postgres=# create table ptest(id integer, name varchar(20));
CREATE TABLE
postgres=# create table ctest01(CHECK(id<5000000)) inherits (ptest);
CREATE TABLE
postgres=# create table ctest02(CHECK(id>=5000000)) inherits (ptest);
CREATE TABLE
postgres=# 
postgres=# create index on ctest01(id);
CREATE INDEX
postgres=# create index on ctest02(id);
CREATE INDEX
postgres=# 
postgres=# 

postgres=# CREATE OR REPLACE FUNCTION ptest_insert_trigger() RETURNS TRIGGER AS $$ 
postgres$# 
postgres$# BEGIN 
postgres$# 
postgres$#    IF ( NEW.id <5000000 ) THEN 
postgres$#        INSERT INTO ctest01 VALUES (NEW.*);
postgres$#    ELSIF ( NEW.id >= 5000000 ) THEN 
postgres$#        INSERT INTO ctest02 VALUES (NEW.*); 
postgres$#    ELSE 
postgres$#        RAISE EXCEPTION 'Error while inserting data';
postgres$#    END IF; 
postgres$#   
postgres$#   RETURN NULL;
postgres$# END; $$ LANGUAGE plpgsql;
CREATE FUNCTION
postgres=# 
postgres=# CREATE TRIGGER insert_ptest_trigger BEFORE INSERT ON ptest FOR EACH ROW 
postgres-#   EXECUTE PROCEDURE ptest_insert_trigger();
CREATE TRIGGER
postgres=#