Primary Keys in PostgreSQL shouldn’t be a Mystery

Posted on by Alex M. Hochberger, MBA

PostgreSQL has so much power, and people think it’s much harder to use.  You can do just about anything with Alter Table in terms of Adding Columns.  The only tricky thing I’ve found is that you can’t add a NOT NULL column without a Default if there is data, since the default would be NULL.  So you can allow Nulls, fill in your data, then tag it Not Null, or give it a useful default and move on.  I saw another post of someone making PostgreSQL way more difficult than needed, creating a Primary Key later.  While there are plenty (or some) reasons to create a sequence manually (I had a need for unique integers for insertion into a third-party system once), there isn’t a need for something simply like a primary key.

Let’s setup our test table, you can obviously do all the inserts on one line, I was playing on an older server for testing.


test_database=# CREATE TABLE test_table (col_A varchar, col_B varchar);
CREATE TABLE
test_database=# INSERT INTO test_table(col_a, col_B) VALUES ('a','A');
INSERT 0 1
test_database=# INSERT INTO test_table(col_a, col_B) VALUES ('b', 'B');
INSERT 0 1
test_database=# INSERT INTO test_table(col_a, col_B) VALUES ('c', 'C');
INSERT 0 1

Now we want to add a Primary Key:

test_database=# ALTER TABLE test_table ADD COLUMN test_id SERIAL PRIMARY KEY;
NOTICE: ALTER TABLE will create implicit sequence "test_table_test_id_seq" for serial column "test_table.test_id"
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index "test_table_pkey" for table "test_table"
ALTER TABLE
test_database=# SELECT * from test_table;
col_a | col_b | test_id
-------+-------+---------
a | A | 1
b | B | 2
c | C | 3
(3 rows)
test_database=# \d test_table
Table "public.test_table"
Column | Type | Modifiers
---------+-------------------+--------------------------------------------------------------
col_a | character varying |
col_b | character varying |
test_id | integer | not null default nextval('test_table_test_id_seq'::regclass)
Indexes:
"test_table_pkey" PRIMARY KEY, btree (test_id)

Look, one line, added a new column test_id, with the sequence, tagged as a primary key.  Not so hard?  Why isn’t that an obvious example under Alter Table, given that it’s something that one might want to do?

PostgreSQL is Powerful, Confuses Newbies

Posted on by Alex M. Hochberger, MBA

Looking at the list of PostgreSQL blog entries, I saw this one,
Add auto increment column in PostgreSQL, and realized why my database of choice isn’t so popular… it confuses people.  Back in the old days, add an auto-numering field to PostgreSQL as a Primary Key was hard, you had to create the sequence and set the default.  This was a common operation, since a numeric Primary Key is often easier than looking for a natural key, and even when we have a natural key, it’s usually easier to index an integer than a string if we are going to use it as a foreign key somewhere.  The SERIAL “type” in PostgreSQL is the same as auto_increment, but it’s implemented as TYPE integer, attached SEQUENCE, and default value.

Example:
CREATE TABLE names_table (name_id SERIAL PRIMARY KEY, name varchar);

This gives you an PRIMARY KEY name_id, that will be auto incremented, and a string column name.  I use these all the time, and the PRIMARY KEY shortcut is an easy way to make a single column the primary key.  You can always use a primary key directive for composite keys in mapping tables and similar constructs.

I find it easy, because I know what a SERIAL is, but given that EVERY newcomer to PostgreSQL is looking for this feature, wouldn’t it make sense to highlight it in the documentation?  Perhaps in the examples given?  In addition, since auto increment is so common, why not support it?  SERIAL is syntactic sugar anyway, why not have it accept the MS SQL/MySQL terminology, you can throw off a notice so people learn the preferred one, but why push off testers?