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postgres/doc/src/sgml/lobj.sgml
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$Header: /cvsroot/pgsql/doc/src/sgml/lobj.sgml,v 1.30 2003/08/31 17:32:19 petere Exp $
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<chapter id="largeObjects">
<title id="largeObjects-title">Large Objects</title>
<indexterm zone="largeobjects"><primary>large object</></>
<indexterm><primary>BLOB</><see>large object</></>
<para>
In <productname>PostgreSQL</productname> 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, <productname>PostgreSQL</productname>
provided and continues to provide a large object interface. This
interface provides file-oriented access to user data that is stored in
a special large-object structure.
</para>
<para>
This chapter describes the implementation and the programming and
query language interfaces to <productname>PostgreSQL</productname>
large object data. We use the <application>libpq</application> C
library for the examples in this chapter, but most programming
interfaces native to <productname>PostgreSQL</productname> support
equivalent functionality. Other interfaces may use the large
object interface internally to provide generic support for large
values. This is not described here.
</para>
<sect1 id="lo-history">
<title>History</title>
<para>
<productname>POSTGRES 4.2</productname>, the indirect predecessor
of <productname>PostgreSQL</productname>, supported three standard
implementations of large objects: as files external to the
<productname>POSTGRES</productname> server, as external files
managed by the <productname>POSTGRES</productname> server, and as
data stored within the <productname>POSTGRES</productname>
database. This caused considerable confusion among users. As a
result, only support for large objects as data stored within the
database is retained in <productname>PostgreSQL</productname>.
Even though this is slower to access, it provides stricter data
integrity. For historical reasons, this storage scheme is
referred to as <firstterm>Inversion large
objects</firstterm>. (You will see the term Inversion used
occasionally to mean the same thing as large object.) Since
<productname>PostgreSQL 7.1</productname>, all large objects are
placed in one system table called
<classname>pg_largeobject</classname>.
</para>
<para>
<indexterm><primary>TOAST</></>
<indexterm><primary>sliced bread</><see>TOAST</></indexterm>
<productname>PostgreSQL 7.1</productname> introduced a mechanism
(nicknamed <quote><acronym>TOAST</acronym></quote>) 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 values up
to 2 GB in size, whereas <acronym>TOAST</acronym> can only handle 1 GB.
</para>
</sect1>
<sect1 id="lo-implementation">
<title>Implementation Features</title>
<para>
The large object implementation breaks large
objects up into <quote>chunks</quote> and stores the chunks in
rows in the database. A B-tree index guarantees fast
searches for the correct chunk number when doing random
access reads and writes.
</para>
</sect1>
<sect1 id="lo-interfaces">
<title>Client Interfaces</title>
<para>
This section describes the facilities that
<productname>PostgreSQL</productname> client interface libraries
provide for accessing large objects. All large object
manipulation using these functions <emphasis>must</emphasis> take
place within an SQL transaction block. (This requirement is
strictly enforced as of <productname>PostgreSQL 6.5</>, though it
has been an implicit requirement in previous versions, resulting
in misbehavior if ignored.)
The <productname>PostgreSQL</productname> large object interface is modeled after
the <acronym>Unix</acronym> file-system interface, with analogues of
<function>open</function>, <function>read</function>,
<function>write</function>,
<function>lseek</function>, etc.
</para>
<para>
Client applications which use the large object interface in
<application>libpq</application> should include the header file
<filename>libpq/libpq-fs.h</filename> and link with the
<application>libpq</application> library.
</para>
<sect2>
<title>Creating a Large Object</title>
<para>
The function
<synopsis>
Oid lo_creat(PGconn *conn, int mode);
</synopsis>
<indexterm><primary>lo_creat</></>
creates a new large object.
<replaceable class="parameter">mode</replaceable> is a bit mask
describing several different attributes of the new
object. The symbolic constants listed here are defined
in the header file <filename>libpq/libpq-fs.h</filename>.
The access type (read, write, or both) is controlled by
or'ing together the bits <symbol>INV_READ</symbol> and
<symbol>INV_WRITE</symbol>. 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 return value is the OID that was assigned to the new large object.
</para>
<para>
An example:
<programlisting>
inv_oid = lo_creat(INV_READ|INV_WRITE);
</programlisting>
</para>
</sect2>
<sect2>
<title>Importing a Large Object</title>
<para>
To import an operating system file as a large object, call
<synopsis>
Oid lo_import(PGconn *conn, const char *filename);
</synopsis>
<indexterm><primary>lo_import</></>
<replaceable class="parameter">filename</replaceable>
specifies the operating system name of
the file to be imported as a large object.
The return value is the OID that was assigned to the new large object.
