LARGE
DESCRIPTION
In Postgres, data values are stored in tuples and individ-
ual tuples cannot span data pages. Since the size of a
data page is 8192 bytes, the upper limit on the size of a
data value is relatively low. To support the storage of
larger atomic values, Postgres provides a large object
interface. This interface provides file-oriented access
to user data that has been declared to be a large type.
This section describes the implementation and the program-
matic and query language interfaces to Postgres large
object data.
Historical Note
Originally, postgres 4.2 supports three standard implemen-
tations of large objects: as files external to Postgres,
as Unix files managed by Postgres, and as data stored
within the Postgres database. It causes considerable con-
fusion among users. As a result, we only support large
objects as data stored within the Postgres database in
Postgres. Even though is is slower to access, it provides
stricter data integrity and time travel. For historical
reasons, they are called Inversion large objects. (We will
use Inversion and large objects interchangeably to mean
the same thing in this section.)
Inversion Large Objects
The Inversion large object implementation breaks large
objects up into "chunks" and stores the chunks in tuples
in the database. A B-tree index guarantees fast searches
for the correct chunk number when doing random access
reads and writes.
Large Object Interfaces
The facilities Postgres provides to access large objects,
both in the backend as part of user-defined functions or
the front end as part of an application using the inter-
face, are described below. (For users familiar with post-
gres 4.2, Postgres has a new set of functions providing a
more coherent interface. The interface is the same for
dynamically-loaded C functions as well as for .
The Postgres large object interface is modeled after the
Unix file system interface, with analogues of open (2),
read (2), write (2), lseek (2), etc. User functions call
these routines to retrieve only the data of interest from
a large object. For example, if a large object type
called mugshot existed that stored photographs of faces,
then a function called beard could be declared on mugshot
data. Beard could look at the lower third of a photo-
graph, and determine the color of the beard that appeared
there, if any. The entire large object value need not be
buffered, or even examined, by the beard function.
Large objects may be accessed from dynamically-loaded C
functions or database client programs that link the Libpq
library. Postgres provides a set of routines that support
opening, reading, writing, closing, and seeking on large
objects.
Creating a Large Object
The routine
Oid lo_creat(PGconn *conn, int mode)
creates a new large object. The mode is a bitmask describ-
ing several different attributes of the new object. The
symbolic constants listed here are defined in
/usr/local/pgsql/src/backend/libpq/libpq-fs.h
The access type (read, write, or both) is controlled by OR
ing together the bits INV_READ and INV_WRITE . If the
large object should be archived - that is, if historical
versions of it should be moved periodically to a special
archive relation - then the INV_ARCHIVE bit should be set.
The low-order sixteen bits of mask are the storage manager
number on which the large object should reside. For sites
other than Berkeley, these bits should always be zero.
The commands below create an (Inversion) large object:
inv_oid = lo_creat(INV_READ|INV_WRITE|INV_ARCHIVE);
Importing a Large Object
To import a UNIX file as a large object, call
Oid
lo_import(PGconn *conn, text *filename)
The filename argument specifies the UNIX pathname of the
file to be imported as a large object.
Exporting a Large Object
To export a large object into UNIX file, call
int
lo_export(PGconn *conn, Oid lobjId, text *filename)
The lobjId argument specifies the Oid of the large object
to export and the filename argument specifies the UNIX
pathname of the file.
Opening an Existing Large Object
To open an existing large object, call
int
lo_open(PGconn *conn, Oid lobjId, int mode, ...)
The lobjId argument specifies the Oid of the large object
to open. The mode bits control whether the object is
opened for reading INV_READ ), ( writing INV_WRITE ), ( or
both.
A large object cannot be opened before it is created.
lo_open returns a large object descriptor for later use in
lo_read , lo_write , lo_lseek , lo_tell , and lo_close .
Writing Data to a Large Object
The routine
int
lo_write(PGconn *conn, int fd, char *buf, int len)
writes len bytes from buf to large object fd . The fd
argument must have been returned by a previous lo_open .
The number of bytes actually written is returned. In the
event of an error, the return value is negative.
Seeking on a Large Object
To change the current read or write location on a large
object, call
int
lo_lseek(PGconn *conn, int fd, int offset, int whence)
This routine moves the current location pointer for the
large object described by fd to the new location specified
by offset . The valid values for .I whence are SEEK_SET
SEEK_CUR and SEEK_END.
Closing a Large Object Descriptor
A large object may be closed by calling
int
lo_close(PGconn *conn, int fd)
where fd is a large object descriptor returned by lo_open
. On success, lo_close returns zero. On error, the
return value is negative.
Built in registered functions
There are two built-in registered functions, lo_import and
lo_export which are convenient for use in SQL queries.
Here is an example of there use
CREATE TABLE image (
name text,
raster oid
);
INSERT INTO image (name, raster)
VALUES ('beautiful image', lo_import('/etc/motd'));
SELECT lo_export(image.raster, '/tmp/motd') from image
WHERE name = 'beautiful image';
Accessing Large Objects from LIBPQ
Below is a sample program which shows how the large object
interface in
can be used. Parts of the program are commented out but
are left in the source for the readers benefit. This pro-
gram can be found in
../src/test/examples
Frontend applications which use the large object interface
in .br
should include the header file libpq/libpq-fs.h and link
with the libpq library.
