Object IDs (OID)

An OID is a 32-bit, positive whole number. Every row^[5] in a PostgreSQL database contains a unique identifier^[6]?the object ID (or OID). Normally, the OID column is hidden. You can see the OID for a row by including the OID column in a the target list of a SELECT statement:

^[5] By default, all tables are created such that every row contains an OID. You can omit the object IDs using the WITHOUT OIDS clause of the CREATE TABLE command.

^[6] The PostgreSQL documentation warns that object IDs are currently unique within a database cluster; but in a future release, an OID may be unique only within a single table.


movies=# SELECT OID, * FROM customers;

  oid  | id |    customer_name     |  phone   | birth_date | balance

-------+----+----------------------+----------+------------+---------

 38333 |  1 | Jones, Henry         | 555-1212 | 1970-10-10 |    0.00

 38334 |  2 | Rubin, William       | 555-2211 | 1972-07-10 |   15.00

 38335 |  3 | Panky, Henry         | 555-1221 | 1968-01-21 |    0.00

 38386 |  5 | Funkmaster, Freddy   | 555-FUNK |            |

 38392 |  7 | Gull, Jonathon LC    | 555-1111 | 1984-02-05 |

 38393 |  8 | Grumby, Jonas        | 555-2222 | 1984-02-21 |

 38336 |  4 | Wonderland, Alice N. | 555-1122 | 1969-03-05 |    3.00

You can create a column of type OID if you want to explicitly refer to another object (usually a row in another table). Think back to the rentals table that you developed in Chapter 1. Each row in the rentals table contains a tape_id, a customer_id, and a rental date. The rentals table currently looks like this:


movies=# \d rentals

            Table "rentals"

  Attribute  |     Type     | Modifier

-------------+--------------+----------

 tape_id     | character(8) | not null

 rental_date | date         | not null

 customer_id | integer      | not null



movies=# SELECT * FROM rentals;

 tape_id  | rental_date | customer_id

----------+-------------+-------------

 AB-12345 | 2001-11-25  |           1

 AB-67472 | 2001-11-25  |           3

 OW-41221 | 2001-11-25  |           1

 MC-68873 | 2001-11-20  |           3

 KJ-03335 | 2001-11-26  |           8

(5 rows)

Each value in the tape_id column refers to a row in the tapes table. Each value in the customer_id column refers to a row in the customers table. Rather than storing the tape_id and customer_id in the rentals table, you could store OIDs for the corresponding rows. The following CREATE TABLE ... AS command creates a new table, rentals2, that is equivalent to the original rentals table:


movies=# CREATE TABLE rentals2 AS

movies-#   SELECT

movies-#     t.oid AS tape_oid, c.oid AS customer_oid, r.rental_date

movies-#   FROM

movies-#     tapes t, customers c, rentals r

movies-#   WHERE

movies-#     t.tape_id = r.tape_id

movies-#        AND

movies-#     c.id = r.customer_id;

This statement (conceptually) works as follows. First, you retrieve a row from the rentals table. Next, you use the rentals.customer_id column to retrieve the matching customers row and the rentals.tape_id column to retrieve the matching tapes row. Finally, you store the OID of the customers row and the OID of the tapes row (and the rental_date) in a new rentals2 row.

Now, when you SELECT from the rentals2 table, you will see the object IDs for the customers row and the tapes row:


movies=# SELECT * FROM rentals2;

 tape_oid | customer_oid | rental_date

----------+--------------+-------------

    38337 |       38333  | 2001-11-25

    38338 |       38335  | 2001-11-25

    38394 |       38393  | 2001-11-26

    38339 |       38335  | 2001-11-20

    38340 |       38333  | 2001-11-25

You can re-create the data in the original table by joining the corresponding customers and tapes records, based on their respective OIDs:


movies=# SELECT t.tape_id, r.rental_date, c.id

movies-#   FROM

movies-#     tapes t, rentals2 r, customers c

movies-#   WHERE

movies-#     t.oid = r.tape_oid AND

movies-#     c.oid = r.customer_oid

movies-#   ORDER BY t.tape_id;



tape_id  | rental_date  | id

----------+-------------+----

 AB-12345 | 2001-11-25  |  1

 AB-67472 | 2001-11-25  |  3

 KJ-03335 | 2001-11-26  |  8

 MC-68873 | 2001-11-20  |  3

 OW-41221 | 2001-11-25  |  1

(5 rows)

Here are a couple of warnings about using OIDs in your own tables.

The first concern has to do with backups. The standard tool for performing a backup of a PostgreSQL database is pg_dump. By default, pg_dump will not archive OIDs. This means that if you back up a table that contains an OID column (referring to another object) and then restore that table from the archive, the relationships between objects will be lost, unless you remembered to tell pg_dump to archive OIDs. This happens because when you restore a row from the archive, it might be assigned a different OID.

The second thing you should consider when using OIDs is that they offer no real performance advantages. If you are coming from an Oracle or Sybase environment, you might be thinking that an OID sounds an awful lot like a ROWID. It's true that an OID and a ROWID provide a unique identifier for a row, but that is where the similarity ends. In an Oracle environment, you can use a ROWID as the fastest possible way to get to a specific row. A ROWID encodes the location (on disk) of the row that it belongs to?when you retrieve a row by ROWID, you can bypass any index^[7] searches and go straight to the data. An OID is just a 32-bit number?you can create an index on the OID column, but you could also create an index on any other (unique) column to achieve the same results. In fact, the only time that it might make sense to use an OID to identify a row is when the primary key^[7] for a table is very long.

^[7] Don't be too concerned if you aren't familiar with the concept of indexes or primary keys, I'll cover each of those topics a bit later.

Finally, I should point out that OIDs can wrap. In an active database cluster, it's certainly possible that 4 billion objects can be created. That doesn't mean that all 4 billion objects have to exist at the same time, just that 4 billion OIDs have been created since the cluster was created. When the OID generator wraps, you end up with duplicate values. This may sound a little far-fetched, but it does happen and it is not easy to recover from. There really is no good reason to use an OID as a primary key?use SERIAL (or BIGSERIAL) instead.

Syntax for Literal Values

The format in which you enter literal OID values is the same that you would use for unsigned INTEGER values. An OID literal is simply a sequence of decimal digits.

Size and Valid Values

As I mentioned earlier, an OID is an unsigned 32-bit (4-byte) integer. An OID column can hold values between 0 and 4294967295. The value 0 represents an invalid OID.

Supported Operators

You can compare two OID values, and you can compare an OID value against an INTEGER value. Table 2.26 shows which operators you can use with the OID data type.