Re: lazy_truncate_heap()

Simon Riggs wrote:
> On Thu, 2009-01-01 at 12:00 +0200, Heikki Linnakangas wrote:
>> Greg Stark wrote:
>>> On 31 Dec 2008, at 13:21, Simon Riggs <simon@2ndQuadrant.com> wrote:
>>>> Both of these bugs are minor, but the effect of either/both of them is
>>>> to cause more AccessExclusiveLocks than we might expect.
>>>>
>>>> For Hot Standby this means that many VACUUMs take AccessExclusiveLocks
>>>> on relations, which would potentially lead to having queries cancelled
>>>> for no reason at all.
>>> Well by default it would just cause wal to pause briefly until the 
>>> queries with those locks finish, no?
>> Wait a minute. Why does an AccessExclusiveLock lead to cancelled queries 
>> or pausing WAL application? I thought it'd just block other queries 
>> trying to acquire a conflicting lock in the standby, just like holding 
>> an AccessExclusiveLock on the primary does. It's unrelated to the xmin 
>> horizon issue.
> 
> Yes, it is unrelated to the xmin horizon issue. There are two reasons
> for delaying WAL apply:
> * locks
> * xmin horizon
> 
> When a lock is acquired on the primary it almost always precedes an
> action which cannot occur concurrently. For example, if VACUUM did
> truncate a table then queries could get errors because parts of their
> table disappear from under them. Others are drop table etc..

Have you implemented the query cancellation mechanism for that scenario 
too? (I'm cool either way, just curious..)

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com