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<H1>13. LARGE OBJECTS</H1>
<HR>
In POSTGRES, data values are stored in tuples and
individual 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
programmatic and query language interfaces to POSTGRES
large object data.
<H2><A NAME="historical-note">13.1. Historical Note</A></H2>
Originally, <B>POSTGRES 4.2</B> supports three standard
implementations of large objects: as files external
to POSTGRES, as <B>UNIX</B> files managed by POSTGRES, and as data
stored within the POSTGRES database. It causes
considerable confusion among users. As a result, we only
support large objects as data stored within the POSTGRES
database in <B>POSTGRES95</B>. 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.)
<H2><A NAME="inversion-large-objects">13.2. Inversion Large Objects</A></H2>
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.
<H2><A NAME="large-object-interfaces">13.3. Large Object Interfaces</A></H2>
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 interface, are described below. (For users
familiar with <B>POSTGRES 4.2</B>, <B>POSTGRES95</B> 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 <B>UNIX</B> file system interface, with analogues of
<B>open(2), read(2), write(2), lseek(2)</B>, 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 <B>C</B>
functions or database client programs that link the
library. POSTGRES provides a set of routines that
support opening, reading, writing, closing, and seeking on
large objects.
<p>
<H3><A NAME="creating-large-objects">13.3.1. Creating a Large Object</A></H3>
The routine
<pre> Oid lo_creat(PGconn *conn, int mode)
</pre>
creates a new large object. The mode is a bitmask
describing several different attributes of the new
object. The symbolic constants listed here are defined
in
<pre> /usr/local/postgres95/src/backend/libpq/libpq-fs.h
</pre>
The access type (read, write, or both) is controlled by
OR ing together the bits <B>INV_READ</B> and <B>INV_WRITE</B>. 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 <B>INV_ARCHIVE</B> 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:
<pre> inv_oid = lo_creat(INV_READ|INV_WRITE|INV_ARCHIVE);
</pre>
<H3><A NAME="importing-a-large-object">13.3.2. Importing a Large Object</A></H3>
To import a <B>UNIX</B> file as
a large object, call
<pre> Oid
lo_import(PGconn *conn, text *filename)
</pre>
The filename argument specifies the <B>UNIX</B> pathname of
the file to be imported as a large object.
<p>
<H3><A NAME="exporting-a-large-object">13.3.3. Exporting a Large Object</A></H3>
To export a large object
into <B>UNIX</B> file, call
<pre> int
lo_export(PGconn *conn, Oid lobjId, text *filename)
</pre>
The lobjId argument specifies the Oid of the large
object to export and the filename argument specifies
the <B>UNIX</B> pathname of the file.
<p>
<H3><A NAME="opening-an-existing-large-object">13.3.4. Opening an Existing Large Object</A></H3>
To open an existing large object, call
<pre> int
lo_open(PGconn *conn, Oid lobjId, int mode, ...)
</pre>
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 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.
<p>
<H3><A NAME="writing-data-to-a-large-object">13.3.5. Writing Data to a Large Object</A></H3>
The routine
<pre> int
lo_write(PGconn *conn, int fd, char *buf, int len)
</pre>
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.
<p>
<H3><A NAME="seeking-on-a-large-object">13.3.6. Seeking on a Large Object</A></H3>
To change the current read or write location on a large
object, call
<pre> int
lo_lseek(PGconn *conn, int fd, int offset, int whence)
</pre>
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.
<p>
<H3><A NAME="closing-a-large-object-descriptor">13.3.7. Closing a Large Object Descriptor</A></H3>
A large object may be closed by calling
<pre> int
lo_close(PGconn *conn, int fd)
</pre>
where fd is a large object descriptor returned by
lo_open. On success, <B>lo_close</B> returns zero. On error,
the return value is negative.
<H2><A NAME="built-in-registered-functions">13.4. Built in registered functions</A></H2>
There are two built-in registered functions, <B>lo_import</B>
and <B>lo_export</B> which are convenient for use in <B>SQL</B>
queries.
Here is an example of there use
<pre> 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';
</pre>
<H2><A NAME="accessing-large-objects-from-libpq">13.5. Accessing Large Objects from LIBPQ</A></H2>
Below 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 readers
benefit. This program can be found in
<pre> ../src/test/examples
</pre>
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.
