Problem with background worker

Hi,

I'm trying to write a background writer, and I'm facing a problem with
timestamps. The following code is where I'm having a problem (it's just a demo for
the problem):

BackgroundWorkerInitializeConnection("test", NULL);
while (!got_sigterm)
{
int ret;
/* Wait 1s */
ret = WaitLatch(&MyProc->procLatch,
WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
1000L);
ResetLatch(&MyProc->procLatch);
/* Insert dummy for now */
StartTransactionCommand();
SPI_connect();
PushActiveSnapshot(GetTransactionSnapshot());
ret = SPI_execute("INSERT INTO log VALUES (now(),statement_timestamp(),clock_timestamp())", false, 0);
SPI_finish();
PopActiveSnapshot();
CommitTransactionCommand();
}

\d log

Column | Type | Modifiers
---------------------+--------------------------+---------------
now | timestamp with time zone |
statement_timestamp | timestamp with time zone |
clock_timestamp | timestamp with time zone |

Here is its content. Only the clock_timestamp is right. There are missing records at the
beginning because i truncated the table for this example.

now | statement_timestamp | clock_timestamp
-------------------------------+-------------------------------+-------------------------------
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:52.77683+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:53.784301+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:54.834212+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:55.848497+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:56.872671+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:57.888461+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:58.912448+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:59.936335+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:02:00.951247+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:02:01.967937+01
2013-03-20 15:01:44.618623+01 | 2013-03-20 15:01:44.618623+01 | 2013-03-20 15:02:02.983613+01

Most of the code is copy/paste from worker_spi (I don't really understand the
PushActiveSnapshot(GetTransactionSnapshot()) and PopActiveSnapshot() but the
behaviour is the same with or without them, and they were in worker_spi).

I guess I'm doing something wrong, but I really dont't know what it is.

Should I attach the whole code ?

Regards,

Marc

Marc Cousin escribió:
> Hi,
>
> I'm trying to write a background writer, and I'm facing a problem with
> timestamps. The following code is where I'm having a problem (it's just a demo for
> the problem):
>
> BackgroundWorkerInitializeConnection("test", NULL);
> while (!got_sigterm)
> {
> int ret;
> /* Wait 1s */
> ret = WaitLatch(&MyProc->procLatch,
> WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
> 1000L);
> ResetLatch(&MyProc->procLatch);
> /* Insert dummy for now */
> StartTransactionCommand();
> SPI_connect();
> PushActiveSnapshot(GetTransactionSnapshot());
> ret = SPI_execute("INSERT INTO log VALUES (now(),statement_timestamp(),clock_timestamp())", false, 0);
> SPI_finish();
> PopActiveSnapshot();
> CommitTransactionCommand();
> }

Ah. The reason for this problem is that the statement start time (which
also sets the transaction start time, when it's the first statement) is
set by postgres.c, not the transaction-control functions in xact.c. So
you'd need to add a SetCurrentStatementStartTimestamp() call somewhere
in your loop.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

On 20/03/2013 16:33, Alvaro Herrera wrote:
> Marc Cousin escribió:
>> Hi,
>>
>> I'm trying to write a background writer, and I'm facing a problem with
>> timestamps. The following code is where I'm having a problem (it's just a demo for
>> the problem):
>>
>> BackgroundWorkerInitializeConnection("test", NULL);
>> while (!got_sigterm)
>> {
>> int ret;
>> /* Wait 1s */
>> ret = WaitLatch(&MyProc->procLatch,
>> WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
>> 1000L);
>> ResetLatch(&MyProc->procLatch);
>> /* Insert dummy for now */
>> StartTransactionCommand();
>> SPI_connect();
>> PushActiveSnapshot(GetTransactionSnapshot());
>> ret = SPI_execute("INSERT INTO log VALUES (now(),statement_timestamp(),clock_timestamp())", false, 0);
>> SPI_finish();
>> PopActiveSnapshot();
>> CommitTransactionCommand();
>> }
>
> Ah. The reason for this problem is that the statement start time (which
> also sets the transaction start time, when it's the first statement) is
> set by postgres.c, not the transaction-control functions in xact.c. So
> you'd need to add a SetCurrentStatementStartTimestamp() call somewhere
> in your loop.
>

Yes, that works. Thanks a lot !

Maybe this should be added to the worker_spi example ?

Regards

Marc Cousin escribió:
> On 20/03/2013 16:33, Alvaro Herrera wrote:

> >Ah. The reason for this problem is that the statement start time (which
> >also sets the transaction start time, when it's the first statement) is
> >set by postgres.c, not the transaction-control functions in xact.c. So
> >you'd need to add a SetCurrentStatementStartTimestamp() call somewhere
> >in your loop.
>
> Yes, that works. Thanks a lot !
>
> Maybe this should be added to the worker_spi example ?

Yeah, I think I need to go over the postgres.c code and figure out what
else needs to be called. I have a pending patch from Guillaume to
improve worker_spi some more; I'll add this bit too.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

On Wed, 2013-03-20 at 13:13 -0300, Alvaro Herrera wrote:
> Marc Cousin escribió:
> > On 20/03/2013 16:33, Alvaro Herrera wrote:
>
> > >Ah. The reason for this problem is that the statement start time (which
> > >also sets the transaction start time, when it's the first statement) is
> > >set by postgres.c, not the transaction-control functions in xact.c. So
> > >you'd need to add a SetCurrentStatementStartTimestamp() call somewhere
> > >in your loop.
> >
> > Yes, that works. Thanks a lot !
> >
> > Maybe this should be added to the worker_spi example ?
>
> Yeah, I think I need to go over the postgres.c code and figure out what
> else needs to be called. I have a pending patch from Guillaume to
> improve worker_spi some more; I'll add this bit too.
>

Happy to know that you still remember it :) Thanks.

--
Guillaume
http://blog.guillaume.lelarge.info
http://www.dalibo.com

Marc Cousin escribió:
> On 20/03/2013 16:33, Alvaro Herrera wrote:

> >Ah. The reason for this problem is that the statement start time (which
> >also sets the transaction start time, when it's the first statement) is
> >set by postgres.c, not the transaction-control functions in xact.c. So
> >you'd need to add a SetCurrentStatementStartTimestamp() call somewhere
> >in your loop.
> >
>
> Yes, that works. Thanks a lot !
>
> Maybe this should be added to the worker_spi example ?

Done in
http://git.postgresql.org/pg/commitdiff/e543631f3c162ab5f6020b1d0209e0353ca2229a

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services