PostgreSQL Naming Rules

When you create an object in PostgreSQL, you give that object a name. Every table has a name, every column has a name , and so on. PostgreSQL uses a single type to define all object names : the name type.

A value of type name is a string of 31 or fewer characters ^[1] . A name must start with a letter or an underscore ; the rest of the string can contain letters , digits, and underscores.

^[1] You can increase the length of the name data type by changing the value of the NAMEDATALEN symbol before compiling PostgreSQL.

If you examine the entry corresponding to name in the pg_type table, you will find that a name is really 32 characters long. Because the name type is used internally by the PostgreSQL engine, it is a null- terminated string. So, the maximum length of name value is 31 characters. You can enter more than 31 characters for an object name, but PostgreSQL stores only the first 31 characters.

Both SQL and PostgreSQL reserve certain words and normally, you cannot use those words to name objects. Examples of reserved words are

 ANALYZE BETWEEN CHARACTER INTEGER CREATE 

You cannot create a table named INTEGER or a column named BETWEEN . A complete list of reserved words can be found in Appendix B of the PostgreSQL User's Guide .

If you find that you need to create an object that does not meet these rules, you can enclose the name in double quotes. Wrapping a name in quotes creates a quoted identifier. For example, you could create a table whose name is "3.14159" ”the double quotes are required, but are not actually a part of the name (that is, they are not stored and do not count against the 31-character limit). When you create an object whose name must be quoted, you have to include the quotes not only when you create the object, but every time you refer to that object. For example, to select from the table mentioned previously, you would have to write

 SELECT filling, topping, crust FROM "3.14159"; 

Here are a few examples of both valid and invalid names:

 my_table       -- valid my_2nd_table   -- valid chanciers    -- valid: accented and non-Latin letters are allowed "2nd_table"    -- valid: quoted identifier "create table" -- valid: quoted identifier "1040Forms"    -- valid: quoted identifier 2nd_table      -- invalid: does not start with a letter or an underscore 

Quoted names are case-sensitive. "1040Forms" and "1040FORMS" are two distinct names. Unquoted names are converted to lowercase, as shown here:

 movies=# CREATE TABLE FOO(BAR INTEGER); CREATE movies=# CREATE TABLE foo(BAR INTEGER); ERROR: Relation 'foo' already exists movies=# \d             List of relations        Name        Type       Owner ------------------+-------+---------------  1040FORMS         table  bruce  1040Forms         table  sheila  customers         table  bruce  distributors      table  bruce  foo               table  bruce  rentals           table  bruce  returns           table  John Whorfin  tapes             table  bruce  (6 rows) 

The names of all objects must be unique within some scope. Every database must have a unique name; the name of a table must be unique within the scope of a single database ^[2] , and column names must be unique within a table. The name of an index must be unique within a database.

^[2] PostgreSQL version 7.3 introduces a new naming context, the schema . Table names must be unique within a schema.