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fuy | 2 years ago

One other thing about JIT that I feel is pretty crazy is that the generated code is not cached. I mean it's the most expensive part of the query execution a lot of the time, how come it's not cached? I couldn't find good reasons for this looking through Postgres mailing lists discussion around JIT.

Disabling JIT is the way to go for OLTP workloads.

discuss

order

davidrowley|2 years ago

There's some information about why that does not happen in https://www.postgresql.org/message-id/20211104234742.ao2qzqf...

In particular:

> The immediate goal is to be able to generate JITed code/LLVM-IR that doesn't > contain any absolute pointer values. If the generated code doesn't change > regardless of any of the other contents of ExprEvalStep, we can still cache > the JIT optimization / code emission steps - which are the expensive bits.

A colleague is working on getting this patch into shape. So we might see some caching work get done after the relative pointer work is in.

SigmundA|2 years ago

Unlike say MSSQL or Oracle PG does not cache plans at all. I think this is mostly due to its multiprocess architecture vs just sharing in memory plans between threads. In MSSQL a plan can take a while to optimize including jitting if needed but it doesn't matter that much because all plans are cached so when that statement comes in again the plan is ready to go.

nextaccountic|2 years ago

> I think this is mostly due to its multiprocess architecture vs just sharing in memory plans between threads

You can share stuff with a multiprocess architecture just fine (either through IPC or just plain shared memory + synchronization)

It's true that threads share memory by default, but processes can opt into sharing memory if they wish. And it appears that Postgres already makes use of shared memory for some things

https://www.instaclustr.com/blog/postgresql-docker-and-share...

https://stackoverflow.com/questions/32930787/understanding-p...

(random links from Google just to illustrate the point)