Postgresql insert trigger to set value

Question

Assume in Postgresql, I have a table T and one of its column is C1.

I want to trigger a function when a new record is adding to the table

Answer 1

It is possible but you are likely to be better off setting a default constraint on the column instead. When creating the table that would look like:

create table mytable as (
    C1 thetype not null default X
);

This says that if you add a row to the table and don't specify the value for C1 then X will be used instead. The not null is not necessary, but prevents updates from nulling that column assuming that's what you want.

EDIT: This only works for constant X, from your comments it seems there are two possible solutions.

Using a trigger would look something like this:

create function update_row_trigger() returns trigger as $$
begin
    if new.C1 is NULL then
        new.C1 := X;
    end if;
    return new;
end
$$ language plpgsql;

create trigger mytrigger before insert on mytable for each row execute procedure update_row_trigger();

The new variable in a trigger function is special, representing the row being inserted. Specifying the trigger as a before insert trigger means you can modify the row before it is written to the table.

The second solution would be to use a computed column which Postgres defines in an unusual way:

create or replace function C1(row mytable) returns columntype immutable as $$
begin
    return X; -- where X is an expression using values from `row`
end
$$ language plpgsql;

This creates a function that takes a row of your table and returns a value, you can call it using . notation though, meaning you can do:

select
    *,
    t.C1
from
    mytable t;

The declaration of the function being immutable is optional, but it's needed if you want to index the "column". You would be able to index this column like this:

create index on mytable (C1(mytable));