<H2><A NAME="sample-program">13.6. Sample Program</A></H2>
<pre> /*--------------------------------------------------------------
*
* testlo.c--
* test using large objects with libpq
*
* Copyright (c) 1994, Regents of the University of California
*
*
* IDENTIFICATION
* /usr/local/devel/pglite/cvs/src/doc/manual.me,v 1.16 1995/09/01 23:55:00 jolly Exp
*
*--------------------------------------------------------------
*/
#include <stdio.h>
#include "libpq-fe.h"
#include "libpq/libpq-fs.h"
<p>
#define BUFSIZE 1024
<p>
/*
* 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;
<p>
/*
* open the file to be read in
*/
fd = open(filename, O_RDONLY, 0666);
if (fd < 0) { /* error */
fprintf(stderr, "can't open unix file
}
<p>
/*
* create the large object
*/
lobjId = lo_creat(conn, INV_READ|INV_WRITE);
if (lobjId == 0) {
fprintf(stderr, "can't create large object");
}
<p>
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
}
}
<p>
(void) close(fd);
(void) lo_close(conn, lobj_fd);
<p>
return lobjId;
}
<p>
void pickout(PGconn *conn, Oid lobjId, int start, int len)
{
int lobj_fd;
char* buf;
int nbytes;
int nread;
<p>
lobj_fd = lo_open(conn, lobjId, INV_READ);
if (lobj_fd < 0) {
fprintf(stderr,"can't open large object %d",
lobjId);
}
<p>
lo_lseek(conn, lobj_fd, start, SEEK_SET);
buf = malloc(len+1);
<p>
nread = 0;
while (len - nread > 0) {
nbytes = lo_read(conn, lobj_fd, buf, len - nread);
buf[nbytes] = ' ';
fprintf(stderr,">>> %s", buf);
nread += nbytes;
}
fprintf(stderr,"0);
lo_close(conn, lobj_fd);
}
<p>
void overwrite(PGconn *conn, Oid lobjId, int start, int len)
{
int lobj_fd;
char* buf;
int nbytes;
int nwritten;
int i;
<p>
lobj_fd = lo_open(conn, lobjId, INV_READ);
if (lobj_fd < 0) {
fprintf(stderr,"can't open large object %d",
lobjId);
}
<p>
lo_lseek(conn, lobj_fd, start, SEEK_SET);
buf = malloc(len+1);
<p>
for (i=0;i<len;i++)
buf[i] = 'X';
buf[i] = ' ';
<p>
nwritten = 0;
while (len - nwritten > 0) {
nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
nwritten += nbytes;
}
fprintf(stderr,"0);
lo_close(conn, lobj_fd);
}
<p>
/*
* 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;
<p>
/*
* 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);
}
<p>
/*
* 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
filename);
}
<p>
/*
* 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
filename);
}
}
<p>
(void) lo_close(conn, lobj_fd);
(void) close(fd);
<p>
return;
}
<p>
void
exit_nicely(PGconn* conn)
{
PQfinish(conn);
exit(1);
}
<p>
int
main(int argc, char **argv)
{
char *in_filename, *out_filename;
char *database;
Oid lobjOid;
PGconn *conn;
PGresult *res;
<p>
if (argc != 4) {
fprintf(stderr, "Usage: %s database_name in_filename out_filename0,
argv[0]);
exit(1);
}
<p>
database = argv[1];
in_filename = argv[2];
out_filename = argv[3];
<p>
/*
* set up the connection
*/
conn = PQsetdb(NULL, NULL, NULL, NULL, database);
<p>
/* check to see that the backend connection was successfully made */
if (PQstatus(conn) == CONNECTION_BAD) {
fprintf(stderr,"Connection to database '%s' failed.0, database);
fprintf(stderr,"%s",PQerrorMessage(conn));
exit_nicely(conn);
}
<p>
res = PQexec(conn, "begin");
PQclear(res);
printf("importing file
/* lobjOid = importFile(conn, in_filename); */
lobjOid = lo_import(conn, in_filename);
/*
printf("as large object %d.0, lobjOid);
<p>
printf("picking out bytes 1000-2000 of the large object0);
pickout(conn, lobjOid, 1000, 1000);
<p>
printf("overwriting bytes 1000-2000 of the large object with X's0);
overwrite(conn, lobjOid, 1000, 1000);
*/
<p>
printf("exporting large object to file
/* exportFile(conn, lobjOid, out_filename); */
lo_export(conn, lobjOid,out_filename);
<p>
res = PQexec(conn, "end");
PQclear(res);
PQfinish(conn);
exit(0);
}
</pre>
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