A PostgreSQL database cluster (installation)
contains one or more named databases. Users and groups of users are
shared across the entire cluster, but no other data is shared across
databases. Any given client connection to the server can access
only the data in a single database, the one specified in the connection
request.
Note: Users of a cluster do not necessarily have the privilege to access every
database in the cluster. Sharing of user names means that there
cannot be different users named, say, joe in two databases
in the same cluster; but the system can be configured to allow
joe access to only some of the databases.
A database contains one or more named schemas, which
in turn contain tables. Schemas also contain other kinds of named
objects, including data types, functions, and operators. The same
object name can be used in different schemas without conflict; for
example, both schema1 and myschema may
contain tables named mytable. Unlike databases,
schemas are not rigidly separated: a user may access objects in any
of the schemas in the database he is connected to, if he has
privileges to do so.
There are several reasons why one might want to use schemas:
To allow many users to use one database without interfering with
each other.
To organize database objects into logical groups to make them
more manageable.
Third-party applications can be put into separate schemas so
they cannot collide with the names of other objects.
Schemas are analogous to directories at the operating system level,
except that schemas cannot be nested.
To create a separate schema, use the command CREATE
SCHEMA. Give the schema a name of your choice. For
example:
CREATE SCHEMA myschema;
To create or access objects in a schema, write a
qualified name consisting of the schema name and
table name separated by a dot:
schema.table
Actually, the even more general syntax
database.schema.table
can be used too, but at present this is just for pro-forma compliance
with the SQL standard; if you write a database name it must be the
same as the database you are connected to.
So to create a table in the new schema, use
CREATE TABLE myschema.mytable (
...
);
This works anywhere a table name is expected, including the table
modification commands and the data access commands discussed in
the following chapters.
To drop a schema if it's empty (all objects in it have been
dropped), use
DROP SCHEMA myschema;
To drop a schema including all contained objects, use
DROP SCHEMA myschema CASCADE;
See Section 2.10 for a description of the general
mechanism behind this.
Often you will want to create a schema owned by someone else
(since this is one of the ways to restrict the activities of your
users to well-defined namespaces). The syntax for that is:
CREATE SCHEMA schemaname AUTHORIZATION username;
You can even omit the schema name, in which case the schema name
will be the same as the user name. See Section 2.8.6 for how this can be useful.
Schema names beginning with pg_ are reserved for
system purposes and may not be created by users.
In the previous sections we created tables without specifying any
schema names. By default, such tables (and other objects) are
automatically put into a schema named "public". Every new
database contains such a schema. Thus, the following are equivalent:
CREATE TABLE products ( ... );
and
CREATE TABLE public.products ( ... );
Qualified names are tedious to write, and it's often best not to
wire a particular schema name into applications anyway. Therefore
tables are often referred to by unqualified names,
which consist of just the table name. The system determines which table
is meant by following a search path, which is a list
of schemas to look in. The first matching table in the search path
is taken to be the one wanted. If there is no match in the search
path, an error is reported, even if matching table names exist
in other schemas in the database.
The first schema named in the search path is called the current schema.
Aside from being the first schema searched, it is also the schema in
which new tables will be created if the CREATE TABLE
command does not specify a schema name.
To show the current search path, use the following command:
SHOW search_path;
In the default setup this returns:
search_path
--------------
$user,public
The first element specifies that a schema with the same name as
the current user is to be searched. Since no such schema exists
yet, this entry is ignored. The second element refers to the
public schema that we have seen already.
The first schema in the search path that exists is the default
location for creating new objects. That is the reason that by
default objects are created in the public schema. When objects
are referenced in any other context without schema qualification
(table modification, data modification, or query commands) the
search path is traversed until a matching object is found.
Therefore, in the default configuration, any unqualified access
again can only refer to the public schema.
To put our new schema in the path, we use
SET search_path TO myschema,public;
(We omit the $user here because we have no
immediate need for it.) And then we can access the table without
schema qualification:
DROP TABLE mytable;
Also, since myschema is the first element in
the path, new objects would by default be created in it.
