An Emacs 25 module for accessing PostgreSQL via the libpq client library.

Using libpq for client connections has various advantages over the wire-protocol speaking pure elisp implementations. For example, it has better performance and supports all features of the protocol like full TLS support and new authentication methods like scram-sha-256.

It doesn’t expose many libpq features yet, but what’s there should be crash-safe no matter what you do in the lisp world. I’ve been using it for three years now for reading mail through my custom Gnus backend without incidents. If you make it crash, please report.

Basic usage

ELISP> (setq *pq* (pq:connectdb "dbname=andreas"))
#<user-ptr ptr=0x55b466c02780 finalizer=0x7f7d50112236>
ELISP> (pq:query *pq* "select version()")
("PostgreSQL 9.6.7 on x86_64-pc-linux-gnu, compiled by gcc (Debian 6.3.0-18) 6.3.0 20170516, 64-bit")
ELISP> (pq:query *pq* "create table local_variables(name text, value text)")
nil
ELISP> (dolist (el (buffer-local-variables))
	 (pq:query *pq* "insert into local_variables values ($1, $2)"
		   (car el) (cdr el)))
nil
ELISP> (pq:query *pq* "select name, length(value) from local_variables where value ~ 'mode'")
(["major-mode" 24]
 ["change-major-mode-hook" 86]
 ["hi-lock-mode-major-mode" 24]
 ["eldoc-mode-major-mode" 24]
 ["font-lock-major-mode" 24]
 ["font-lock-mode-major-mode" 24])

Error Handling

pq raises SQL errors as error signal pq:error. This provides the SQLSTATE error code in an additional string in the error data list. For example, you can reliably catch unique violations like this:

(condition-case err (pq:query *pq* "insert into t values (666)")
  (pq:error
   (if (string= "23505" (nth 2 err))
       (progn
	 (message "Caught a unique violation"))
     ;; re-throw anything else
     (signal (car err) (cdr err)))))

Conversion of data types from SQL to Emacs Lisp

pq converts bigints and numerics your queries return to lisp floats because they don’t fit into a lisp integer. This looses precision on big values. If you need the full precision, cast them to text and use, e.g., calc-eval to do arbitrary precision things with them. All other data types are returned as utf-8 strings.

Conversion of data types from Emacs Lisp to SQL

Strings and the query text itself is converted to utf-8 by the module interface. If this conversion fails, the behavior is undefined by the module interface. If you want to send strings that are not valid utf-8, you need to work around this. For example, I’m using code like the following to store raw bytes into a table with a bytea column:

 (pq:query *con*
   "insert into t (blob) values (decode($1, 'base64'))"
   (base64-encode-string my-random-bytes))

Any non-string parameter to pq:query is turned into an emacs string using prin1-to-string first. This works quite well to store arbitrary lisp data and read it back with read. All other aspects of prin1-to-string apply too. For example, when print-length or print-level are set to non-nil, these would be applied as well.

Notifications

After a =LISTEN= statement, PostgreSQL will deliver notifications asynchronously over the connection. Since the emacs-module interface does not allow for asynchronous callbacks, you have to check for these periodically after a LISTEN statement by calling pq:notifies. Calling it will not cause any traffic on the connection itself.

See the testsuite ./test.el for more implemented features.