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$Header: /cvsroot/pgsql/doc/src/sgml/lobj.sgml,v 1.26 2002/01/20 22:19:56 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</></>
<sect1 id="lo-intro">
<title>Introduction</title>
<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 has been
declared to be a large object.
</para>
<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 random access to the data, i.e., the ability to read or
write small chunks of a large value. It is planned to equip
<acronym>TOAST</acronym> with such functionality in the future.
</para>
<para>
This section 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 section, 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>
<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
tuples 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>Interfaces</title>
<para>
The facilities <productname>PostgreSQL</productname> 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 <productname>POSTGRES 4.2</productname>,
<productname>PostgreSQL</productname> has a new set of
functions providing a more coherent interface.
<note>
<para>
All large object manipulation <emphasis>must</emphasis> take
place within an SQL transaction. 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.
</para>
</note>
</para>
<para>
The <productname>PostgreSQL</productname> large object interface is modeled after
the <acronym>Unix</acronym> file-system interface, with analogues of
<function>open(2)</function>, <function>read(2)</function>,
<function>write(2)</function>,
<function>lseek(2)</function>, 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 <type>mugshot</type> existed that stored
photographs of faces, then a function called <function>beard</function> could
be declared on <type>mugshot</type> data. <function>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 <function>beard</function> function.
Large objects may be accessed from dynamically-loaded <acronym>C</acronym>
functions or database client programs that link the
library. <productname>PostgreSQL</productname> provides a set of routines that
support opening, reading, writing, closing, and seeking on
large objects.
</para>
<sect2>
<title>Creating a Large Object</title>
<para>
The routine
<synopsis>
Oid lo_creat(PGconn *<replaceable class="parameter">conn</replaceable>, int <replaceable class="parameter">mode</replaceable>)
</synopsis>
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 commands below create a large object:
<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 *<replaceable class="parameter">conn</replaceable>, const char *<replaceable class="parameter">filename</replaceable>)
</synopsis>
<replaceable class="parameter">filename</replaceable>
specifies the operating system name of
the file to be imported as a 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 *<replaceable class="parameter">conn</replaceable>, Oid <replaceable class="parameter">lobjId</replaceable>, const char *<replaceable class="parameter">filename</replaceable>)
</synopsis>
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>
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>.
</para>
</sect2>
<sect2>
<title>Writing Data to a Large Object</title>
<para>
The routine
<programlisting>
int lo_write(PGconn *conn, int fd, const char *buf, size_t len)
</programlisting>
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 routine
<programlisting>
int lo_read(PGconn *conn, int fd, char *buf, size_t len)
</programlisting>
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
<programlisting>
int lo_lseek(PGconn *conn, int fd, int offset, int whence)
</programlisting>
This routine 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</>, <symbol>SEEK_CUR</>, and <symbol>SEEK_END</>.
</para>
</sect2>
<sect2>
<title>Closing a Large Object Descriptor</title>
<para>
A large object may be closed by calling
<programlisting>
int lo_close(PGconn *conn, int fd)
</programlisting>
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>
</sect2>
<sect2>
<title>Removing a Large Object</title>
<para>
To remove a large object from the database, call
<synopsis>
Oid lo_unlink(PGconn *<replaceable class="parameter">conn</replaceable>, Oid lobjId)
</synopsis>
The <parameter>lobjId</parameter> argument specifies the OID of the large
object to remove.
</para>
</sect2>
</sect1>
<sect1 id="lo-funcs">
<title>Server-side Built-in Functions</title>
<para>
There are two built-in registered functions, <function>lo_import</function>
and <function>lo_export</function> which are convenient for use
in <acronym>SQL</acronym>
queries.
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>
</sect1>
<sect1 id="lo-libpq">
<title>Accessing Large Objects from <application>Libpq</application></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 be found in
<filename>src/test/examples/testlo.c</filename> in the source distribution.
Frontend 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>
<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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