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
[[https://www.postgresql.org/docs/current/errcodes-appendix.html][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 [[https://www.postgresql.org/docs/current/sql-listen.html][=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.

[[https://api.travis-ci.org/anse1/emacs-libpq.svg]]
