Managing a Database
This section is currently a thinly disguised copy of the
Tutorial. Needs to be augmented.
- thomas 1998-01-12
Although the site administrator is responsible for overall management
of the Postgres installation, some databases within the
installation may be managed by another person,
designated the database administrator.
This assignment of responsibilities occurs when a database is created.
A user may be assigned explicit privileges to create databases and/or to create new users.
A user assigned both privileges can perform most administrative task
within Postgres, but will
not by default have the same operating system privileges as the site administrator.
The Database Administrator's Guide covers these topics in more detail.
Database Creation
Databases are created by the create database issued from
within Postgres. createdb is a command-line
utility provided to give the same functionality from outside Postgres.
The Postgres backend must be running for either method
to succeed, and the user issuing the command must be the Postgres
superuser or have been assigned database creation privileges by the
superuser.
To create a new database named mydb from the command line, type
% createdb mydb
and to do the same from within psql type
=> CREATE DATABASE mydb;
If you do not have the privileges required to create a database, you will see
the following:
ERROR: CREATE DATABASE: Permission denied.
Postgres allows you to create any number of databases
at a given site and you automatically become the
database administrator of the database you just created.
Database names must have an alphabetic first
character and are limited to 32 characters in length.
Alternate Database Locations
It is possible to create a database in a location other than the default
location for the installation. Remember that all database access actually
occurs through the database backend, so that any location specified must
be accessible by the backend.
Alternate database locations are created and referenced by an environment variable
which gives the absolute path to the intended storage location.
This environment variable must have been defined before the postmaster was started
and the location it points to must be writable by the postgres administrator account.
Consult with the site administrator
regarding preconfigured alternate database locations.
Any valid environment variable name may be used to reference an alternate location,
although using variable names with a prefix of PGDATA is recommended
to avoid confusion
and conflict with other variables.
In previous versions of Postgres,
it was also permissable to use an absolute path name to specify
an alternate storage location.
Although the environment variable style of specification
is to be preferred since it allows the site administrator more flexibility in
managing disk storage, it is also possible to use an absolute path
to specify an alternate location.
The administrator's guide discusses how to enable this feature.
For security and integrity reasons,
any path or environment variable specified has some
additional path fields appended.
Alternate database locations must be prepared by running
initlocation.
To create a data storage area using the environment variable
PGDATA2 (for this example set to /alt/postgres),
ensure that /alt/postgres already exists and is writable by
the Postgres administrator account.
Then, from the command line, type
% initlocation PGDATA2
Creating Postgres database system directory /alt/postgres/data
Creating Postgres database system directory /alt/postgres/data/base
To create a database in the alternate storage area PGDATA2
from the command line, use the following command:
% createdb -D PGDATA2 mydb
and to do the same from within psql type
=> CREATE DATABASE mydb WITH LOCATION = 'PGDATA2';
If you do not have the privileges required to create a database, you will see
the following:
ERROR: CREATE DATABASE: permission denied
If the specified location does not exist or the database backend does not have
permission to access it or to write to directories under it, you will see
the following:
ERROR: The database path '/no/where' is invalid. This may be due to a character that is not allowed or because the chosen path isn't permitted for databases.
Accessing a Database
Once you have constructed a database, you can access it
by:
running the PostgreSQL interactive terminal
psql which allows you to interactively
enter, edit, and execute SQL commands.
writing a C program using the LIBPQ subroutine
library. This allows you to submit SQL commands
from C and get answers and status messages back to
your program. This interface is discussed further
in The PostgreSQL Programmer's Guide.
You might want to start up psql,
to try out the examples in this manual.
It can be activated for the mydb
database by typing the command:
% psql mydb
You will be greeted with the following message:
Welcome to psql, the PostgreSQL interactive terminal.
Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit
mydb=>
This prompt indicates that psql is listening
to you and that you can type SQL queries into a
workspace maintained by the terminal monitor.
The psql program responds to escape codes that begin
with the backslash character, "\". For example, you
can get help on the syntax of various
PostgreSQL SQL commands by typing:
mydb=> \h
Once you have finished entering your queries into the
workspace, you can pass the contents of the workspace
to the Postgres server by typing:
mydb=> \g
This tells the server to process the query. If you
terminate your query with a semicolon, the "\g" is not
necessary.
psql will automatically process semicolon terminated queries.
To read queries from a file, say myFile, instead of
entering them interactively, type:
mydb=> \i fileName
To get out of psql and return to Unix, type
mydb=> \q
and psql will quit and return you to your command
shell. (For more escape codes, type \? at the psql
prompt.)
White space (i.e., spaces, tabs and newlines) may be
used freely in SQL queries. Single-line comments are denoted by
"--". Everything after the dashes up to the end of the
line is ignored. Multiple-line comments, and comments within a line,
are denoted by "/* ... */".
Destroying a Database
If you are the owner of the database
mydb, you can destroy it using the following Unix command:
% dropdb mydb
This action physically removes all of the Unix files
associated with the database and cannot be undone, so
this should only be done with a great deal of forethought.