993/08/23 09:03:16 aoki Exp $
DESCRIPTION
In , 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, 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 large object data.
Originally, postgres 4.2 supports three standard implementations of large objects: as files external to , as files managed by , and as data stored within the database. It causes considerable confusion among users. As a result, we only support large objects as data stored within the database in . 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 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, 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 large object interface is modeled after the file system interface, with analogues of 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 existed that stored photographs of faces, then a function called could be declared on data. 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 function.
Large objects may be accessed from dynamically-loaded C functions or database client programs that link the library.
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 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 ing together the bits and 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 bit should be set. The low-order sixteen bits of 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 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 argument specifies the Oid of the large object to export and the 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 argument specifies the Oid of the large object to open. The mode bits control whether the object is opened for reading writing INV_WRITE ), ( or both.
A large object cannot be opened before it is created. returns a large object descriptor for later use in and
The routine int lo_write(PGconn *conn, int fd, char *buf, int len) writes bytes from to large object The argument must have been returned by a previous
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 to the new location specified by The valid values for .i whence are and
A large object may be closed by calling int lo_close(PGconn *conn, int fd) where is a large object descriptor returned by On success, returns zero. On error, the return value is negative.
There are two built-in registered functions, and 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 should include the header file and link with the library. /*-------------------------------------------------------------------------
*
* testlo.c--
* test using large objects with libpq
*
* Copyright (c) 1994, Regents of the University of California
*
*
* IDENTIFICATION
* $Header: /usr/local/devel/pglite/cvs/src/doc/ref/large_objects.3pqsrc,v 1.1 1996/02/24 01:16:24 jolly 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
}
/*
* 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
}
}
(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] = ' ';
fprintf(stderr,">>> %s", buf);
nread += nbytes;
}
fprintf(stderr,"0);
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] = ' ';
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); }
/*
* 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
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
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_filename0,
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.0, database);
fprintf(stderr,"%s",PQerrorMessage(conn));
exit_nicely(conn);
}
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);
printf("picking out bytes 1000-2000 of the large object0);
pickout(conn, lobjOid, 1000, 1000);
printf("overwriting bytes 1000-2000 of the large object with X's0);
overwrite(conn, lobjOid, 1000, 1000); */
printf("exporting large object to file /* exportFile(conn, lobjOid, out_filename); */
lo_export(conn, lobjOid,out_filename);
res = PQexec(conn, "end");
PQclear(res);
PQfinish(conn);
exit(0); }