Re: [HACKERS] [PATCH] New command to monitor progression of longrunning queries - Mailing list pgsql-hackers

Hi!

On 17.04.2017 15:09, Remi Colinet wrote:

Hello,

I've implemented a new command named PROGRESS to monitor progression of
long running SQL queries in a backend process.


Use case
=======

A use case is shown in the below example based on a table named t_10m
with 10 millions rows.

The table has been created with :

CREATE TABLE T_10M ( id integer, md5 text);
INSERT INTO T_10M SELECT generate_series(1,10000000) AS id,
md5(random()::text) AS md5;

1/ Start a first psql session to run long SQL queries:

[pgadm@rco ~]$ psql -A -d test
psql (10devel)
Type "help" for help.
test=#

The option -A is used to allow rows to be output without formatting work.

Redirect output to a file in order to let the query run without terminal
interaction:
test=# \o out

Start a long running query:
test=# select * from t_10M order by md5;

2/ In a second psql session, list the backend pid and their SQL query

[pgadm@rco ~]$ psql -d test
psql (10devel)
Type "help" for help.

test=# select pid, query from pg_stat_activity ;
  pid  |                   query
-------+-------------------------------------------
 19081 |
 19084 |
 19339 | select pid, query from pg_stat_activity ;
 19341 | select * from t_10m order by md5;
 19727 | select * from t_10m order by md5;
 19726 | select * from t_10m order by md5;
 19079 |
 19078 |
 19080 |
(9 rows)

test=#

Chose the pid of the backend running the long SQL query to be monitored.
Above example is a parallel SQL query. Lowest pid is the main backend of
the query.

test=# PROGRESS 19341;
                                       PLAN
PROGRESS
-------------------------------------------------------------------------------------------
 Gather Merge
   ->  Sort=> dumping tuples to tapes
         rows r/w merge 0/0 rows r/w effective 0/2722972 0%
         Sort Key: md5
         ->  Parallel Seq Scan on t_10m => rows 2751606/3954135 69% blks
125938/161222 78%
(5 rows)

test=#

The query of the monitored backend is:
test=# select * from t_10M order by md5;

Because the table has 10 millions of rows, the sort is done on tapes.


Design of the command
=================

The design of the patch/command is:
- the user issue the "PROGRESS pid" command from a psql session. The pid
is the one of the backend which runs the SQL query for which we want to
get a progression report. It can be determined from the view
pg_stat_activity.
- the monitoring backend, upon receiving the "PROGRESS pid" command from
psql utility used in step above, sends a signal to the backend whose
process pid is the one provided in the PROGRESS command.
- the monitored backend receives the signal and notes the request as for
any interrupt. Then, it continues its execution of its SQL query until
interrupts can be serviced.
- when the monitored process can service the interrupts, it deals with
the progress request by collecting its execution tree with the execution
progress of each long running node. At this time, the SQL query is no
more running. The progression of each node is calculated during the
execution of the SQL query which is at this moment stopped. The
execution tree is dumped in shared memory pages allocated at the start
of the server. Then, the monitored backend set a latch on which the
monitoring process is waiting for. It then continues executing its SQL
query.
- the monitoring backend collects the share memory data dumped by the
monitored backed, and sends it to its psql session, as a list of rows.

The command PROGRESS does not incur any slowness on the running query
because the execution progress is only computed upon receiving the
progress request which is supposed to be seldom used.

The code heavily reuses the one of the explain command. In order to
share as much code as possible with the EXPLAIN command, part of the
EXPLAIN code which deals with reporting quals for instance, has been
moved to a new report.c file in the src/backend/commands folder. This
code in report.c is shared between explain.c source code and PROGRESS
command source code which is in progress.c file.

The progression reported by PROGRESS command is given in terms of rows,
blocks, bytes and percents. The values displayed depend on the node type
in the execution plan.

The current patch implements all the possible nodes which could take a
lot of time:
- Sequential scan nodes with rows and block progress (node type
T_SeqScan, T_SampleScan, T_BitmapHeaepScan, T_SubqueryScan,
T_FunctionScan, T_ValuesScan, T_CteScan, T_WorkTableScan)
- Tuple id scan node with rows and blocks progress (T_TidScan)
- Limit node with rows progress (T_Limit)
- Foreign and custom scan with rows and blocks progress (T_ForeignScan,
T_CustomScan)
- Index scan, index only scan and bitmap index scan with rows and blocks
progress

Use cases
========

Some further examples of use are shown below in the test_v1.txt file.


What do you make of this idea/patch?

Does it make sense?

Any suggestion is welcome.

The current patch is still work in progress. It is meanwhile stable. It
can be used with regular queries. Utilities commands are not supported
for the moment.
Documentation is not yet written.

Regards
Remi

I had implemented analogous feature as extension *pg_query_state* [1] the idea of which I proposed in the thread [2]. Together with this extension I provided some patches to postgres core to enable to send custom signals to working backend (similar to your PROCSIG_PROGRESS) and to print the current query state through patches in 'ExplainNode' function.

I had implemented the same mechanics as you:
1) interrupt the working backend through ProcSignal;
2) handle progress request in the CHECK_FOR_INTERRUPTS entry;
3) transfer query state through shared memory to caller.
But criticism of my approach was that the structure 'QueryDesc' on basis of which query state is formed can be inconsistent in the places where CHECK_FOR_INTERRUPTS appears [3].

I plan to propose the custom_signal patch to community as soon as possible and as consequence release *pg_query_state* from dependency on patches to postgres core. In perspective, I want to resolve the problem related to binding to the CHECK_FOR_INTERRUPTS entries perhaps through patching the executor and implement the robust PROGRESS command.


1. https://github.com/postgrespro/pg_query_state
2. https://www.postgresql.org/message-id/dbfb1a42-ee58-88fd-8d77-550498f52bc5%40postgrespro.ru
3. https://www.postgresql.org/message-id/24182.1472745492%40sss.pgh.pa.us


--
Maksim Milyutin
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company