pg.el

Eric Marsden’s pg.el is a socket-level Emacs interface to the PostgreSQL object-relational DBMS. It is capable of automatic coercion from a range of SQL types to the appropriate Emacs Lisp types. Supports large objects (BLOBs).

Overview

This module lets you access the PostgreSQL object-relational DBMS from Emacs, using its socket-level frontend/backend protocol. The module is capable of automatic type coercions from a range of SQL types to the equivalent Emacs Lisp type. This is a low level API, and won’t be useful to end users. Should work with GNU Emacs 19.34 and up, and XEmacs 20 and up. Performance is very poor when not byte-compiled.

Entry points

(with-pg-connection con (dbname user [password host port]) &body body)

A macro which opens a connection to database DBNAME, executes the BODY forms then disconnects. See function `pg:connect’ for details of the connection arguments.

(with-pg-transaction con &body body)

A macro which executes the BODY forms wrapped in an SQL transaction. CON is a connection to the database. If an error occurs during the execution of the forms, a ROLLBACK instruction is executed.

(pg:connect dbname user [password host port]) -> connection

Connect to the database DBNAME on HOST (defaults to localhost) at PORT (defaults to 5432) via TCP/IP and log in as USER. If the database requires a password, send PASSWORD as clear text. Set the output date type to ‘ISO’, and initialize our type parser tables.

(pg:exec connection &rest sql) -> pgresult

Concatenate the SQL strings and send to the backend. Retrieve all the information returned by the database and return it in an opaque record PGRESULT.

(pg:result pgresult what &rest args) -> info

Extract information from the PGRESULT.

The WHAT keyword can be one of

• :connection
• :status
• :attributes
• :tuples
• :tuple tupleNumber
• :oid

:connection allows you to retrieve the database connection.

:status

is a string returned by the backend to indicate the status of the command it is something like “SELECT” for a select command, “DELETE 1” if the deletion affected a single row, etc.

:attributes

is a list of tuples providing metadata:

the first component of each tuple is the attribute’s name as a string, the second an integer representing its PostgreSQL type, and the third an integer representing the size of that type.

:tuples

returns all the data retrieved from the database, as a list of lists, each list corresponding to one row of data returned by the backend.

:tuple num

can be used to extract a specific tuple (numbering starts at 0).

:oid

allows you to retrieve the OID returned by the backend if the command was an insertion; the OID is a unique identifier for that row in the database (this is PostgreSQL-specific, please refer to the documentation for more details).

(pg:disconnect connection) -> nil

Close the database connection.

(pg:for-each connection select-form callback)

Calls CALLBACK on each tuple returned by SELECT-FORM. Declares a cursor for SELECT-FORM, then fetches tuples using repeated executions of FETCH 1, until no results are left. The cursor is then closed. The work is performed within a transaction. When you have a large amount of data to handle, this usage is more efficient than fetching all the tuples in one go.

If you wish to browse the results, each one in a separate buffer, you could have the callback insert each tuple into a buffer created with (generate-new-buffer “myprefix”), then use ibuffer’s “/ n” to list/visit/delete all buffers whose names match myprefix.

(pg:databases connection) -> list of strings

Return a list of the databases available at this site (a database is a set of tables; in a virgin PostgreSQL installation there is a single database named “template1”).

(pg:tables connection) -> list of strings

Return a list of the tables present in the database to which we are currently connected. Only include user tables: system tables are excluded.

(pg:columns connection table) -> list of strings

Return a list of the columns (or attributes) in TABLE, which must be a table in the database to which we are currently connected. We only include the column names; if you want more detailed information (attribute types, for example), it can be obtained from `pg:result’ on a SELECT statement for that table.

(pg:lo-create conn . args) -> oid

Create a new large object (BLOB, or binary large object in other DBMSes parlance) in the database to which we are connected via CONN. Returns an OID (which is represented as an elisp integer) which will allow you to use the large object. Optional ARGS are a Unix-style mode string which determines the permissions of the newly created large object, one of “r” for read-only permission, “w” for write-only, “rw” for read+write. Default is “r”.

Large-object functions MUST be used within a transaction (see the macro `with-pg-transaction’).

(pg:lo-open conn oid . args) -> fd

Open a large object whose unique identifier is OID (an elisp integer) in the database to which we are connected via CONN. Optional ARGS is a Unix-style mode string as for pg:lo-create; which defaults to “r” read-only permissions. Returns a file descriptor (an elisp integer) which can be used in other large-object functions.

