Custom Aggregate Postgresql C extension with input as initial condition

Question

Is there a way to write a custom aggregate Postgresql C extension function that takes an input from the user and sets it as the initial condition? If so, how?

I’ve written several Postgresql Extensions in the past. The inputs are always columns of the table, but now it is requirement —for theoretical reasons- that I need to make an aggregate extension dependent on one user input that changes a lot. In particular, this values cannot be stored in the database either.

Edit 1: For example, say I want to run the function

aggregate fancy_select( int tmp_accum_value, int column, int *fancy_math_stuff){

return (row *** fancy_math_stuff)    
}

where *** is a fancy math operation that uses the entire fancy_math_stuff array and said array is passed on the first call but it is not stored as column in the database. tmp_accum_value is the intermediary state of the accumulation.

Answer 1

If you need to pass an argument to an aggregate function, just do it. That argument does not have to be a table column.

For example:

SELECT myagg(col1, 'initial value') FROM atable;

True, you could also call the function as

SELECT myagg(col1, col2) FROM atable;

but nobody forces you to do that, if you want the second argument to be a constant used for initializing then state.