Simultaneous calls to the same function: how are deadlocks occurring?

My function new_customer is called several times per second (but only once per session) by a web application. The very first thing it does is lock the customer table (to do an 'insert if not exists'—a simple variant of an upsert).

My understanding of the docs is that other calls to new_customer should simply queue until all previous calls have finished:


LOCK TABLE obtains a table-level lock, waiting if necessary for any conflicting locks to be released.

Why is it sometimes deadlocking instead?

definition:

create function new_customer(secret bytea) returns integer language sql 
                security definer set search_path = postgres,pg_temp as $
  lock customer in exclusive mode;
  --
  with w as ( insert into customer(customer_secret,customer_read_secret)
              select secret,decode(md5(encode(secret, 'hex')),'hex') 
              where not exists(select * from customer where customer_secret=secret)
              returning customer_id )
  insert into collection(customer_id) select customer_id from w;
  --
  select customer_id from customer where customer_secret=secret;
$;

error from log:

2015-07-28 08:02:58 BST DETAIL: Process 12380 waits for ExclusiveLock on relation 16438 of database 12141; blocked by process 12379.
Process 12379 waits for ExclusiveLock on relation 16438 of database 12141; blocked by process 12380.
Process 12380: select new_customer(decode($1::text, 'hex'))
Process 12379: select new_customer(decode($1::text, 'hex'))
2015-07-28 08:02:58 BST HINT: See server log for query details.
2015-07-28 08:02:58 BST CONTEXT: SQL function "new_customer" statement 1
2015-07-28 08:02:58 BST STATEMENT: select new_customer(decode($1::text, 'hex'))

relation:

postgres=# select relname from pg_class where oid=16438;
┌──────────┐
│ relname  │
├──────────┤
│ customer │
└──────────┘

edit:

I've managed to get a simple-ish reproducible test case. To me this looks like a bug due to some sort of ra

I posted this to pgsql-bugs and the reply there from Tom Lane indicates this is a lock escalation issue, disguised by the mechanics of the way SQL language functions are processed. Essentially, the lock generated by the insert is obtained before the exclusive lock on the table:


I believe the issue with this is that a SQL function will do parsing (and
maybe planning too; don't feel like checking the code right now) for the
entire function body at once. This means that due to the INSERT command
you acquire RowExclusiveLock on the "test" table during function body
parsing, before the LOCK command actually executes. So the LOCK
represents a lock escalation attempt, and deadlocks are to be expected.


This coding technique would be safe in plpgsql, but not in a SQL-language
function.


There have been discussions of reimplementing SQL-language functions so
that parsing occurs one statement at a time, but don't hold your breath
about something happening in that direction; it doesn't seem to be a
high priority concern for anybody.


regards, tom lane

This also explains why locking the table outside the function in a wrapping plpgsql block (as suggested by @ypercube) prevents the deadlocks.

StackExchange Database Administrators Q#108290, answer score: 11