact_start is not NULL order by xact_start;

(the pg_stat_activity is changed a bit in every major pg release, try this for elder versions):

select * from pg_stat_activity 
where 
not waiting and xact_start is not NULL order by xact_start;

The first row to show up is probably the one causing problems. It is often an "idle in transaction" - this may very well hold locks, and if it's an old transaction it may as well kill performance. Probably the programmer forgot to ensure ending the transaction with "commit" or "rollback", or maybe some db session got stuck due to network problems.

To terminate transaction with pid 1234, use select pg_cancel_backend(1234);, if that fails, select pg_terminate_backend(1234). With shell access, the equivalent commands are kill -INT 1234 and kill 1234. (keep in mind, kill -9 1234 is a really bad idea).

There is also a view pg_locks which may give some insight, though it may probably not be that easy to get any useful info out from it. If granted is true, the lock is held, when granted is false it means the query is waiting for the lock. Here are some more hints here on how to extract useful info from pg_locks: http://wiki.postgresql.org/wiki/Lock_Monitoring

If everything else fails, then it's probably time to go for the simple solution, restart that database server.