'CREATE OR REPLACE FUNCTION' behavior whenever a transaction is running

On Wed, Apr 20, 2022 at 12:50 PM PG Doc comments form <
noreply@postgresql.org> wrote:

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/14/sql-createfunction.html
Description:

The page does not mention what is the default behavior whenever a function
is replaced while the same function is being used in another
query/transaction.

Does the query fail? Does the 'CREATE OR REPLACE' operation fail? Does the
query finish before the function is replaced?

As a side note, my interest in this came from our need to use 'C-Language
Functions', aka binary functions, so it would be neat to also add what is
the behavior on those as well, if there's any difference.

IIUC pg_proc is administered using MVCC behavior just like any other
table. What you experience will be subject to your isolation mode but in
no case will a single command's execution see different versions nor will
such execution prevent the "replace"ment of the function with a newer
version.

Absent guidance to the contrary I wouldn't expect C language functions to
behave any differently than any others. But that just pertains to the
"REPLACE" aspect. You need to read the notes about C language functions to
get the rest of the story (and you likely wouldn't need to "replace" a
C-langauge function entry because of this different loading/compiling
mechanism). Namely:

https://www.postgresql.org/docs/current/xfunc-c.html#XFUNC-C-DYNLOAD

In short, C-language functions, referenced simply by name and module, and
pre-compiled, do not have a mechanism to invalidate the cache like pl/pgsql
functions do. Other languages may involve yet other dynamics.

David J.