I m a new comer to postgresql and database part. I wish to check function exit or otherwise in postgresql using trigger before place statement. any body understand how to create trigger this?

Produce a special trigger function:

CREATE OR REPLACE FUNCTION some_function() 
RETURNS trigger AS $$
    BEGIN
        IF some_condition THEN
            RAISE EXCEPTION 'Some message for id %', NEW.id;
        END IF;
        RETURN NEW;
    END;
$$ LANGUAGE plpgsql;

Then define a trigger to fireplace it

CREATE TRIGGER some_trigger 
BEFORE INSERT OR UPDATE ON some_table
FOR EACH ROW EXECUTE PROCEDURE some_function();

Once the exception is elevated, the update will fail.

The SQL performed might be or arbitrary complexity, including any query or function you want. This may also alter the NEW values via SET NEW.some_column = some_value, as well as update other tables.

Note the NEW.id is definitely an example column only. Go to whichever column you want in the NEW row (or none) within the message.

When the check is localized towards the row, get a column constraint, eg:

CREATE TABLE my_table (
    some_column int CHECK (some_column between 0 and 10),
    other_column int CHECK (other_column < some_column)
)