Re: Potential "AIO / io workers" inter-worker locking issue in PG18?

Last attempt to get the mail out in a proper format. I hate it when software gets "smart" with formatting... ;-(

I added one minor detail to the last question of Andres at the entire bottom, see below.

Even just knowing whether the "normal query plan" is the same one as we see in
profiles of "stuck" backends is valuable. Even if the query plan is perfectly
normal, it *still* is very important to know in which order the joins are
evaluated etc. But there also might be changes in the query plan between 17
and 18 that trigger the issue...

Without more details about what is expected to be run and what is actually
happening, it's just about impossible for us to debug this without a
reproducer that we can run and debug ourselves.

https://www.postgresql.org/docs/current/auto-explain.html

Making vacuum more aggressive won't really help much if you have

longrunning queries/sessions, since vacuum can't clean up row versions that
are still visibile to some of the transactions.

My code batches the updates in sets of 2000 records at a time and then
COMMITs, so the transactions themselves are limited in time and size, which
should allow vacuum to do its job.

Are the "stuck" backends stuck within one 2000 record batch, or are they
"just" slower processing each batch?

Note that this is just a simplified version of the different processing modes I developed for different challenges and geoprocessing steps.

    26.48%  postgres  postgres           [.] LWLockAttemptLock
            |
            ---LWLockAttemptLock
               |
               |--23.15%--heapam_index_fetch_tuple.lto_priv.0
               |          index_fetch_heap
               |          IndexNext
               |          ExecScan
               |          ExecNestLoop
               |          ExecNestLoop
               |          ExecModifyTable
               |          standard_ExecutorRun
               |          ProcessQuery

So the query plan we have is a nested loop between at least three tables
(there are two joins, c.f. the two ExecNestLoop calls), where there presumably
are a lot of row [versions] on the inner side of the innermost join.

In [1] you showed a query. Reformated that looks like this:

UPDATE osm.landcover_scrubs_small_scale_2_ply AS t1
SET area_geo = t2.area_geo,    perim_geo = t2.perim_geo,    compact_geo = CASE WHEN t2.area_geo > 0 THEN ((power(t2.perim_geo,2) / t2.area_geo) / (4 * pi())) ELSE 0 END,    npoints_geo = t2.npoints_geo,    comp_npoints_geo = CASE WHEN t2.npoints_geo > 0 THEN (CASE WHEN t2.area_geo > 0 THEN ((power(t2.perim_geo,2) / t2.area_geo) / (4 * pi())) ELSE 0 END / t2.npoints_geo) ELSE 0 END,    convex_ratio_geo = CASE WHEN ST_Area(ST_ConvexHull(way)::geography,true) > 0 THEN (t2.area_geo / ST_Area(ST_ConvexHull(way)::geography,true)) ELSE 1 END
FROM (    SELECT        objectid,        ST_Area(way::geography,true) AS area_geo,        ST_Perimeter(way::geography,true) AS perim_geo,        ST_NPoints(way) AS npoints_geo    FROM osm.landcover_scrubs_small_scale_2_ply) AS t2
WHERE (t2.objectid = t1.objectid)   AND t1.objectid IN (SELECT t3.objectid FROM mini_test.osm.osm_tmp_28128_ch5 AS t3)


Which certainly fits with two nested loops, although I don't think I can infer
which order it the joins are in.


Is osm.landcover_scrubs_small_scale_2_ply.object_id unique?

Yes.

Can there be multiple rows for one object_id in
mini_test.osm.osm_tmp_28128_ch5?

No. This table contains the records to process, which are unique. It is the job.

It is a one-to-one join.

Are there indexes on mini_test.osm.osm_tmp_28128_ch5.unique_id and
osm.landcover_scrubs_small_scale_2_ply?

Yes, the unique ids / objectid fields are indexed to allow an efficient join.

Actually, the "*_ch<number>" database object that represents the records to process for one job, references a database view. Each thread gets its own view. All views reference the same secondary table that has an index on the objectid.