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

2017-07-26 15:27 GMT+02:00 Robert Haas <robertmhaas@gmail.com>:

On Wed, Jun 21, 2017 at 10:01 AM, Remi Colinet <remi.colinet@gmail.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.

I had initially used the same output as for the ANALYZE command:

test=# PROGRESS 14611;
                                      PLAN PROGRESS                                     
-----------------------------------------------------------------------------------------
 Gather Merge
   ->  Sort=> dumping tuples to tapes
         rows r/w merge 0/0 rows r/w effective 0/1464520 0%
         Sort Key: md5
         ->  Parallel Seq Scan on t_10m => rows 1464520/4166700 35% blks 36011/83334 43%
(5 rows)

test=#
 

But this restricts the use to "human consumers". Using a table output with name/value pairs, allows the use by utilities for instance, without parsing. This is less handy for administrators, but far better for 3rd party utilities. One solution is otherwise to create a PL/SQL command on top of pg_progress() SQL function to produce an output similar to the one of the ANALYZE command.

> 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.

Yes, I had once considered this solution but then moved away as I was unsure of the exact need for the transfert of the progress report between the monitored and the monitoring backends.

I'am going to switch to shm_mq.

Thx & Rgds

 

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