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 Q 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 Q.

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/postgres95/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

Exporting a Large Object

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

Frontend applications which use the large object interface in P should include the header file libpq/libpq-fs.h and link with the libpq library.

Sample Program

#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");
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, "end");
PQclear(res);
PQfinish(conn);
exit(0);
}

Table of Contents

• DESCRIPTION
• Historical Note
• Inversion Large Objects
• Large Object Interfaces
• Creating a Large Object
• Importing a Large Object
• Exporting a Large Object
• Opening an Existing Large Object
• Writing Data to a Large Object
• Seeking on a Large Object
• Closing a Large Object Descriptor
• Built in registered functions
• Accessing Large Objects from LIBPQ
• Sample Program