Re: Centralizing protective copying of utility statements

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2021-06-16 21:39:49 -0400, Tom Lane wrote:
>> Although this adds some overhead in the form of copying of
>> utility node trees that won't actually mutate during execution,
>> I think that won't be too bad because those trees tend to be
>> small and hence cheap to copy.

> Have you evaluated the cost in some form? I don't think it a relevant
> cost for most utility statements, but there's a few exceptions that *do*
> worry me. In particular, in some workloads transaction statements are
> very frequent.

I hadn't, but since you mention it, I tried this test case:

$ cat trivial.sql
begin;
commit;
$ pgbench -n -M prepared -f trivial.sql -T 60

I got these results on HEAD:
tps = 23853.244130 (without initial connection time)
tps = 23810.759969 (without initial connection time)
tps = 23167.608493 (without initial connection time)
tps = 23784.432746 (without initial connection time)

and adding the v2 patch:
tps = 23298.081147 (without initial connection time)
tps = 23614.466755 (without initial connection time)
tps = 23475.297853 (without initial connection time)
tps = 23530.826527 (without initial connection time)

So if you squint there might be a sub-one-percent difference
there, but it's barely distinguishable from the noise. In
any situation where the transactions are doing actual work,
I doubt you could measure a difference.

(In any case, if someone does get excited about this, they
could rearrange things to push the copyObject calls into the
individual arms of the switch in ProcessUtility. Personally
though I doubt it could be worth the code bloat.)

regards, tom lane