Parallel workers stats in pg_stat_database

Hi hackers,

This patch introduces four new columns in pg_stat_database:

* parallel_worker_planned
* parallel_worker_launched
* parallel_maint_worker_planned
* parallel_maint_worker_launched

The intent is to help administrators evaluate the usage of parallel
workers in their databases and help sizing max_worker_processes,
max_parallel_workers or max_parallel_maintenance_workers).

Here is a test script:

psql << _EOF_

-- Index creation
DROP TABLE IF EXISTS test_pql;
CREATE TABLE test_pql(i int, j int);
INSERT INTO test_pql SELECT x,x FROM generate_series(1,1000000) as F(x);

-- 0 planned / 0 launched
EXPLAIN (ANALYZE)
SELECT 1;

-- 2 planned / 2 launched
EXPLAIN (ANALYZE)
SELECT i, avg(j) FROM test_pql GROUP BY i;

SET max_parallel_workers TO 1;
-- 4 planned / 1 launched
EXPLAIN (ANALYZE)
SELECT i, avg(j) FROM test_pql GROUP BY i
UNION
SELECT i, avg(j) FROM test_pql GROUP BY i;

RESET max_parallel_workers;
-- 1 planned / 1 launched
CREATE INDEX ON test_pql(i);

SET max_parallel_workers TO 0;
-- 1 planned / 0 launched
CREATE INDEX ON test_pql(j);
-- 1 planned / 0 launched
CREATE INDEX ON test_pql(i, j);

SET maintenance_work_mem TO '96MB';
RESET max_parallel_workers;
-- 2 planned / 2 launched
VACUUM (VERBOSE) test_pql;

SET max_parallel_workers TO 1;
-- 2 planned / 1 launched
VACUUM (VERBOSE) test_pql;

-- TOTAL: parallel workers: 6 planned / 3 launched
-- TOTAL: parallel maint workers: 7 planned / 4 launched
_EOF_

And the output in pg_stat_database a fresh server without any
configuration change except thoses in the script:

[local]:5445 postgres(at)postgres=# SELECT datname,
parallel_workers_planned, parallel_workers_launched,
parallel_maint_workers_planned, parallel_maint_workers_launched FROM pg
_stat_database WHERE datname = 'postgres' \gx

-[ RECORD 1 ]-------------------+---------

datname | postgres

parallel_workers_planned | 6

parallel_workers_launched | 3

parallel_maint_workers_planned | 7

parallel_maint_workers_launched | 4

Thanks to: Jehan-Guillaume de Rorthais, Guillaume Lelarge and Franck
Boudehen for the help and motivation boost.

---
Benoit Lobréau
Consultant
http://dalibo.com