lobj.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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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>
    <productname>PostgreSQL</productname> has a <firstterm>large object</>
    facility, which provides stream-style access to user data that is stored
    in a special large-object structure.  Streaming access is useful
    when working with data values that are too large to manipulate
    conveniently as a whole.
   </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</primary>
     <secondary>versus large objects</secondary>
    </indexterm>
    <productname>PostgreSQL</productname> 7.1 introduced a mechanism
    (nicknamed <quote><acronym>TOAST</acronym></quote>) that allows
    data values to be much larger than single pages.  This
    makes the large object facility partially obsolete.  One
    remaining advantage of the large object facility is that it allows values
    up to 2 GB in size, whereas <acronym>TOAST</acronym>ed fields can be at
    most 1 GB.  Also, large objects can be manipulated piece-by-piece much more
    easily than ordinary data fields, so the practical limits are considerably
    different.
   </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.  
     The return value is the OID that was assigned to the new large object,
     or InvalidOid (zero) on failure.

     <replaceable class="parameter">mode</replaceable> is unused and
     ignored as of <productname>PostgreSQL</productname> 8.1; however, for
     backwards compatibility with earlier releases it is best to
     set it to <symbol>INV_READ</symbol>, <symbol>INV_WRITE</symbol>,
     or <symbol>INV_READ</symbol> <literal>|</> <symbol>INV_WRITE</symbol>.
     (These symbolic constants are defined
     in the header file <filename>libpq/libpq-fs.h</filename>.)
    </para>

    <para>
     An example:
<programlisting>
inv_oid = lo_creat(conn, INV_READ|INV_WRITE);
</programlisting>
    </para>

    <para>
     The function
<synopsis>
Oid lo_create(PGconn *conn, Oid lobjId);
</synopsis>
     <indexterm><primary>lo_create</></>
     also creates a new large object.  The OID to be assigned can be
     specified by <replaceable class="parameter">lobjId</replaceable>;
     if so, failure occurs if that OID is already in use for some large
     object.  If <replaceable class="parameter">lobjId</replaceable>
     is InvalidOid (zero) then <function>lo_create</> assigns an unused
     OID (this is the same behavior as <function>lo_creat</>).
     The return value is the OID that was assigned to the new large object,
     or InvalidOid (zero) on failure.
    </para>

    <para>
     <function>lo_create</> is new as of <productname>PostgreSQL</productname>
     8.1; if this function is run against an older server version, it will
     fail and return InvalidOid.
    </para>

    <para>
     An example:
<programlisting>
inv_oid = lo_create(conn, desired_oid);
</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,
     or InvalidOid (zero) on failure.
     Note that the file is read by the client interface library, not by
     the server; so it must exist in the client filesystem and be readable
     by the client application.
    </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 of the file.  Note that the file is
     written by the client interface library, not by the server.  Returns 1
     on success, -1 on failure.
    </para>
   </sect2>

   <sect2>
    <title>Opening an Existing Large Object</title>

    <para>
     To open an existing large object for reading or writing, 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.
     (These symbolic constants are defined
     in the header file <filename>libpq/libpq-fs.h</filename>.)
     A large object cannot be opened before it is created.
     <function>lo_open</function> returns a (non-negative) 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.
     On failure, -1 is returned.
    </para>

    <para>
     The server currently does not distinguish between modes
     <symbol>INV_WRITE</symbol> and <symbol>INV_READ</> <literal>|</>
     <symbol>INV_WRITE</symbol>: you are allowed to read from the descriptor
     in either case.  However there is a significant difference between
     these modes and <symbol>INV_READ</> alone: with <symbol>INV_READ</>
     you cannot write on the descriptor, and the data read from it will
     reflect the contents of the large object at the time of the transaction
     snapshot that was active when <function>lo_open</> was executed,
     regardless of later writes by this or other transactions.  Reading
     from a descriptor opened with <symbol>INV_WRITE</symbol> returns
     data that reflects all writes of other committed transactions as well
     as writes of the current transaction.  This is similar to the behavior
     of <literal>SERIALIZABLE</> versus <literal>READ COMMITTED</> transaction
     modes for ordinary SQL <command>SELECT</> commands.
    </para>

    <para>
     An example:
<programlisting>
inv_fd = lo_open(conn, inv_oid, INV_READ|INV_WRITE);
</programlisting>
    </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 descriptor <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 descriptor
     <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 in a Large Object</title>

<para>
     To change the current read or write location associated with a
     large object descriptor, 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 descriptor identified 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, or -1 on error.
</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 descriptor,
     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 descriptor 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>