Large Objects
large object>>
BLOB>large object>>
Introduction
In PostgreSQL releases prior to 7.1,
the size of any row in the database could not exceed the size of a
data page. Since the size of a data page is 8192 bytes (the
default, which can be raised up to 32768), the upper limit on the
size of a data value was relatively low. To support the storage of
larger atomic values, PostgreSQL
provided and continues to provide a large object interface. This
interface provides file-oriented access to user data that has been
declared to be a large object.
POSTGRES 4.2, the indirect predecessor
of PostgreSQL, supported three standard
implementations of large objects: as files external to the
POSTGRES server, as external files
managed by the POSTGRES server, and as
data stored within the POSTGRES
database. This caused considerable confusion among users. As a
result, only support for large objects as data stored within the
database is retained in PostgreSQL.
Even though this is slower to access, it provides stricter data
integrity. For historical reasons, this storage scheme is
referred to as Inversion large
objects. (You will see the term Inversion used
occasionally to mean the same thing as large object.) Since
PostgreSQL 7.1, all large objects are
placed in one system table called
pg_largeobject.
TOAST>>
sliced bread>TOAST>
PostgreSQL 7.1 introduced a mechanism
(nicknamed TOAST
) that allows
data rows to be much larger than individual data pages. This
makes the large object interface partially obsolete. One
remaining advantage of the large object interface is that it
allows random access to the data, i.e., the ability to read or
write small chunks of a large value. It is planned to equip
TOAST with such functionality in the future.
This section describes the implementation and the programming and
query language interfaces to PostgreSQL
large object data. We use the libpq C
library for the examples in this section, but most programming
interfaces native to PostgreSQL support
equivalent functionality. Other interfaces may use the large
object interface internally to provide generic support for large
values. This is not described here.
Implementation Features
The 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.
Interfaces
The facilities PostgreSQL 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 interface, are described below. For users
familiar with POSTGRES 4.2,
PostgreSQL has a new set of
functions providing a more coherent interface.
All large object manipulation must take
place within an SQL transaction. This requirement is strictly
enforced as of PostgreSQL 6.5>, though it has been an
implicit requirement in previous versions, resulting in
misbehavior if ignored.
The PostgreSQL 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 photograph, 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
library. PostgreSQL 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.
mode is a bit mask
describing several different attributes of the new
object. The symbolic constants listed here are defined
in the header file libpq/libpq-fs.h.
The access type (read, write, or both) is controlled by
or'ing together the bits INV_READ and
INV_WRITE. The low-order sixteen bits of the mask have
historically been used at Berkeley to designate the storage manager number on which the large object
should reside. These
bits should always be zero now.
The commands below create a large object:
inv_oid = lo_creat(INV_READ|INV_WRITE);
Importing a Large Object
To import an operating system file as a large object, call
Oid lo_import(PGconn *conn, const char *filename)
filename
specifies the operating system name of
the file to be imported as a large object.
Exporting a Large Object
To export a large object
into an operating system file, call
int lo_export(PGconn *conn, Oid lobjId, const char *filename)
The lobjId argument specifies the Oid of the large
object to export and the filename argument specifies
the operating system name name 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, const char *buf, size_t 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.
Reading Data from a Large Object
The routine
int lo_read(PGconn *conn, int fd, char *buf, size_t len)
reads len bytes from large object fd into buf. The fd
argument must have been returned by a previous lo_open.
The number of bytes actually read 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 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.
Removing a Large Object
To remove a large object from the database, call
Oid lo_unlink(PGconn *conn, Oid lobjId)
The lobjId argument specifies the Oid of the large
object to remove.
Server-side Built-in 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 their 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
is a sample program which shows how the large object
interface
in libpq> can be used. Parts of the program are
commented out but are left in the source for the reader's
benefit. This program can be found in
src/test/examples/testlo.c in the source distribution.
Frontend applications which use the large object interface
in libpq should include the header file
libpq/libpq-fs.h and link with the libpq library.
Large Objects with Libpq Example Program
/*--------------------------------------------------------------
*
* testlo.c--
* test using large objects with libpq
*
* Copyright (c) 1994, Regents of the University of California
*
*--------------------------------------------------------------
*/
#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\n");
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 large object\n");
}
(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\n",
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] = ' ';
fprintf(stderr, ">>> %s", buf);
nread += nbytes;
}
free(buf);
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\n",
lobjId);
}
lo_lseek(conn, lobj_fd, start, SEEK_SET);
buf = malloc(len + 1);
for (i = 0; i < len; i++)
buf[i] = 'X';
buf[i] = ' ';
nwritten = 0;
while (len - nwritten > 0)
{
nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
nwritten += nbytes;
}
free(buf);
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\n",
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\n",
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\n",
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");
PQclear(res);
printf("importing file %s\n", in_filename);
/* lobjOid = importFile(conn, in_filename); */
lobjOid = lo_import(conn, in_filename);
/*
printf("as 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, "end");
PQclear(res);
PQfinish(conn);
exit(0);
}