lobj.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/lobj.sgml,v 1.21 2001/09/15 16:08:59 petere Exp $
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 <chapter id="largeObjects">
  <title id="largeObjects-title">Large Objects</title>

  <para>
   In <productname>Postgres</productname>,
   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, 
   <productname>Postgres</productname> 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 
   programming and query  language  interfaces  to 
   <productname>Postgres</productname>
   large object data.
  </para>

  <sect1 id="lo-history">
   <title>Historical Note</title>

   <para>
    Originally, <productname>Postgres 4.2</productname> supported three standard 
    implementations of large objects: as files external 
    to <productname>Postgres</productname>,  as
    external files managed by <productname>Postgres</productname>, and as data
    stored within the <productname>Postgres</productname> database. It causes  
    considerable confusion among users. As a result, we only 
    support large objects as data stored within  the  <productname>Postgres</productname>
    database  in  <productname>PostgreSQL</productname>.  Even  though it is slower to
    access, it provides stricter data  integrity.
    For historical reasons, this storage scheme is referred to as 
    Inversion large objects. (We will use  Inversion  and  large
    objects  interchangeably to mean the same thing in this
    section.)
    Since <productname>PostgreSQL 7.1</productname> all large objects are placed in
    one system table called <classname>pg_largeobject</classname>.
   </para>
  </sect1>

  <sect1 id="lo-implementation">
   <title>Implementation Features</title>

   <para>
    The Inversion 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>Postgres</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 Postgres 6.5, though it has been an
      implicit requirement in previous versions, resulting in
      misbehavior if ignored.
     </para>
    </note>
   </para>

   <para>
    The  <productname>Postgres</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 beard could
    be declared on <type>mugshot</type> 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 <acronym>C</acronym>
    functions  or  database  client  programs that link the
    library.  <productname>Postgres</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
     <filename>$<envar>PGROOT</envar>/src/backend/libpq/libpq-fs.h</filename>
     The access type (read, write, or both) is controlled by
     OR'ing together the bits <acronym>INV_READ</acronym>  and
     <acronym>INV_WRITE</acronym>.  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:
     <programlisting>
inv_oid = lo_creat(INV_READ|INV_WRITE);
     </programlisting>
    </para>
   </sect2>

   <sect2>
    <title>Importing a Large Object</title>

    <para>
     To import a <acronym>UNIX</acronym> 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  <acronym>Unix</acronym>  path 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 <acronym>UNIX</acronym> 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 <acronym>UNIX</acronym> path 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  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 fd to the new location specified 
     by offset.  The valid values for whence are
     SEEK_SET, SEEK_CUR, and 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  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, <acronym>lo_import</acronym>
     and <acronym>lo_export</acronym> 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>
     Below 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  readers
     benefit.  This program can be found in
<filename>
../src/test/examples
</filename>
     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>
</sect1>

<sect1 id="lo-example">
<title>Example Program</title>

  <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
 *
 *
 * IDENTIFICATION
 *    /usr/local/devel/pglite/cvs/src/doc/manual.me,v 1.16 1995/09/01 23:55:00 jolly Exp
 *
 *--------------------------------------------------------------
 */
#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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