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.

Including support for:

• SCRAM-SHA-256 authentication (the default authentication method since PostgreSQL version 14)

• MD5 authentication

• Encrypted (TLS) connections with PostgreSQL, though currently not for authentication using client-side certificates. This assumes your Emacs has been built with GnuTLS support.

• Connections over TCP or (on Unix machines) a local Unix socket

• Parsing PostgreSQL JSON, JSONB and HSTORE objects

• Parsing PostgreSQL array types

• Parsing PostgreSQL integer and numerical range types

The code has been tested with PostgreSQL versions 15.4, 13.8, 11.17, and 10.22 on Linux. It is also tested via GitHub actions on MacOS and Windows, using the PostgreSQL version which is pre-installed in the virtual images (currently 14.8). This library also works against other databases that implement the PostgreSQL wire protocol:

• CockroachDB: tested with CockroachDB CCL v22.1.7. Note that this database does not implement the large object functionality, and its interpretation of SQL occasionally differs from that of PostgreSQL.

• CrateDB: tested with version 5.0.1.

• QuestDB: tested against version 6.5.4

• YugabyteDB: tested against version 2.15

Tested with Emacs versions 29.1, 28.2, 27.2 and 26.3. Emacs versions older than 26.1 will not work against a recent PostgreSQL version (whose default configuration requires SCRAM-SHA-256 authentication), because they don't include the GnuTLS support which we use to calculate HMACs. They may however work against a database set up to allow unauthenticated local connections.

You may be interested in an alternative library emacs-libpq that enables access to PostgreSQL from Emacs by binding to the libpq library.

Install via the MELPA package archive by including the following in your Emacs initialization file (.emacs.el or init.el):

(require 'package)
(add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)

then saying

 M-x package-install RET pg

To install manually, place the file pg.el in a directory on your load-path, byte-compile it (using for example B in dired) and add the following to your Emacs initialization file:

(require 'pg)

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

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

(with-pg-connection-local con (path dbname user [password]) &body body)

A macro which opens a connection to database DBNAME over a local Unix socket at PATH, executes the BODY forms then disconnects. See function pg-connect-local 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 authenticate as USER with PASSWORD. This library currently supports SCRAM-SHA-256 authentication (the default method from PostgreSQL version 14 onwards), MD5 authentication and cleartext password authentication. This function also sets the output date type to 'ISO', and initializes our type parser tables.

(pg-connect-local path dbname user [password])

Initiate a connection with the PostgreSQL backend over local Unix socket PATH. Connect to the database DBNAME with the username USER, providing PASSWORD if necessary. Return a connection to the database (as an opaque type). PASSWORD defaults to an empty string.

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

Concatenate the SQL strings and send to the PostgreSQL backend. Retrieve 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 returned by pg-exec. The WHAT keyword can be one of

• :connection: retrieve the database connection.

• :status: 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: 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: 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 tuple-number: return 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-cancel con) -> nil

Ask the server to cancel the command currently being processed by the backend. The cancellation request concerns the command requested over database connection CON.

(pg-disconnect con) -> nil

Close the database connection CON.

(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 con) -> list of strings

Return a list of the databases available over PostgreSQL connection CON. A database is a set of tables; in a fresh PostgreSQL installation there is a single database named "template1".

(pg-tables con) -> list of strings

Return a list of the tables present in the database to which we are currently connected over CON. Only include user tables: system tables are not included in this list.

(pg-columns con 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 connected over CON. 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-hstore-setup con)

Prepare for the use of HSTORE datatypes over database connection CON. This function must be called before using the HSTORE extension. It loads the extension if necessary, and sets up the parsing support for HSTORE datatypes.

(pg-lo-create con . 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 CON. 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 con oid . args) -> fd

Open a large object whose unique identifier is OID (an elisp integer) in the database to which we are connected via CON. 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 con 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 con 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 con fd buf)

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

(pg-lo-lseek cnn fd offset whence)

Do the equivalent of a lseek(2) on the file descriptor FD which is associated with a large object; i.e. 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 con 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 con 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 con 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 this 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.

Variable pg-parameter-change-functions is a list of handlers to be called when the backend informs us of a parameter change, for example a change to the session time zone. Each handler is called with three arguments: the connection to the backend, the parameter name and the parameter value. It is initially set to a function that looks out for client_encoding messages and updates the value recorded in the connection.

Variable pg-handle-notice-functions is a list of handlers to be called when the backend sends us a NOTICE message. Each handler is called with one argument, the notice, as a pgerror struct.

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

For more information about PostgreSQL see https://www.PostgreSQL.org/.

Security note: setting up PostgreSQL to accept TCP/IP connections has security implications; please consult the documentation for details. 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. 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.

Thanks to Eric Ludlam 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.

The latest version of this package should be available from

<https://github.com/emarsden/pg-el/>