How can I use a PostgreSQL triggers to store changes (SQL statements and row changes)

example of an audit trigger from https://www.postgresql.org/docs/current/static/plpgsql-trigger.html

CREATE TABLE emp (
    empname           text NOT NULL,
    salary            integer
);

CREATE TABLE emp_audit(
    operation         char(1)   NOT NULL,
    stamp             timestamp NOT NULL,
    userid            text      NOT NULL,
    empname           text      NOT NULL,
    salary integer
);

CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
    BEGIN
        --
        -- Create a row in emp_audit to reflect the operation performed on emp,
        -- make use of the special variable TG_OP to work out the operation.
        --
        IF (TG_OP = 'DELETE') THEN
            INSERT INTO emp_audit SELECT 'D', now(), user, OLD.*;
            RETURN OLD;
        ELSIF (TG_OP = 'UPDATE') THEN
            INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*;
            RETURN NEW;
        ELSIF (TG_OP = 'INSERT') THEN
            INSERT INTO emp_audit SELECT 'I', now(), user, NEW.*;
            RETURN NEW;
        END IF;
        RETURN NULL; -- result is ignored since this is an AFTER trigger
    END;
$emp_audit$ LANGUAGE plpgsql;

CREATE TRIGGER emp_audit
AFTER INSERT OR UPDATE OR DELETE ON emp
    FOR EACH ROW EXECUTE PROCEDURE process_emp_audit();

Do you actually need the audit log of queries stored in a table? The easiest way to get a file with all the queries that have been executed is to use postgresql's built-in logging.

In your postgresql.conf (usually in the $PG_DATA dir), set the following options appropriately:

log_directory '/path/to/log/dir'
log_filename = 'filename.log'
log_statement = 'mod'

That last option makes it log all the INSERT, UPDATE, DELETE, TRUNCATE, and COPY FROM statements.

More details in the Postgres docs: http://www.postgresql.org/docs/current/static/runtime-config-logging.html

The link below should point you in the right direction.

https://www.postgresql.org/docs/current/sql-createtrigger.html

Depending on what you want to do, it probably is better to turn on logging.