Re: sequences vs. synchronous replication

On 12/21/21 02:01, Tom Lane wrote:
> Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
>> OK, I did a quick test with two very simple benchmarks - simple select
>> from a sequence, and 'pgbench -N' on scale 1. Benchmark was on current
>> master, patched means SEQ_LOG_VALS was set to 1.
>
> But ... pgbench -N doesn't use sequences at all, does it?
>
> Probably inserts into a table with a serial column would constitute a
> plausible real-world case.
>

D'oh! For some reason I thought pgbench has a sequence on the history
table, but clearly I was mistaken. There's another thinko, because after
inspecting pg_waldump output I realized "SEQ_LOG_VALS 1" actually logs
only every 2nd increment. So it should be "SEQ_LOG_VALS 0".

So I repeated the test fixing SEQ_LOG_VALS, and doing the pgbench with a
table like this:

create table test (a serial, b int);

and a script doing

insert into test (b) values (1);

The results look like this:

1) select nextval('s');

clients 1 4
------------------------------
master 39533 124998
patched 3748 9114
------------------------------
diff -91% -93%

2) insert into test (b) values (1);

clients 1 4
------------------------------
master 3718 9188
patched 3698 9209
------------------------------
diff 0% 0%

So the nextval() results are a bit worse, due to not caching 1/2 the
nextval calls. The -90% is roughly expected, due to generating about 32x
more WAL (and having to wait for commit).

But results for the more realistic insert workload are about the same as
before (i.e. no measurable difference). Also kinda expected, because
those transactions have to wait for WAL anyway.

regards

--
Tomas Vondra
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