Re: postgresql13-llvm jit-13.5-1PGDG.rhel8.x86_64

On Mon, Nov 15, 2021 at 8:57 AM Mladen Gogala <gogala(dot)mladen(at)gmail(dot)com>
wrote:

>
> On 11/15/21 00:04, Pavel Stehule wrote:
>
>
> Hi
>
>
>> There is a CentOS8-stream version which solves the problem but I cannot
>> use that in the office. I will probably have to wait for another month
>> before OL8 has everything that I need in its repositories. Now, the
>> question is what kind of an impact will running without llvm-jit have?
>> According to the links below, llvm-jit effects are quite spectacular:
>>
>> https://llvm.org/devmtg/2016-09/slides/Melnik-PostgreSQLLLVM.pdf
>>
>> https://www.pgcon.org/2017/schedule/events/1092.en.html
>
>
> When JIT was used on very large query with a lot of CASE expr, then JIT
> has a positive effect about 50%. On usual large queries, the effect of JIT
> was about 20%. Unfortunately, JIT is sensitive to estimation, and the JIT
> sometimes increases seconds to queries, although without JIT this query is
> executed in ms. When you use a query that can be well calculated in
> parallel, then positive effect of JIT is less.
>
> Regards
>
> Pavel
>
>
> Thanks Pavel, you answered my question. I'll wait with the upgrade.
>
>
>
FWIW, there was a lively discussion on the postgresql subreddit over the
weekend on JIT:
https://www.reddit.com/r/PostgreSQL/comments/qtsif5/cascade_of_doom_jit_and_how_a_postgres_update_led/

(lively for that subreddit anyway)