The general parameter setting command; this is the most important (and powerful) formatting command of the list. It encapsulates a variety of display options, and it could easily be the only formatting slash command you ever use. You may pass it various parameters to accomplish different formatting functions.

Within its syntax, parameter is one of the following valid parameters:

The align command; this toggles psql between aligned and unaligned mode. This is equivalent to successive uses of \pset format aligned and \pset format unaligned.

The query title command; this allows you to set a title that will be displayed at the top of any displayed result set, and is equivalent to \pset title.

The field delimiter command; this sets the field delimiter when using the unaligned formatting mode, and is equivalent to \pset fieldsep.

The HTML output command; this toggles between HTML output formatting and the default aligned formatting, and is equivalent to successive uses of \pset format HTML and \pset format aligned.

The table information command; this toggles the display of optional table information, and is equivalent to \pset tuples_only.

The table attribute command; this defines extra table attributes you wish to be inserted into the table tags of table data displaying while in HTML formatting mode. It is equivalent to \pset tableattr.

The toggle expanded command; this toggles expanded row formatting and off. It is equivalent to \pset expanded.

The general display command; it is used to view various pieces of information about a specified relation. The relation you specify may be an index, sequence, table, or view. When issued, the command will display all of the relation's columns, types, and special attributes or defaults. When executed without a specified relation, it displays each of the relations available within the currently connected database.

The aggregate display command; with it, you may retrieve the list of the connected database's aggregate functions, and their accepted data types. If you specify a parameter following the slash command, it will display only the list of aggregate functions whose names begin with the aggregate_name pattern in a case-insensitive comparison.

The general database object display command; it is used to display the descriptions of any specified database object. The object you specify may be any defined aggregate, function, operator, relation, rule, or trigger. If you do not specify an object name , or a partial name, all objects in the database will be displayed.

The function display command; it is used to display information about a function defined within the database, including its arguments, and return types. You can either specify a function to display, or specify none, and list information about all functions. Like \da and \dd, a full or partial function_name may be supplied for a case-insensitive comparison against all functions from the beginning of each function name.

A scoped version of the general display command; you may specify any of the options within the brackets:

The large object display command; this command is equivalent to the \lo_list command, which displays the list of large objects within the current database.

The operator display command; this displays the list of defined operators within the current database, along with their operands (arguments), and return types. You may specify a complete or partial operator_name to examine, or retrieve information about all available operators.

The permissions display command; this retrieves the list of all database objects (or objects at least partially matching an object_name , if provided) currently defined within the database, along with all their associated access permissions (public, user, and group).

The data type display command; this displays the list of all available data types. You may again specify a type_name , or partial data type name, or view all available data types in the current database.

The database display command; this lists all defined databases on the server, and their ownership information, and multibyte encoding type. Entering \l+ will display any comments the databases may have (see the Section called Documenting a Database in Chapter 9" in Chapter 9 for how to comment on a database).

The large object display command; this displays the list of all existing large objects within the current database, along with any comments that have been attached to them.

The permissions display command, equivalent to \dp.

The help command; this prints out the list of slash commands documented in this chapter.

The copyright command; this displays copyright information about PostgreSQL.

The encoding command; if multibyte encoding is enabled, this can set the client encoding. If you do not supply an argument, the current encoding will be displayed on the screen.

The general help command; used without an argument, it will print a list of all commands for which greater help is available. Used with an argument, it will print more information (if there is data available) for the subject. Used with an asterisk (*) as the argument, it will retrieve syntax information for all documented SQL commands.

The copy command; this can be used to copy from the client application (and thus, use the permissions of the user who started the client) instead of using the SQL COPY command to copy from the server. This slash command can also accept any of the standard COPY clauses. For more information on the syntax of this command, refer to the COPY entry in the command reference section at the back of this book.

The difference between using \copy over COPY are important to understand and include:

• Data you \copy transfers first through the client (via your connection), which may be quite a bit slower than if it were done directly through the server (i.e., the backend) process.

• You have access to files on the local filesystem under whatever permissions the user account you are using has, which means you may have more (or less) accessibility to needed files than the backend process.

• The terms stdin and stdout (standard input and output) have a different meaning; they refer to psql  's input and output stream. On the backend process they are used differently: stdin represents where the COPY was issued from, and stdout represents the query output stream.

The echo command; this sends a string to the standard output. This can be useful for scripting, because you can add non-database–supplied information into script output (such as comments).

The buffer execution command; this is essentially the same as using the semicolon (;) in that it sends the current query buffer to the backend to be processed. Optionally, you can save the result set to a file of your choice, or have psql pipe it to a separate shell command by following the \g with either a filename or piped command name.

The file input command; this reads input from a file (the name of which you supply as an argument after the \i) and causes psql to parse its content as if it were typed directly into the program's prompt.

The large object export command; this lets you export the large object with OID lo_oid to filename on your local filesystem. This is different from the lo_export() server function in the same way the \copy and the SQL COPY commands are different.

The large object import command; this imports large objects into the database from files on your local filesystem. Optionally, you can attach a comment to the object; this is recommended, as otherwise it will be identifiable only by an OID, which you will need to remember if you wish to access it again. If you attach a comment to the object, issuing the \lo_list command displays your comment with the OID of the object, thus making it easier to find once imported.

The output command; this redirects future output (i.e., data retrieved after this command is issued) to either a file of your choice or a pipe to a system command . If not given any arguments, the output channel will reset to standard output; use no arguments when you wish to stop sending output elsewhere. One of the most useful features of this command is the ability to pipe output to commands such as grep , which can then search for a pattern of your choosing, allowing you to search against database and slash command output (which will, of course, only work if grep is installed on your system).

The buffer display command; this prints the psql input currently buffered. If no SQL input has been entered since the last executed statement, the last executed statement is displayed.

The query-output echo command; this sends a string to your chosen query output channel (which is set with the \o command), instead of stdout. This command can be useful when you need to send non-database–related information into the psql output.

The buffer output command; this outputs the current query buffer to a specified file , or piped system command .

The database re-connection command; this connects you to another database from within psql . You may specify the database to connect to and the username to use (if it is not the same as the current username) and omitting this parameter will cause the current username to be used.

The external editor command; with this, you can either edit a file of your choice or (if no file is specified) the current query buffer. After you are done editing, the new buffer is input to the query buffer, and executed if terminated with a semi-colon.

When opening a file for editing with this command, psql searches your environment variables for the following fields (in this order) to find out what editor to use: PSQL_EDITOR, EDITOR, and VISUAL. If none of these are present, it will attempt to launch /bin/vi .

The quit command; this exits the program. You may also use CTRL-D in most terminal applications to quit.

The variable setting command; used without arguments, this displays all set variables within psql . Otherwise, it sets the variable name to value . If no value is passed, name is set with an empty value. If multiple values are passed, name is assigned the concatenation of each value .

The variable unsetting command; this unsets a specified variable from memory. This is different from assigning a variable with an empty value, which is still technically set.

The shell execution command; without arguments, this opens a shell which overrides the psql prompt until it is exited. Otherwise, it executes a specified shell command from within psql , and displays its results to stdout.