Alter data type of a column to serial

Question

In pgsql, is there a way to have a table of several values, and choose one of them (say, other_id), find out what its highest value is and make every new entry that is put i

Answer 1

A quick glance at the docs tells you that

The data types smallserial, serial and bigserial are not true types but merely a notational convenience for creating unique identifier columns

If you want to make an existing (integer) column to work as a "serial", just create the sequence by hand (the name is arbitrary), set its current value to the maximum (or bigger) of your current address.new_id value, at set it as default value for your address.new_id column.

To set the value of your sequence see here.

SELECT setval('address_new_id_seq', 10000);

This is just an example, use your own sequence name (arbitrary, you create it), and a number greater than the maximum current value of your column.

---

Update: as pointed out by Lucas' answer (which should be the acccepted one) you should also specify to which column the sequence "belongs to" by using CREATE/ALTER SEQUENCE ... OWNED BY ...

Answer 2

Look into postgresql documentation of datatype serial. Serial is only short hand.

CREATE TABLE tablename (
    colname SERIAL
);

is equivalent to specifying:

CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename (
    colname integer NOT NULL DEFAULT nextval('tablename_colname_seq')
);
ALTER SEQUENCE tablename_colname_seq OWNED BY tablename.colname;

Answer 3

This happened because you may use serial data type only when you are creating new table or adding new column to a table. If you'll try to ALTER existing table using this data type you'll get an error. Because serial is not a true data type, but merely an abbreviation or alias for a bit longer query.

In case you would like to reach the same effect, as you are expecting from using serial data type when you are altering existing table you may do this:

CREATE SEQUENCE my_serial AS integer START 1 OWNED BY address.new_id;

ALTER TABLE address ALTER COLUMN new_id SET DEFAULT nextval('my_serial');

1. First line of query creates your own sequence with name my_serial. OWEND BY statement connects newly created sequence with exact column of your table. In your exact case table is address and column is new_id. START statement defines what value should this sequence start from.

2. Second line alters your table with new default value, which will be determined by previously created sequence.

It will brings you to the same result as you were expecting from using serial.