Re: Using timestamp(tz) in C functions

On Fri, Jul 29, 2016 at 12:53 AM, Vitaly Burovoy <vitaly(dot)burovoy(at)gmail(dot)com>
wrote:

> On 7/28/16, Keith Fiske <keith(at)omniti(dot)com> wrote:
> > Working on trying to get a C version of the maintenance function for my
> > pg_partman extension working so I can hopefully make it more flexible and
> > efficient.
> >
> >
> https://github.com/keithf4/pg_partman/blob/master/sql/functions/run_maintenance.sql
> >
> > Up until this point I've just been using SPI and the Datum conversion
> > functions (DatumGetCString, DatumGetInt32) to get string and numeric data
> > out of tables and things have been working fine. I'm at the first point
> in
> > that function where I'll need to start dealing with timestamp data.
> >
> >
> https://gist.github.com/keithf4/81c32bf8b689c74b20c10ad8c91d45a3#file-pg_partman_bgw-c-L532
> >
> > There's what I've got working so far and links directly to the area where
> > I'm having a problem. I found the DatumGetTimeTzADTP() function and the
> > TimeTzADT data type looking through the source and that seems to be
> exactly
> > what I'm looking for. However, when I get to the point of trying to
> simply
> > use the time value in that variable (line 544), Postgres segfaults. So
> far
> > I've just been trying to print the value out to the log to ensure I'm
> > pulling it out correctly. The "time" value of the struct appears to be an
> > int64, so I thought %ld would be the correct, but even using %d or %s
> > fails.
> >
> > You can see in my original plpgsql function how I eventually intend to
> use
> > the timestamp values. Here's a link directly to the equivalent section
> >
> >
> https://github.com/keithf4/pg_partman/blob/master/sql/functions/run_maintenance.sql#L139
> >
> > So, not sure if I'm even going about this the right manner for the way I
> > intend to use the timestamp values. Still fairly new to C and getting use
> > to postgres internals. Any help, or even an example of using timstamp
> data
> > pulled from a table in C with SPI, would be great.
> >
> > Thanks!
> >
> > --
> > Keith Fiske
> > Database Administrator
> > OmniTI Computer Consulting, Inc.
> > http://www.keithf4.com
> >
>
> I think it is not about timestamp(tz), but about usage of SPI.
> Since DatumGetTimeTzADTP is just a macros implements type conversion
> (declared at src/include/utils/date.h:60 (or 75)) you get segfault not
> in it but when the code tries to get value by dereference pointer
> (last_partition_timestamp->time).
>
> Please, answer questions:
> 1. How many rows SPI_execute returns (value of "ret" variable)?
> 2. Is last_partition_timestamp != NULL? Where it points to?
> 3. Try to check SPI_result just after SPI_getbinval. Has it error code?
>
> --
> Best regards,
> Vitaly Burovoy
>

It returns a single row. Here's an example of the results of the two
queries that are run that lead to providing the timestamp value

keith=# select partition_tablename from
partman.show_partitions('partman_test.time_taptest_table', 'DESC') limit
1; partition_tablename
--------------------------------
time_taptest_table_p2016_08_02
(1 row)

keith=# select child_start_time from
partman.show_partition_info('partman_test.time_taptest_table_p2016_08_02',
'1 day', 'partman_test.time_taptest_table');
child_start_time
------------------------
2016-08-02 00:00:00-04
(1 row)

So there is valid data. As you're pointing out, this may just be a
misunderstanding of how to actually use the Datum retrieval function and C
in general. Appreciate the assistance.

Keith