If the Postgres postmaster is up and running we can create some databases to experiment with. Here, we describe the basic commands for managing a database. Let's say you want to create a database named mydb. You can do this with the following command: % createdb dbname 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 31 characters in length. Not every user has authorization to become a database administrator. If Postgres refuses to create databases for you, then the site administrator needs to grant you permission to create databases. Consult your site administrator if this occurs. Once you have constructed a database, you can access it by: running the Postgres terminal monitor program (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 dbname database by typing the command: psql dbname 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 dbname=> This prompt indicates that the terminal monitor 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 Postgres SQL commands by typing: dbname=> \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: dbname=> \g This tells the server to process the query. If you terminate your query with a semicolon, the backslash-g is not necessary. psql will automatically process semicolon terminated queries. To read queries from a file, instead of entering them interactively, type: dbname=> \i filename To get out of psql and return to Unix, type dbname=> \q and psql will quit and return you to your command shell. (For more escape codes, type backslash-h at the monitor prompt.) White space (i.e., spaces, tabs and newlines) may be used freely in SQL queries. Single-line comments are denoted by two dashes ("--"). Everything after the dashes up to the end of the line is ignored. Multiple-line comments, and comments within a line, are denoted by "/* ... */", a convention borrowed from Ingres. If you are the database administrator for the database mydb, you can destroy it using the following Unix command: % dropdb dbname 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. It is also possible to destroy a database from within an SQL session by using > drop database dbname Every database should be backed up on a regular basis. Since Postgres manages it's own files in the file system, it is not advisable to rely on system backups of your file system for your database backups; there is no guarantee that the files will be in a usable, consistant state after restoration. Postgres provides two utilities to backup your system: pg_dump to backup individual databases and pg_dumpall to backup your installation in one step. An individual database can be backed up using the following command: % pg_dump dbname > dbname.pgdump and can be restored using cat dbname.pgdump | psql dbname This technique can be used to move databases to new locations, and to rename existing databases. Written by Hannu Krosing on 1999-06-19. Since Postgres allows tables larger than the maximum file size on your system, it can be problematic to dump the table to a file, since the resulting file will likely be larger than the maximum size allowed by your system. As pg_dump writes to stdout, you can just use standard *nix tools to work around this possible problem: Use compressed dumps: % pg_dump dbname | gzip > filename.dump.gz reload with % createdb dbname % gunzip -c filename.dump.gz | psql dbname or % cat filename.dump.gz | gunzip | psql dbname Use split: % pg_dump dbname | split -b 1m - filename.dump. reload with % createdb dbname % cat filename.dump.* | pgsql dbname Of course, the name of the file (filename) and the content of the pg_dump output need not match the name of the database. Also, the restored database can have an arbitrary new name, so this mechanism is also suitable for renaming databases.