A PostgreSQL database cluster 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.
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
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
can contain tables named
Unlike databases, schemas are not rigidly separated: a user can
access objects in any of the schemas in the database they are
connected to, if they have 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 do not 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 schema, use the CREATE SCHEMA command. 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:
This works anywhere a table name is expected, including the table modification commands and the data access commands discussed in the following chapters. (For brevity we will speak of tables only, but the same ideas apply to other kinds of named objects, such as types and functions.)
Actually, the even more general syntax
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 ( ... );
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 5.13 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:
You can even omit the schema name, in which case the schema name will be the same as the user name. See Section 5.8.6 for how this can be useful.
Schema names beginning with
reserved for system purposes and cannot 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 ( ... );
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 ability to create like-named objects in different schemas
complicates writing a query that references precisely the same
objects every time. It also opens up the potential for users to
change the behavior of other users' queries, maliciously or
accidentally. Due to the prevalence of unqualified names in queries
and their use in PostgreSQL
internals, adding a schema to
search_path effectively trusts all users having
CREATE privilege on that schema. When
you run an ordinary query, a malicious user able to create objects
in a schema of your search path can take control and execute
arbitrary SQL functions as though you executed them.
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
To show the current search path, use the following command:
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. If no such schema exists, the 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;
myschema is the first
element in the path, new objects would by default be created in
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 9.25 for other ways to manipulate 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
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 access any objects in schemas they do
not own. To allow that, the owner of the schema must grant the
USAGE privilege on the schema. To
allow users to make use of the objects in the schema, additional
privileges might need to be granted, as appropriate for the
A user can also be allowed to create objects in someone else's
schema. To allow that, the
privilege on the schema needs to be granted. Note that by default,
USAGE privileges on the schema
public. This allows all users that are able to
connect to a given database to create objects in its
public schema. Some usage patterns call for revoking
REVOKE CREATE ON SCHEMA 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 key word, hence the different capitalization; recall the guidelines from Section 4.1.1.)
In addition to
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 can explicitly place
the end of your search path if you prefer to have user-defined
names override built-in names.
Since system table names begin with
pg_, it is best to avoid such names to ensure that
you won't suffer a conflict if some future version defines a system
table named the same as your table. (With the default search path,
an unqualified reference to your table name would then be resolved
as the system table instead.) System tables 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
Schemas can be used to organize your data in many ways. There are a few usage patterns easily supported by the default configuration, only one of which suffices when database users mistrust other database users:
Constrain ordinary users to user-private schemas. To implement
REVOKE CREATE ON SCHEMA public
FROM PUBLIC, and create a schema for each user with the same
name as that user. If affected users had logged in before this,
consider auditing the public schema for objects named like objects
pg_catalog. Recall that the
default search path starts with
which resolves to the user name. Therefore, if each user has a
separate schema, they access their own schemas by default.
Remove the public schema from each user's default search path
ALTER ROLE . Everyone retains the ability to create objects in
the public schema, but only qualified names will choose those
objects. While qualified table references are fine, calls to
functions in the public schema will be unsafe or
unreliable. Also, a user holding the
user SET search_path =
CREATEROLE privilege can undo this setting and
issue arbitrary queries under the identity of users relying on the
setting. If you create functions or extensions in the public schema
CREATEROLE to users not
warranting this almost-superuser ability, use the first pattern
Remove the public schema from
postgresql.conf. The ensuing user experience matches the
previous pattern. In addition to that pattern's implications for
CREATEROLE, this trusts
database owners like
you create functions or extensions in the public schema or assign
CREATEDB privilege or individual database
ownership to users not warranting almost-superuser access, use the
first pattern instead.
Keep the default. All users access the public schema implicitly. This simulates the situation where schemas are not available at all, giving a smooth transition from the non-schema-aware world. However, any user can issue arbitrary queries under the identity of any user not electing to protect itself individually. This pattern is acceptable only when the database has a single user or a few mutually-trusting users.
For any pattern, 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 search 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 do not 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
. 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 the
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.
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