---
name: "large_objects"
section: "3"
platform: "linux"
package: "postgresql-dev"
version: "6.3.2-15"
release: "Debian 2.1 (Slink)"
platforms: ["linux"]
url: "https://new.carta.tech/large_objects"
---

# large\_objects(3)

## DESCRIPTION

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.

## Historical Note

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.)

## 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  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.

## Creating a Large Object

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);
```

## 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 program can be found in

```
../src/test/examples
```

Frontend applications which use the large object interface in  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<len;i++)
	buf[i] = 'X';
    buf[i] = '\0';
    nwritten = 0;
    while (len - nwritten > 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);
}
```