Interfaces

PostgreSQL
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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, PostgreSQL has a new set of functions providing a more coherent interface.

Note: All large object manipulation must take place within an SQL transaction. This requirement is strictly enforced as of Postgres v6.5, though it has been an implicit requirement in previous versions, resulting in misbehavior if ignored.

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 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. mode is a bitmask describing several different attributes of the new object. The symbolic constants listed here are defined in $PGROOT/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, const char *filename)

filename 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, const char *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 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, 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.

Reading Data from a Large Object

The routine

int lo_read(PGconn *conn, int fd, char *buf, size_t len)

reads len bytes from large object fd into byf. 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.

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 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.

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