BUG #19505: Some weird spikes postgresql processes in database (up to 200k sometime) without apparent reasons.

The following bug has been logged on the website:

Bug reference: 19505
Logged by: Maxim Boguk
Email address: maxim(dot)boguk(at)gmail(dot)com
PostgreSQL version: 18.4
Operating system: Ubuntu 24.04.4 LTS
Description:

I started investigation of this issue after found that process count of
postgresql on my replica sometime jump to 200k+ (with max_connections=1000
and real connections under 100 most time).
Somehow single (seems random by always heavy/analytical) query spawn
thousands of the threads and tens thousands of parallel workers.

After some logging I caught one snapshot (ps -u postgres -L -o
pid,tid,ppid,lstart,args -ww 2 ) with 39257 processes:

[postgres(at)db ~/tmp]$ zcat ps-L-2026-06-02_17-40-22.gz | wc -l
39257

Main content is:
PID TID PPID StartTime
command
2158552 2158552 948705 Tue Jun 2 17:40:17 2026 postgres: 18/main:
background_shared db [local] SELECT

Then:
The same PID but 1620 different TIDS.
PID TID PPID StartTime
command
#main process
2158557 2158557 948705 Tue Jun 2 17:40:18 2026 postgres: 18/main:
background_shared db [local] SELECT
#1620 threads
2158557 2158607 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main:
background_shared db [local] SELECT
2158557 2158608 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main:
background_shared db [local] SELECT
2158557 2158609 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main:
background_shared db [local] SELECT

Then, 37571 rows!!! of:
PID TID PPID StartTime
command
2158579 2159176 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main: parallel
worker for PID 2158557
2158579 2159179 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main: parallel
worker for PID 2158557
2158579 2159183 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main: parallel
worker for PID 2158557
2158579 2159196 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main: parallel
worker for PID 2158557
2158579 2159198 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main: parallel
worker for PID 2158557
2158579 2159202 948705 Tue Jun 2 17:40:20 2026 postgres: 18/main: parallel
worker for PID 2158557

I double checked the query (it had been logged in database log): it run with
6 worker processes and without any issues on manual run.

Related db configuration:
max_connections = 1000
max_worker_processes = 128 # (change requires restart)
max_parallel_workers_per_gather = 16 # limited by max_parallel_workers
max_parallel_workers = 64
io_method = io_uring # worker, io_uring, sync
io_max_concurrency = -1 # Max number of IOs that one process
jit = on (usual suspect in case of weird things going on)

Given that situation happens like 1-10 times per hour (and lead for short LA
spikes up to 10000) - it's seriously affect the database replica
performance.

No external/non-standard/C extensions except of pgq and postgis loaded into
the database.

I can look for any additional information and perform any local research
but currently I'm out of ideas what my next steps should be.

PS: it's seems that the issue could be triggered by different queries, but
not the one particular.