Re: AutoCommit and DDL

On 2005-03-01, Don Drake <dondrake@gmail.com> wrote:
> I don't think it would be easy to duplicate since our code base is
> quite extensive.
>
> Basically, what was happening was a script would first open a database
> connection (AutoCommit turned off by default), create a few objects
> (that also opened independent db connections), the objects would run
> queries so they have data populated, an insert is done and committed,
> then we call a generic function that will create a new table (using
> inherits, part of our partitioning) as well as adding indexes and
> constraints to this new table.  It would get to a point in the
> function where it was adding a FK constraint and every query against
> the table would "hang" which appeared to be some exclusive lock not
> being released.  Activity on the DB would be 100% idle during this
> period, the alter table never came back so we killed it each time.  I
> commented out the code doing the FK constraint add and everything
> worked just fine.

This sounds as though your application deadlocked against itself - by
using multiple connections without autocommit, you can easily get into
situations where you are waiting for completion on one connection, which
is blocked waiting for a lock held by another connection - the lock remains
until the second connection commits, which never happens since the app is
waiting on the first. The DB can't detect this as a deadlock because it
does not know that one session is waiting on another on the client side;
deadlock detection considers only sessions waiting _inside the server_.

This situation isn't specific to DDL, but is easier to produce that way
since most DDL operations acquire very high level locks (often
AccessExclusive, which blocks queries).

> As a test I moved the partition function call to the beginning of the
> script (before the objects  were created) and it worked just fine.  I
> then changed the object declarations passing in the single DB handle,
> and every now works just fine.

This is consistent with it being a client-side deadlock.

-- 
Andrew, Supernews
http://www.supernews.com - individual and corporate NNTP services