</para>
</sect2>
<sect2>
<title>Exporting a Large Object</title>
<para>
To export a large object
into an operating system file, call
<synopsis>
int lo_export(PGconn *conn, Oid lobjId, const char *filename);
</synopsis>
<indexterm><primary>lo_export</></>
The <parameter>lobjId</parameter> argument specifies the OID of the large
object to export and the <parameter>filename</parameter> argument specifies
the operating system name name of the file.
</para>
</sect2>
<sect2>
<title>Opening an Existing Large Object</title>
<para>
To open an existing large object, call
<synopsis>
int lo_open(PGconn *conn, Oid lobjId, int mode);
</synopsis>
<indexterm><primary>lo_open</></>
The <parameter>lobjId</parameter> argument specifies the OID of the large
object to open. The <parameter>mode</parameter> bits control whether the
object is opened for reading (<symbol>INV_READ</>), writing (<symbol>INV_WRITE</symbol>), or
both.
A large object cannot be opened before it is created.
<function>lo_open</function> returns a large object descriptor
for later use in <function>lo_read</function>, <function>lo_write</function>,
<function>lo_lseek</function>, <function>lo_tell</function>, and
<function>lo_close</function>. The descriptor is only valid for
the duration of the current transaction.
</para>
</sect2>
<sect2>
<title>Writing Data to a Large Object</title>
<para>
The function
<synopsis>
int lo_write(PGconn *conn, int fd, const char *buf, size_t len);
</synopsis>
<indexterm><primary>lo_write</></> writes
<parameter>len</parameter> bytes from <parameter>buf</parameter>
to large object <parameter>fd</>. The <parameter>fd</parameter>
argument must have been returned by a previous
<function>lo_open</function>. The number of bytes actually
written is returned. In the event of an error, the return value
is negative.
</para>
</sect2>
<sect2>
<title>Reading Data from a Large Object</title>
<para>
The function
<synopsis>
int lo_read(PGconn *conn, int fd, char *buf, size_t len);
</synopsis>
<indexterm><primary>lo_read</></> reads
<parameter>len</parameter> bytes from large object
<parameter>fd</parameter> into <parameter>buf</parameter>. The
<parameter>fd</parameter> argument must have been returned by a
previous <function>lo_open</function>. The number of bytes
actually read is returned. In the event of an error, the return
value is negative.
</para>
</sect2>
<sect2>
<title>Seeking on a Large Object</title>
<para>
To change the current read or write location on a large
object, call
<synopsis>
int lo_lseek(PGconn *conn, int fd, int offset, int whence);
</synopsis>
<indexterm><primary>lo_lseek</></> This function moves the
current location pointer for the large object described by
<parameter>fd</> to the new location specified by
<parameter>offset</>. The valid values for <parameter>whence</>
are <symbol>SEEK_SET</> (seek from object start),
<symbol>SEEK_CUR</> (seek from current position), and
<symbol>SEEK_END</> (seek from object end). The return value is
the new location pointer.
</para>
</sect2>
<sect2>
<title>Obtaining the Seek Position of a Large Object</title>
<para>
To obtain the current read or write location of a large object,
call
<synopsis>
int lo_tell(PGconn *conn, int fd);
</synopsis>
<indexterm><primary>lo_tell</></> If there is an error, the
return value is negative.
</para>
</sect2>
<sect2>
<title>Closing a Large Object Descriptor</title>
<para>
A large object may be closed by calling
<synopsis>
int lo_close(PGconn *conn, int fd);
</synopsis>
<indexterm><primary>lo_close</></> where <parameter>fd</> is a
large object descriptor returned by <function>lo_open</function>.
On success, <function>lo_close</function> returns zero. On
error, the return value is negative.
</para>
<para>
Any large object descriptors that remain open at the end of a
transaction will be closed automatically.
</para>
</sect2>
<sect2>
<title>Removing a Large Object</title>
<para>
To remove a large object from the database, call
<synopsis>
int lo_unlink(PGconn *conn, Oid lobjId);
</synopsis>
<indexterm><primary>lo_unlink</></> The
<parameter>lobjId</parameter> argument specifies the OID of the
large object to remove. In the event of an error, the return
value is negative.
</para>
</sect2>
</sect1>
<sect1 id="lo-funcs">
<title>Server-side Functions</title>
<para>
There are two built-in server-side functions,
<function>lo_import</function><indexterm><primary>lo_import</></>
and
<function>lo_export</function>,<indexterm><primary>lo_export</></>
for large object access, which are available for use in
<acronym>SQL</acronym> commands. Here is an example of their
use:
<programlisting>
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';
</programlisting>
</para>
<para>
These functions read and write files in the server's filesystem, using the
permissions of the database's owning user. Therefore, their use is restricted
to superusers. (In contrast, the client-side import and export functions
read and write files in the client's filesystem, using the permissions of
the client program. Their use is not restricted.)
