large object BLOBlarge object In PostgreSQL releases prior to 7.1, the size of any row in the database could not exceed the size of a data page. Since the size of a data page is 8192 bytes (the default, which can be raised up to 32768), the upper limit on the size of a data value was relatively low. To support the storage of larger atomic values, PostgreSQL provided and continues to provide a large object interface. This interface provides file-oriented access to user data that has been declared to be a large object. POSTGRES 4.2, the indirect predecessor of PostgreSQL, supported three standard implementations of large objects: as files external to the POSTGRES server, as external files managed by the POSTGRES server, and as data stored within the POSTGRES database. This caused considerable confusion among users. As a result, only support for large objects as data stored within the database is retained in PostgreSQL. Even though this is slower to access, it provides stricter data integrity. For historical reasons, this storage scheme is referred to as Inversion large objects. (You will see the term Inversion used occasionally to mean the same thing as large object.) Since PostgreSQL 7.1, all large objects are placed in one system table called pg_largeobject. TOAST sliced breadTOAST PostgreSQL 7.1 introduced a mechanism (nicknamed TOAST) that allows data rows to be much larger than individual data pages. This makes the large object interface partially obsolete. One remaining advantage of the large object interface is that it allows random access to the data, i.e., the ability to read or write small chunks of a large value. It is planned to equip TOAST with such functionality in the future. This section describes the implementation and the programming and query language interfaces to PostgreSQL large object data. We use the libpq C library for the examples in this section, but most programming interfaces native to PostgreSQL support equivalent functionality. Other interfaces may use the large object interface internally to provide generic support for large values. This is not described here. The 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 PostgreSQL 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, PostgreSQL has a new set of functions providing a more coherent interface. All large object manipulation must take place within an SQL transaction. This requirement is strictly enforced as of PostgreSQL 6.5, though it has been an implicit requirement in previous versions, resulting in misbehavior if ignored. The PostgreSQL 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 library. PostgreSQL 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. mode is a bit mask describing several different attributes of the new object. The symbolic constants listed here are defined in the header file libpq/libpq-fs.h. The access type (read, write, or both) is controlled by or'ing together the bits INV_READ and INV_WRITE. The low-order sixteen bits of the mask have historically been used at Berkeley to designate the storage manager number on which the large object should reside. These bits should always be zero now. The commands below create a large object: inv_oid = lo_creat(INV_READ|INV_WRITE); To import an operating system file as a large object, call Oid lo_import(PGconn *conn, const char *filename) filename specifies the operating system name of the file to be imported as a large object. To export a large object into an operating system file, call int lo_export(PGconn *conn, Oid lobjId, const char *filename) The lobjId argument specifies the OID of the large object to export and the filename argument specifies the operating system name name 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, const char *buf, size_t 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. The routine int lo_read(PGconn *conn, int fd, char *buf, size_t len) reads len bytes from large object fd into buf. The fd argument must have been returned by a previous lo_open. The number of bytes actually read 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 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. To remove a large object from the database, call int lo_unlink(PGconn *conn, Oid lobjId) The lobjId argument specifies the OID of the large object to remove. In the event of an 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 their 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'; 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 reader's benefit. This program can be found in src/test/examples/testlo.c in the source distribution. 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. /*-------------------------------------------------------------- * * testlo.c-- * test using large objects with libpq * * Copyright (c) 1994, Regents of the University of California * *-------------------------------------------------------------- */ #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\n"); 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 large object\n"); } (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\n", 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; } free(buf); 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\n", 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; } free(buf); 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\n", 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\n", 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\n", 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("as 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); }