lobj.sgml

PostgreSQL7.4.6 for Linux

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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