Re: RecoveryInProgress() has critical side effects

Hi,

On 2021-11-15 16:29:28 -0500, Robert Haas wrote:
> v1: 0.378 ms
> v2: 0.391 ms
> common base commit (10eae82b2): 0.376 ms

Is this with assertions enabled? Because on my workstation (which likely is
slower on a single-core basis than your laptop) I get around half of this with
an optimized build (and a non-optimized build rarely is worth using as a
measuring stick). It could also be that your psqlrc does something
heavyweight?

> methodology:
> for i in `seq 1 1000`; do psql -c '\timing' -c 'select
> txid_current();'; done | grep '^Time:' | awk '{total+=$2} END {print
> total/NR}'
> run twice, discarding the first result, since the very first
> connection attempt after starting a new server process is notably
> slower

Hm. I think this might included a bunch of convoluting factors that make it
hard to judge the actual size of the performance difference. It'll
e.g. include a fair bit of syscache initialization overhead that won't change
between the two approaches. This could be addressed by doing something like -c
"SELECT 'txid_current'::regproc;" first.

Also, the psql invocations itself use up a fair bit of time, even if you
ignored them from the result with the \timing trick. A pgbench -C doing 1k
SELECT 1;s is about ~1.5s for me, whereas psql is ~5.7s.

Just to size up the order of magnitude of overhead of the connection
establishment and syscache initialization, I ran a pgbench with a script of

SELECT 1;
SELECT 1;
SELECT 'txid_current()'::regprocedure;
SELECT 'txid_current()'::regprocedure;
SELECT txid_current();
SELECT txid_current();

and ran that with pgbench -n -f /tmp/txid.sql -C -t1000 -P1 --report-latencies
and got
statement latencies in milliseconds:
0.125 SELECT 1;
0.024 SELECT 1;
0.095 SELECT 'txid_current()'::regprocedure;
0.025 SELECT 'txid_current()'::regprocedure;
0.033 SELECT txid_current();
0.024 SELECT txid_current();

which nicely shows how much of the overhead is not related to the actual
txid_current() invocation.

Greetings,

Andres Freund