Thanks for the insight on the internals. Regarding your questions:
On Tue, 31 Jan 2023, Tom Lane wrote:
Do you get the same 10 rows when you repeat the command?
Yes. Just tested with both cold and hot caches. The first 10 rows are
exactly the same, either they return slowly or immediately.
If turning synchronize_seqscans off changes the behavior, that'd
be a good clue that this is the right theory.
Turning it off makes the query slow no matter how many times I re-run it.
The system is doing lots of read I/O with both hot and cold caches. Here
is the EXPLAIN output from the hot cache run (that previously had only 14
hits and no reads):
Limit (cost=0.00..0.29 rows=10 width=42) (actual time=620510.813..620510.821
rows=10 loops=1)
Output: run_n, test_name_n, workitem_n, started_on, duration_ms,
test_result_n, test_executable_n, test_function_n, test_datatag_n
Buffers: shared hit=64 read=2334462
I/O Timings: shared/local read=567846.559
-> Seq Scan on public.test_runs_raw (cost=0.00..9250235.80 rows=317603680
width=42) (actual time=620510.800..620510.804 rows=10 loops=1)
Output: run_n, test_name_n, workitem_n, started_on, duration_ms,
test_result_n, test_executable_n, test_function_n, test_datatag_n
Buffers: shared hit=64 read=2334462
I/O Timings: shared/local read=567846.559
Settings: effective_cache_size = '2GB', max_parallel_workers_per_gather = '0',
work_mem = '64MB'
Planning Time: 0.099 ms
Execution Time: 620510.855 ms
After reading the docs, I'm surprised this setting affects my case given
that I have no parallelism in my setup.
As for a real fix,
it might be time for a VACUUM FULL or CLUSTER on that table.
Regarding CLUSTER, would it help with a seqscan on a bloated table?
Furthermore, given that the table is expected to grow every day by a few
million rows, do you suggest running CLUSTER every night? Will postgres
remember that the rows up to N are clustered, even after appending more
rows?
Dimitris
