Re: [PATCH v3] pg_progress() SQL function to monitor progression of long running SQL queries/utilities

On Wed, Jun 21, 2017 at 10:01 AM, Remi Colinet <remi(dot)colinet(at)gmail(dot)com> wrote:
> test=# SELECT pid, ppid, bid, concat(repeat(' ', 3 * indent),name), value,
> unit FROM pg_progress(0,0);
> pid | ppid | bid | concat | value | unit
> -------+------+-----+------------------+------------------+---------
> 14106 | 0 | 4 | status | query running |
> 14106 | 0 | 4 | relationship | progression |
> 14106 | 0 | 4 | node name | Sort |
> 14106 | 0 | 4 | sort status | on tapes writing |
> 14106 | 0 | 4 | completion | 0 | percent
> 14106 | 0 | 4 | relationship | Outer |
> 14106 | 0 | 4 | node name | Seq Scan |
> 14106 | 0 | 4 | scan on | t_10m |
> 14106 | 0 | 4 | fetched | 25049 | block
> 14106 | 0 | 4 | total | 83334 | block
> 14106 | 0 | 4 | completion | 30 | percent
> (11 rows)
>
> test=#

Somehow I imagined that the output would look more like what EXPLAIN produces.

> If the one shared memory page is not enough for the whole progress report,
> the progress report transfert between the 2 backends is done with a series
> of request/response. Before setting the latch, the monitored backend write
> the size of the data dumped in shared memory and set a status to indicate
> that more data is to be sent through the shared memory page. The monitoring
> backends get the result and sends an other signal, and then wait for the
> latch again. The monitored backend does not collect a new progress report
> but continues to dump the already collected report. And the exchange goes on
> until the full progress report has been dumped.

This is basically what shm_mq does. We probably don't want to
reinvent that code, as it has taken a surprising amount of debugging
to get it fully working.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company