(pg:lo-close conn fd)

Close the file descriptor FD which was associated with a large object. Note that this does not delete the large object; use `pg:lo-unlink’ for that.

(pg:lo-read conn fd bytes) -> string

Read BYTES from the file descriptor FD which is associated with a large object. Return an elisp string which should be BYTES characters long.

(pg:lo-write connection fd buf)

Write the bytes contained in the elisp string BUF to the large object associated with the file descriptor FD.

(pg:lo-lseek conn fd offset whence)

Do the equivalent of a lseek(2) on the file descriptor FD which is associated with a large object; ie reposition the read/write file offset for that large object to OFFSET (an elisp integer). WHENCE has the same significance as in lseek(); it should be one of SEEK_SET (set the offset to the absolute position), SEEK_CUR (set the offset relative to the current offset) or SEEK_END (set the offset relative to the end of the file). WHENCE should be an elisp integer whose values can be obtained from the header file <unistd.h> (probably 0, 1 and 2 respectively).

(pg:lo-tell conn oid) -> integer

Do the equivalent of an ftell(3) on the file associated with the large object whose unique identifier is OID. Returns the current position of the file offset for the object’s associated file descriptor, as an elisp integer.

(pg:lo-unlink conn oid)

Remove the large object whose unique identifier is OID from the system (in the current implementation of large objects in PostgreSQL, each large object is associated with an object in the filesystem).

(pg:lo-import conn filename) -> oid

Create a new large object and initialize it to the data contained in the file whose name is FILENAME. Returns an OID (as an elisp integer). Note that is operation is only syntactic sugar around the basic large-object operations listed above.

(pg:lo-export conn oid filename)

Create a new file named FILENAME and fill it with the contents of the large object whose unique identifier is OID. This operation is also syntactic sugar.

Boolean variable `pg:disable-type-coercion’ which can be set to non-nil (before initiating a connection) to disable the library’s type coercion facility. Default is t.

The interface is pretty slow (byte compiling helps a lot). Maybe someone can suggest a better way of reading input from the network stream. Please note that your postmaster has to be started with the `-i’ option in order to accept TCP/IP connections (this is not the default). For more information about PostgreSQL see URL:http://www.PostgreSQL.org/.

Thanks to Eric Ludlam <[email protected]> for discovering a bug in the date parsing routines, to Hartmut Pilch and Yoshio Katayama for adding multibyte support, and to Doug McNaught and Pavel Janik for bug fixes.

SECURITY NOTE:

setting up PostgreSQL to accept TCP/IP connections has security implications; please consult the documentation for details. pg.el supports neither the crypt authentication method, nor Kerberos (support for these can’t be added to Emacs due to silly US crypto export regulations). However, it is possible to use the port forwarding capabilities of ssh to establish a connection to the backend over TCP/IP, which provides both a secure authentication mechanism and encryption (and optionally compression) of data passing through the tunnel. Here’s how to do it (thanks to Gene Selkov, Jr. <[email protected]> for the description):

1. Establish a tunnel to the backend machine, like this:

ssh -L 3333:backend.dom:5432 [email protected]

The first number in the -L argument, 3333, is the port number of your end of the tunnel. The second number, 5432, is the remote end of the tunnel – the port number your backend is using. The name or the address in between the port numbers belongs to the server machine, as does the last argument to ssh that also includes the optional user name. Without the user name, ssh will try the name you are currently logged on as on the client machine. You can use any user name the server machine will accept, not necessarily those related to postgres.

2.

Now that you have a running ssh session, you can point pg.el to the local host at the port number which you specified in step 1. For example,

(pg:connect "dbname" "user" "password" "localhost" 3333)

You can omit the port argument if you chose 5432 as the local end of the tunnel, since pg.el defaults to this value.

INSTALL

Place this file in a directory somewhere in the load-path, then byte-compile it (do a `B’ on it in dired, for example). Place a line such as `(require ‘pg)’ in your emacs initialization file.

• add a mechanism for parsing user-defined types. The user should be able to define a parse function and a type-name; we query pg_type to get the type’s OID and add the information to pg:parsers.
• in a future release I will probably modify the numeric conversion routines to return elisp floating point values instead of elisp integers, in order to work around possible overflow problems.

Version history

• 0.11 - Bugfixes (see issues 2, 3, 4, and 5)
• 0.10 - 2003-02-12 by Eric Marsden, under GNU GPL.