Re: Need help identifying a periodic performance issue.

[Robert Creager <robertc@xxxxxxxxxxxxxxxx>]

I forgot, I had reloaded postgres, but had not re-started our app, so the connections wouldn’t have that plan setting on them. Re-doing now.

On Nov 24, 2021, at 2:13 PM, Robert Creager <robertc@xxxxxxxxxxxxxxxx> wrote:

On Nov 19, 2021, at 11:47 AM, Robert Creager <robertc@xxxxxxxxxxxxxxxx> wrote:

On Nov 18, 2021, at 2:42 PM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:

This message originated outside your organization.

Thomas Munro <thomas.munro@xxxxxxxxx> writes:

This is all guesswork though, since we don't know quite what's
happening on Robert's system.  It might be worth setting
"log_autovacuum_min_duration = 0" (either globally, or as a
reloption on the relevant tables), and seeing if there seems
to be any correlation between autovacuum/autoanalyze activity
and the occurrences of poor plan choices.

Ok, doing a SET plan_cache_mode=force_custom_plan before the COPY and resetting it after appears to fix the problem.  We’re going to run it over the weekend to make sure.

We are at it again.  I have a DELETE operation that’s taking 48 minutes so far.  I had set plan_cache_mode = force_custom_plan for the entire server before this happened, as we started seeing the COPY slowdown again.  I have dtrace information again, but primarily shows the nested scan operation.

pid,client_port,runtime,query_start,datname,state,wait_event_type,query,usename

40665,15978,0 years 0 mons 0 days 0 hours 48 mins 49.62347 secs,2021-11-24 20:13:30.017188 +00:00,tapesystem,active,,DELETE FROM ds3.blob WHERE EXISTS (SELECT * FROM ds3.s3_object WHERE id = ds3.blob.object_id AND (bucket_id = $1)),Administrator

So how do we avoid this query plan? Do we need to start doing explicit analyzes after every delete?

EXPLAIN DELETE
FROM ds3.blob
WHERE EXISTS(SELECT * FROM ds3.s3_object WHERE id = ds3.blob.object_id AND (bucket_id = '85b9e793-2141-455c-a752-90c2346cdfe1'));

250k objects in blob
256k objects in s3_object

QUERY PLAN
Delete on blob  (cost=10117.05..16883.09 rows=256002 width=12)
  ->  Hash Join  (cost=10117.05..16883.09 rows=256002 width=12)
        Hash Cond: (blob.object_id = s3_object.id)
        ->  Seq Scan on blob  (cost=0.00..6094.02 rows=256002 width=22)
        ->  Hash  (cost=6917.02..6917.02 rows=256002 width=22)
              ->  Seq Scan on s3_object  (cost=0.00..6917.02 rows=256002 width=22)
                    Filter: (bucket_id = '8a988c6c-ef98-465e-a148-50054c739212'::uuid)

’Normal’ explain, very few objects with that bucket.

QUERY PLAN
Delete on blob  (cost=0.71..6.76 rows=1 width=12)
  ->  Nested Loop  (cost=0.71..6.76 rows=1 width=12)
        ->  Index Scan using ds3_s3_object__bucket_id on s3_object  (cost=0.29..2.31 rows=1 width=22)
              Index Cond: (bucket_id = '85b9e793-2141-455c-a752-90c2346cdfe1'::uuid)
        ->  Index Scan using ds3_blob__object_id on blob  (cost=0.42..4.44 rows=1 width=22)
              Index Cond: (object_id = s3_object.id)