We could also have written
SET search_path TO myschema;
Then we no longer have access to the public schema without
explicit qualification. There is nothing special about the public
schema except that it exists by default. It can be dropped, too.
See also Section 6.13 for other ways to access
the schema search path.
The search path works in the same way for data type names, function names,
and operator names as it does for table names. Data type and function
names can be qualified in exactly the same way as table names. If you
need to write a qualified operator name in an expression, there is a
special provision: you must write
OPERATOR(schema.operator)
This is needed to avoid syntactic ambiguity. An example is
SELECT 3 OPERATOR(pg_catalog.+) 4;
In practice one usually relies on the search path for operators,
so as not to have to write anything so ugly as that.
By default, users cannot see the objects in schemas they do not
own. To allow that, the owner of the schema needs to grant the
USAGE privilege on the schema. To allow users
to make use of the objects in the schema, additional privileges
may need to be granted, as appropriate for the object.
A user can also be allowed to create objects in someone else's
schema. To allow that, the CREATE privilege on
the schema needs to be granted. Note that by default, everyone
has the CREATE privilege on the schema
public. This allows all users that manage to
connect to a given database to create objects there. If you do
not want to allow that, you can revoke that privilege:
REVOKE CREATE ON public FROM PUBLIC;
(The first "public" is the schema, the second
"public" means "every user". In the
first sense it is an identifier, in the second sense it is a
reserved word, hence the different capitalization; recall the
guidelines from Section 1.1.1.)
In addition to public and user-created schemas, each
database contains a pg_catalog schema, which contains
the system tables and all the built-in data types, functions, and
operators. pg_catalog is always effectively part of
the search path. If it is not named explicitly in the path then
it is implicitly searched before searching the path's
schemas. This ensures that built-in names will always be
findable. However, you may explicitly place
pg_catalog at the end of your search path if you
prefer to have user-defined names override built-in names.
In PostgreSQL versions before 7.3,
table names beginning with pg_ were reserved. This is
no longer true: you may create such a table name if you wish, in
any non-system schema. However, it's best to continue to avoid
such names, to ensure that you won't suffer a conflict if some
future version defines a system catalog named the same as your
table. (With the default search path, an unqualified reference to
your table name would be resolved as the system catalog instead.)
System catalogs will continue to follow the convention of having
names beginning with pg_, so that they will not
conflict with unqualified user-table names so long as users avoid
the pg_ prefix.
Schemas can be used to organize your data in many ways. There are
a few usage patterns that are recommended and are easily supported by
the default configuration:
If you do not create any schemas then all users access the
public schema implicitly. This simulates the situation where
schemas are not available at all. This setup is mainly
recommended when there is only a single user or a few cooperating
users in a database. This setup also allows smooth transition
from the non-schema-aware world.
You can create a schema for each user with the same name as
that user. Recall that the default search path starts with
$user, which resolves to the user name.
Therefore, if each user has a separate schema, they access their
own schemas by default.
If you use this setup then you might also want to revoke access
to the public schema (or drop it altogether), so users are
truly constrained to their own schemas.
To install shared applications (tables to be used by everyone,
additional functions provided by third parties, etc.), put them
into separate schemas. Remember to grant appropriate
privileges to allow the other users to access them. Users can
then refer to these additional objects by qualifying the names
with a schema name, or they can put the additional schemas into
their path, as they choose.
In the SQL standard, the notion of objects in the same schema
being owned by different users does not exist. Moreover, some
implementations don't allow you to create schemas that have a
different name than their owner. In fact, the concepts of schema
and user are nearly equivalent in a database system that
implements only the basic schema support specified in the
standard. Therefore, many users consider qualified names to
really consist of
username.tablename.
This is how PostgreSQL will effectively behave if you create a per-user
schema for every user.
Also, there is no concept of a public schema in the
SQL standard. For maximum conformance to the standard, you should
not use (perhaps even remove) the public schema.
Of course, some SQL database systems might not implement schemas
at all, or provide namespace support by allowing (possibly
limited) cross-database access. If you need to work with those
systems, then maximum portability would be achieved by not using
schemas at all.
