CREATE DATABASE and the pg_database catalog

The CREATE DATABASE statement is used to create a new PostgreSQL database. It does so by cloning an existing database, a template. PostgreSQL comes with two template databases, template0 and template1. The difference between template0 and template1 is their intended use. template1 is the default template when creating a new database, it can also be connected to and any custom objects that new databases should have can be created there. template0 does not accept connections and is therefore a clean source for those times when template1 has become messy for some reason.

When a new database is created a new row is added to the pg_database catalog. pg_database stores all the database settings that are available in the CREATE DATABASE statement. Collation, encoding and tablespace information along with connectable and connection limits are stored here.

pg_database is a shared catalog that is available and shows the same data no matter what database you are connected to. Interestingly for [comparing PostgreSQL databases] the dependencies a database has on its collation, tablespace and owner are not captured in pg_shdepend, the catalog that contains dependencies between cluster-wide objects.

CREATE DATABASE name
    [ [ WITH ] [ OWNER [=] user_name ]
           [ TEMPLATE [=] template ]
           [ ENCODING [=] encoding ]
           [ LC_COLLATE [=] lc_collate ]
           [ LC_CTYPE [=] lc_ctype ]
           [ TABLESPACE [=] tablespace_name ]
           [ ALLOW_CONNECTIONS [=] allowconn ]
           [ CONNECTION LIMIT [=] connlimit ]
           [ IS_TEMPLATE [=] istemplate ] ]

CREATE DATABASE documentation

Name Type References Description
oid oid   Row identifier (hidden attribute; must be explicitly selected)
datname name   Database name
datdba oid pg_authid.oid Owner of the database, usually the user who created it
encoding int4   Character encoding for this database (pg_encoding_to_char() can translate this number to the encoding name)
datcollate name   LC_COLLATE for this database
datctype name   LC_CTYPE for this database
datistemplate bool   If true, then this database can be cloned by any user with CREATEDB privileges; if false, then only superusers or the owner of the database can clone it.
datallowconn bool   If false then no one can connect to this database. This is used to protect the template0 database from being altered.
datconnlimit int4   Sets maximum number of concurrent connections that can be made to this database. -1 means no limit.
datlastsysoid oid   Last system OID in the database; useful particularly to pg_dump
datfrozenxid xid   All transaction IDs before this one have been replaced with a permanent (“frozen”) transaction ID in this database. This is used to track whether the database needs to be vacuumed in order to prevent transaction ID wraparound or to allow pg_clog to be shrunk. It is the minimum of the per-table pg_class.relfrozenxid values.
datminmxid xid   All multixact IDs before this one have been replaced with a transaction ID in this database. This is used to track whether the database needs to be vacuumed in order to prevent multixact ID wraparound or to allow pg_multixact to be shrunk. It is the minimum of the per-table pg_class.relminmxid values.
dattablespace oid pg_tablespace.oid The default tablespace for the database. Within this database, all tables for which pg_class.reltablespace is zero will be stored in this tablespace; in particular, all the non-shared system catalogs will be there.
datacl aclitem[]   Access privileges; see GRANT and REVOKE for details

pg_class documentation