Sample Program
/*-------------------------------------------------------------------------
*
* testlo.c--
* test using large objects with libpq
*
* Copyright (c) 1994, Regents of the University of California
*
*
* IDENTIFICATION
* $Header: /usr/local/cvsroot/pgsql/src/man/large_objects.3,v 1.6 1998/03/23 15:09:29 momjian Exp $
*
*-------------------------------------------------------------------------
*/
#include <stdio.h>
#include "libpq-fe.h"
#include "libpq/libpq-fs.h"
#define BUFSIZE 1024
/*
* importFile -
* import file "in_filename" into database as large object "lobjOid"
*
*/
Oid importFile(PGconn *conn, char *filename)
{
Oid lobjId;
int lobj_fd;
char buf[BUFSIZE];
int nbytes, tmp;
int fd;
/*
* open the file to be read in
*/
fd = open(filename, O_RDONLY, 0666);
if (fd < 0) { /* error */
fprintf(stderr, "can't open unix file\"%s\"\n", filename);
}
/*
* create the large object
*/
lobjId = lo_creat(conn, INV_READ|INV_WRITE);
if (lobjId == 0) {
fprintf(stderr, "can't create large object");
}
lobj_fd = lo_open(conn, lobjId, INV_WRITE);
/*
* read in from the Unix file and write to the inversion file
*/
while ((nbytes = read(fd, buf, BUFSIZE)) > 0) {
tmp = lo_write(conn, lobj_fd, buf, nbytes);
if (tmp < nbytes) {
fprintf(stderr, "error while reading \"%s\"", filename);
}
}
(void) close(fd);
(void) lo_close(conn, lobj_fd);
return lobjId;
}
void pickout(PGconn *conn, Oid lobjId, int start, int len)
{
int lobj_fd;
char* buf;
int nbytes;
int nread;
lobj_fd = lo_open(conn, lobjId, INV_READ);
if (lobj_fd < 0) {
fprintf(stderr,"can't open large object %d",
lobjId);
}
lo_lseek(conn, lobj_fd, start, SEEK_SET);
buf = malloc(len+1);
nread = 0;
while (len - nread > 0) {
nbytes = lo_read(conn, lobj_fd, buf, len - nread);
buf[nbytes] = '\0';
fprintf(stderr,">>> %s", buf);
nread += nbytes;
}
fprintf(stderr,"\n");
lo_close(conn, lobj_fd);
}
void overwrite(PGconn *conn, Oid lobjId, int start, int len)
{
int lobj_fd;
char* buf;
int nbytes;
int nwritten;
int i;
lobj_fd = lo_open(conn, lobjId, INV_READ);
if (lobj_fd < 0) {
fprintf(stderr,"can't open large object %d",
lobjId);
}
lo_lseek(conn, lobj_fd, start, SEEK_SET);
buf = malloc(len+1);
for (i=0;i 0) {
nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
nwritten += nbytes;
}
fprintf(stderr,"\n");
lo_close(conn, lobj_fd);
}
/*
* exportFile -
* export large object "lobjOid" to file "out_filename"
*
*/
void exportFile(PGconn *conn, Oid lobjId, char *filename)
{
int lobj_fd;
char buf[BUFSIZE];
int nbytes, tmp;
int fd;
/*
* create an inversion "object"
*/
lobj_fd = lo_open(conn, lobjId, INV_READ);
if (lobj_fd < 0) {
fprintf(stderr,"can't open large object %d",
lobjId);
}
/*
* open the file to be written to
*/
fd = open(filename, O_CREAT|O_WRONLY, 0666);
if (fd < 0) { /* error */
fprintf(stderr, "can't open unix file\"%s\"",
filename);
}
/*
* read in from the Unix file and write to the inversion file
*/
while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) > 0) {
tmp = write(fd, buf, nbytes);
if (tmp < nbytes) {
fprintf(stderr,"error while writing \"%s\"",
filename);
}
}
(void) lo_close(conn, lobj_fd);
(void) close(fd);
return;
}
void
exit_nicely(PGconn* conn)
{
PQfinish(conn);
exit(1);
}
int
main(int argc, char **argv)
{
char *in_filename, *out_filename;
char *database;
Oid lobjOid;
PGconn *conn;
PGresult *res;
if (argc != 4) {
fprintf(stderr, "Usage: %s database_name in_filename out_filename\n",
argv[0]);
exit(1);
}
database = argv[1];
in_filename = argv[2];
out_filename = argv[3];
/*
* set up the connection
*/
conn = PQsetdb(NULL, NULL, NULL, NULL, database);
/* check to see that the backend connection was successfully made */
if (PQstatus(conn) == CONNECTION_BAD) {
fprintf(stderr,"Connection to database '%s' failed.\n", database);
fprintf(stderr,"%s",PQerrorMessage(conn));
exit_nicely(conn);
}
res = PQexec(conn, "begin work;");
PQclear(res);
printf("importing file \"%s\" ...\n", in_filename);
/* lobjOid = importFile(conn, in_filename); */
lobjOid = lo_import(conn, in_filename);
/*
printf("\tas large object %d.\n", lobjOid);
printf("picking out bytes 1000-2000 of the large object\n");
pickout(conn, lobjOid, 1000, 1000);
printf("overwriting bytes 1000-2000 of the large object with X's\n");
overwrite(conn, lobjOid, 1000, 1000);
*/
printf("exporting large object to file \"%s\" ...\n", out_filename);
/* exportFile(conn, lobjOid, out_filename); */
lo_export(conn, lobjOid,out_filename);
res = PQexec(conn, "commit;");
PQclear(res);
PQfinish(conn);
exit(0);
}