pg.scm -- socket level interface to the PostgreSQL RDBMS for DrScheme
Version: 0.18
Copyright (C) 1999,2000,2001,2002,2003 Eric Marsden
DrScheme version: Zbigniew Jurkiewicz <zbyszek@mimuw.edu.pl>
Copyright: (C) 2001,2002,2003 Zbigniew Jurkiewicz
== Overview =========================================================
This module lets you access the PostgreSQL object-relational DBMS
from DrScheme and provides a wrapper around the libpq module. The module is
capable of type oercions from a range of SQL types to the equivalent Lisp
type.
Works with MzScheme and DrScheme.
== Entry points =======================================================
(with-pg-connection ((con . open-args) . 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 OPEN-ARGS.
(with-pg-transaction con . 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), and log in as USER. If HOST is #f,
attempt to connect to the localhost using a Unix domain socket;
otherwise the connection is established using TCP/IP. If the
database requires a password, send PASSWORD (as clear text unless
the backend demands crypt() authentication). Set the output date
type to 'ISO', and initialize our type parser tables.
(pg-exec connection . 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 . args) -> info
Extract information from the PGRESULT. WHAT 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 normally "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. `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-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.
(pg-disconnect connection) -> nil
Close the database connection.
(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.
(pglo-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 a
Scheme 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').
(pglo-open conn oid . args) -> fd
Open a large object whose unique identifier is OID (a Scheme integer)
in the database to which we are connected via CONN. Optional ARGS
is a Unix-style mode string as for pglo-create; which defaults to
"r" read-only permissions. Returns a file descriptor (a Scheme integer)
which can be used in other large-object functions.
(pglo-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
PGLO-UNLINK for that.
(pglo-read conn fd bytes) -> string
Read BYTES from the file descriptor FD which is associated with a
large object. Return a string which should be BYTES characters
long.
(pglo-write connection fd buf)
Write the bytes contained in the string BUF to the large object
associated with the file descriptor FD.
(pglo-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 (a Scheme 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
integer whose values can be obtained from the header file
<unistd.h> (probably 0, 1 and 2 respectively).
(pglo-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 a Scheme integer.
(pglo-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).
(pglo-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 a Scheme
integer). Note that is operation is only syntactic sugar around
the basic large-object operations listed above.
(pglo-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
#t (before initiating a connection) to disable the library's
type coercion facility. Default is #f. ;!!!
SECURITY NOTE: please note that your postmaster has to be started
with the `-i' option in order for it to accept TCP/IP connections
(typically this is not the default setting). See the PostgreSQL
documentation at <URL:http://www.PostgreSQL.org/> for more
information.
Setting up PostgreSQL to accept TCP/IP connections has security
implications; please consult the documentation for details. You can
connect to the database using Unix domain sockets if you wish to
avoid setting up PostgreSQL to listen on a TCP socket.
pg.scm is able to use the crypt authentication method to avoid
sending the password in cleartext over the wire (this assumes access
to the `crypt' function via the FFI). It does not support the
Kerberos authentication method, nor OpenSSL connections (though this
should not be difficult if your Scheme implementation is able to
open SSL streams). 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.
<selkovjr@mcs.anl.gov> for the description):
1. Establish a tunnel to the backend machine, like this:
ssh -L 3333:backend.dom:5432 postgres@backend.dom
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.scm 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 last three arguments if you chose 5432 as the local
end of the tunnel, since pg.scm defaults to this value.
This code has been tested or reported to work with
* MzScheme 209, DrScheme 209 on Linux/x86
* PostgreSQL 6.5, 7.0, 7.1.2, 7.2.