</para>
</sect1>
<sect1 id="lo-examplesect">
<title>Example Program</title>
<para>
<xref linkend="lo-example"> is a sample program which shows how the large object
interface
in <application>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 also be found in
<filename>src/test/examples/testlo.c</filename> in the source distribution.
</para>
<example id="lo-example">
<title>Large Objects with <application>libpq</application> Example Program</title>
<programlisting>
/*--------------------------------------------------------------
*
* testlo.c--
* test using large objects with libpq
*
* Copyright (c) 1994, Regents of the University of California
*
*--------------------------------------------------------------
*/
#include &lt;stdio.h&gt;
#include &quot;libpq-fe.h&quot;
#include &quot;libpq/libpq-fs.h&quot;
#define BUFSIZE 1024
/*
* importFile
* import file &quot;in_filename&quot; into database as large object &quot;lobjOid&quot;
*
*/
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 &lt; 0)
{ /* error */
fprintf(stderr, &quot;can't open unix file %s\n&quot;, filename);
}
/*
* create the large object
*/
lobjId = lo_creat(conn, INV_READ | INV_WRITE);
if (lobjId == 0)
fprintf(stderr, &quot;can't create large object\n&quot;);
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)) &gt; 0)
{
tmp = lo_write(conn, lobj_fd, buf, nbytes);
if (tmp &lt; nbytes)
fprintf(stderr, &quot;error while reading large object\n&quot;);
}
(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 &lt; 0)
{
fprintf(stderr, &quot;can't open large object %d\n&quot;,
lobjId);
}
lo_lseek(conn, lobj_fd, start, SEEK_SET);
buf = malloc(len + 1);
nread = 0;
while (len - nread &gt; 0)
{
nbytes = lo_read(conn, lobj_fd, buf, len - nread);
buf[nbytes] = ' ';
fprintf(stderr, &quot;&gt;&gt;&gt; %s&quot;, buf);
nread += nbytes;
}
free(buf);
fprintf(stderr, &quot;\n&quot;);
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 &lt; 0)
{
fprintf(stderr, &quot;can't open large object %d\n&quot;,
lobjId);
}
lo_lseek(conn, lobj_fd, start, SEEK_SET);
buf = malloc(len + 1);
for (i = 0; i &lt; len; i++)
buf[i] = 'X';
buf[i] = ' ';
nwritten = 0;
while (len - nwritten &gt; 0)
{
nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
nwritten += nbytes;
}
free(buf);
fprintf(stderr, &quot;\n&quot;);
lo_close(conn, lobj_fd);
}
/*
* exportFile * export large object &quot;lobjOid&quot; to file &quot;out_filename&quot;
*
*/
void
exportFile(PGconn *conn, Oid lobjId, char *filename)
{
int lobj_fd;
char buf[BUFSIZE];
int nbytes,
tmp;
int fd;
/*
* create an inversion &quot;object&quot;
*/
lobj_fd = lo_open(conn, lobjId, INV_READ);
if (lobj_fd &lt; 0)
{
fprintf(stderr, &quot;can't open large object %d\n&quot;,
lobjId);
}
/*
* open the file to be written to
*/
fd = open(filename, O_CREAT | O_WRONLY, 0666);
if (fd &lt; 0)
{ /* error */
fprintf(stderr, &quot;can't open unix file %s\n&quot;,
filename);
}
/*
* read in from the Unix file and write to the inversion file
*/
while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) &gt; 0)
{
tmp = write(fd, buf, nbytes);
if (tmp &lt; nbytes)
{
fprintf(stderr, &quot;error while writing %s\n&quot;,
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, &quot;Usage: %s database_name in_filename out_filename\n&quot;,
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, &quot;Connection to database '%s' failed.\n&quot;, database);
fprintf(stderr, &quot;%s&quot;, PQerrorMessage(conn));
exit_nicely(conn);
}
res = PQexec(conn, &quot;begin&quot;);
PQclear(res);
printf(&quot;importing file %s\n&quot;, in_filename);
/* lobjOid = importFile(conn, in_filename); */
lobjOid = lo_import(conn, in_filename);
/*
printf(&quot;as large object %d.\n&quot;, lobjOid);
printf(&quot;picking out bytes 1000-2000 of the large object\n&quot;);
pickout(conn, lobjOid, 1000, 1000);
printf(&quot;overwriting bytes 1000-2000 of the large object with X's\n&quot;);
overwrite(conn, lobjOid, 1000, 1000);
*/
printf(&quot;exporting large object to file %s\n&quot;, out_filename);
/* exportFile(conn, lobjOid, out_filename); */
lo_export(conn, lobjOid, out_filename);
res = PQexec(conn, &quot;end&quot;);
PQclear(res);
PQfinish(conn);
exit(0);
}
</programlisting>
</example>
</sect1>
</chapter>
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