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PostgreSQL Identity Columns

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It’s interesting to see the way different databases implement automatic numbering. Oracle Database 12c is the closest to PostgreSQL in some significant ways. However, its probably more accurate to say Oracle Database 12c copied PostgreSQL’s implementation. At least, that’s my conjecture because Oracle added a way to reset the START WITH value of the indirect sequence. However, I prefer the MySQL approach because the automatic numbering sequence is a property of the table and a simple clause of the CREATE TABLE statement.

Both PostgreSQL and Oracle Database 12c implement automatic numbering as indirect sequences. Indirect sequences are those created by a table when you designate a column as an identity column in Oracle or as a serial column in PostgreSQL. The difference is that PostgreSQL doesn’t provide a syntax version inside the CREATE TABLE semantic.

MySQL provides such syntax. You set an auto numbering column in MySQL by appending the AUTO_INCREMENT clause to the table creation statement when you want it to start with a number other than 1, like this:

CREATE TABLE auto
( id           INT UNSIGNED PRIMARY KEY AUTO_INCREMENT
, text_field   VARCHAR(30)  NOT NULL
) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8;

Oracle disallows you to changing a sequence created as a background activity of the CREATE TABLE statement; and Oracle disallows you dropping an indirect sequence without changing the table that created it, which is exactly how they handle indexes created for unique constraints. Unfortunately, Oracle also disallows altering the START WITH value of any sequence.

If you want to change the START WITH value on an Oracle Database 12c indirect sequence, you must export the table, drop the table, and recreate the table with a new START WITH value before importing the data back into the table. The syntax for setting an IDENTITY column value higher than 1 is:

CREATE TABLE auto
( auto_id     NUMBER GENERATED BY DEFAULT AS IDENTITY (START WITH 1001)
, text_field  VARCHAR2(30)
, CONSTRAINT  auto_pk PRIMARY KEY (auto_id));

You can only create a PostgreSQL table with automatic numbering by using the SERIAL data type, which always sets the initial value to 1. You can reset the SERIAL sequence value in PostgreSQL with the ALTER statement. Unlike Oracle Database 12c, PostgreSQL does let you modify the START WITH value of any sequence. The trick is understanding how to find the sequence name. The name is always the combination of the table name, an underscore, an id string, an underscore, and a seq string. This behavior makes a great case for choosing id as the name of any auto numbering columns in a table.

CREATE TABLE auto
( id          SERIAL      CONSTRAINT auto_pk PRIMARY KEY
, text_field  VARCHAR(30));
 
ALTER SEQUENCE auto_id_seq RESTART WITH 1001;

You can see the table and assigned sequence with the following command in PostgreSQL:

\d+ auto

It should display:

                                                      Table "public.auto"
   Column   |         Type          |                     Modifiers                     | Storage  | Stats target | Description 
------------+-----------------------+---------------------------------------------------+----------+--------------+-------------
 id         | integer               | not null default nextval('auto_id_seq'::regclass) | plain    |              | 
 text_field | character varying(30) |                                                   | extended |              | 
Indexes:
    "auto_pk" PRIMARY KEY, btree (id)
Has OIDs: no

As always, I hope this helps those trying to sort through how to start identity columns above the initial value of 1.

Written by maclochlainn

August 4th, 2017 at 12:52 am

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