Global configuration of native Postgres functions at function creation time

Question

My C++ functions for postgres use a parameter (the path to the config) for static singleton Config class, now it is set from Cmake via definitions. I want to make the path specified like an argv parameter in the main function. However, my functions are just a shared (.so) library. Is there any way to achieve such functionality ?

Now I use this vartiant:

add_compile_definitions(CONFIG_PATH="/some/path/to/config.ini)

Also I have sql script for creating postgres functions:

CREATE OR REPLACE FUNCTION some() RETURNS text
     AS 'postgres_Clibrary.so', 'someFunction'
     LANGUAGE C;

And load functions as:

psql -f create_functions.sql

And I want to change this to something like:

psql -f create_functions.sql -DPATH_TO_CONFIG="/some/path/to/config.ini"

Is there any variant to get similar functionality as I want ? How to do it without recompilation ?

I am not certain what you mean. Do you want a fixed value for a certain function argument, defined at the time the function is created in PostgreSQL? — Laurenz Albe
– Laurenz Albe, Commented Sep 21, 2022 at 8:50
@LaurenzAlbe If we use a regular bin file, then we write ./execute "some" "argv" "params". I want that when creating functions using psql or something like that, you can specify a path so as not to compile the library again. — lmidori
– lmidori, Commented Sep 21, 2022 at 9:26
The linked answer shows how to use system settings to accomplish what you want. If needed, that can be overridden on a per-connection basis — Botje
– Botje, Commented Sep 21, 2022 at 9:41
@Botje I don't think that's what is asked for, so I'll vote to reopen. I'd say that default values for a function argument are the solution. A placeholder parameter is not fixed in any way and can always change. — Laurenz Albe
– Laurenz Albe, Commented Sep 21, 2022 at 9:47

Laurenz Albe · Accepted Answer · 2022-09-21 11:02:06Z

0

I think you could use a default value for a parameter here.

Define your function so that it has an additional parameter config_path. That parameter has to be the last one. Then you can define the function as

CREATE FUNCTION some(
   first_param text,
   config_path text DEFAULT '/some/path/to/config.ini'
) RETURNS text
AS 'postgres_Clibrary.so', 'someFunction'
LANGUAGE C;

Now you can simply call the function with

SELECT some('first_arg');

and the default value will be substituted for the second argument.

edited Sep 21, 2022 at 11:02

answered Sep 21, 2022 at 9:51

Laurenz Albe

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4 Comments

lmidori Over a year ago

So, it should work, but my config is a static sigleton class, so I don't want to read or create class object every time

Laurenz Albe Over a year ago

I thought you were talking about a function, not a class... Can you clarify?

lmidori Over a year ago

The function body uses the singleton class, which must be initialized in advance. Something like static Config& getConfig(), which in turn uses configPath. So, I can't init it every time, I should do this only one time

Laurenz Albe Over a year ago

Well, you can check if it is already initialized, can't you? And only initialize it the first time.

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