Re: query against pg_locks leads to large memory alloc

Kevin Grittner <kgrittn(at)ymail(dot)com> writes:
> Dave Owens <dave(at)teamunify(dot)com> wrote:
>> max_connections = 450 ...we have found that we run out of shared
>> memory when max_pred_locks_per_transaction is less than 30k.

> It gathers the information in memory to return for all those locks
> (I think both the normal heavyweight locks and the predicate locks
> do that). 450 * 30000 is 13.5 million predicate locks you could
> have, so they don't need a very big structure per lock to start
> adding up. I guess we should refactor that to use a tuplestore, so
> it can spill to disk when it gets to be more than work_mem.

Seems to me the bigger issue is why does he need such a huge
max_pred_locks_per_transaction setting? It's hard to believe that
performance wouldn't tank with 10 million predicate locks active.
Whether you can do "select * from pg_locks" seems pretty far down
the list of concerns about this setting.

regards, tom lane