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large_objects(3) |
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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.
Originally, postgres 4.2 supports three standard implementations 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 confusion 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.)
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.
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 postgres 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 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 Libpq library. Postgres provides a set of routines that support opening, reading, writing, closing, and seeking on large objects.
The routine
Oid lo_creat(PGconn *conn, int mode)
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
/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);
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.
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.
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 .
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.
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.
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.
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’;
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 program can be found in
../src/test/examples
Frontend
applications which use the large object interface in .sp 40u
should include the header file libpq/libpq-fs.h and
link with the libpq library.
/*-------------------------------------------------------------------------
*
* 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"
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#define BUFSIZE | |
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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 */
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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) {
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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) {
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tmp = lo_write(conn, lobj_fd, buf, nbytes); | |
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if (tmp < nbytes) { | |
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fprintf(stderr, "error while reading \"%s\"", filename); | |
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} |
}
(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) {
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fprintf(stderr,"can’t open large object %d", | |
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lobjId); |
}
lo_lseek(conn,
lobj_fd, start, SEEK_SET);
buf = malloc(len+1);
nread = 0;
while (len - nread > 0) {
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nbytes = lo_read(conn, lobj_fd, buf, len - nread); | |
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buf[nbytes] = ’\0’; | |
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fprintf(stderr,">>> %s", buf); | |
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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) {
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fprintf(stderr,"can’t open large object %d", | |
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lobjId); |
}
lo_lseek(conn,
lobj_fd, start, SEEK_SET);
buf = malloc(len+1);
for (i=0;i<len;i++)
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buf[i] = ’X’; |
buf[i] = ’\0’;
nwritten = 0;
while (len - nwritten > 0) {
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nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten); | |
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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) {
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fprintf(stderr,"can’t open large object %d", | |
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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
*/
}
(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;
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fprintf(stderr, "Usage: %s database_name in_filename out_filename\n", | ||
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argv[0]); | ||
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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 */
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fprintf(stderr,"Connection to database ’%s’ failed.\n", database); | ||
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fprintf(stderr,"%s",PQerrorMessage(conn)); | ||
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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);
}
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large_objects(3) | |