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Commitd3215af

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Update intro in face of TOAST.
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‎doc/src/sgml/lobj.sgml‎

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<!--
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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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$Header: /cvsroot/pgsql/doc/src/sgml/lobj.sgml,v 1.22 2001/09/16 22:53:52 petere Exp $
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-->
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<chapter id="largeObjects">
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<title id="largeObjects-title">Large Objects</title>
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<para>
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In <productname>Postgres</productname>,
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data values are stored in tuples and
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individual tuples cannot span data pages. Since the size of
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a data page is 8192 bytes, the upper limit on the size
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of a data value is relatively low. To support the storage
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of larger atomic values,
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<productname>Postgres</productname> provides a large
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object interface. This interface provides file
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oriented access to user data that has been declared to
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be a large type.
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This section describes the implementation and the
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programming and query language interfaces to
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<productname>Postgres</productname>
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large object data.
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</para>
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<sect1 id="lo-history">
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<title>Historical Note</title>
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<sect1 id="lo-intro">
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<title>Introduction</title>
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<para>
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Originally, <productname>Postgres 4.2</productname> supported three standard
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implementations of large objects: as files external
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to <productname>Postgres</productname>, as
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external files managed by <productname>Postgres</productname>, and as data
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stored within the <productname>Postgres</productname> database. It causes
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considerable confusion among users. As a result, we only
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support large objects as data stored within the <productname>Postgres</productname>
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database in <productname>PostgreSQL</productname>. Even though it is slower to
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access, it provides stricter data integrity.
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For historical reasons, this storage scheme is referred to as
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Inversion large objects. (We will use Inversion and large
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objects interchangeably to mean the same thing in this
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section.)
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Since <productname>PostgreSQL 7.1</productname> all large objects are placed in
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one system table called <classname>pg_largeobject</classname>.
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In <productname>PostgreSQL</productname> releases prior to 7.1,
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the size of any row in the database could not exceed the size of a
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data page. Since the size of a data page is 8192 bytes (the
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default, which can be raised up to 32768), the upper limit on the
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size of a data value was relatively low. To support the storage of
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larger atomic values, <productname>PostgreSQL</productname>
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provided and continues to provide a large object interface. This
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interface provides file-oriented access to user data that has been
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declared to be a large object.
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</para>
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<para>
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<productname>POSTGRES 4.2</productname>, the indirect predecessor
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of <productname>PostgreSQL</productname>, supported three standard
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implementations of large objects: as files external to the
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<productname>POSTGRES</productname> server, as external files
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managed by the <productname>POSTGRES</productname> server, and as
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data stored within the <productname>POSTGRES</productname>
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database. This caused considerable confusion among users. As a
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result, only support for large objects as data stored within the
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database is retained in <productname>PostgreSQL</productname>.
33+
Even though this is slower to access, it provides stricter data
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integrity. For historical reasons, this storage scheme is
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referred to as <firstterm>Inversion large
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objects</firstterm>. (You will see the term Inversion used
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occasionally to mean the same thing as large object.) Since
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<productname>PostgreSQL 7.1</productname>, all large objects are
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placed in one system table called
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<classname>pg_largeobject</classname>.
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</para>
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<para>
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<productname>PostgreSQL 7.1</productname> introduced a mechanism
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(nicknamed <quote><acronym>TOAST</acronym></quote>) that allows
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data rows to be much larger than individual data pages. This
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makes the large object interface partially obsolete. One
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remaining advantage of the large object interface is that it
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allows random access to the data, i.e., the ability to read or
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write small chunks of a large value. It is planned to equip
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<acronym>TOAST</acronym> with such functionality in the future.
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</para>
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<para>
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This section describes the implementation and the programming and
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query language interfaces to <productname>PostgreSQL</productname>
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large object data. We use the <application>libpq</application> C
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library for the examples in this section, but most programming
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interfaces native to <productname>PostgreSQL</productname> support
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equivalent functionality. Other interfaces may use the large
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object interface internally to provide generic support for large
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values. This is not described here.
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</para>
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</sect1>
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<sect1 id="lo-implementation">
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<title>Implementation Features</title>
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<para>
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TheInversionlarge object implementation breaks large
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The large object implementation breaks large
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objects up into <quote>chunks</quote> and stores the chunks in
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tuples in the database. A B-tree index guarantees fast
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searches for the correct chunk number when doing random
@@ -60,27 +80,27 @@ $Header: /cvsroot/pgsql/doc/src/sgml/lobj.sgml,v 1.21 2001/09/15 16:08:59 petere
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<title>Interfaces</title>
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<para>
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The facilities <productname>Postgres</productname> provides to
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The facilities <productname>PostgreSQL</productname> provides to
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access large objects, both in the backend as part of user-defined
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functions or the front end as part of an application
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using the interface, are described below. For users
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familiar with <productname>Postgres 4.2</productname>,
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familiar with <productname>POSTGRES 4.2</productname>,
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<productname>PostgreSQL</productname> has a new set of
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functions providing a more coherent interface.
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<note>
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<para>
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All large object manipulation <emphasis>must</emphasis> take
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place within an SQL transaction. This requirement is strictly
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enforced as ofPostgres 6.5, though it has been an
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enforced as of<productname>PostgreSQL 6.5</>, though it has been an
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implicit requirement in previous versions, resulting in
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misbehavior if ignored.
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</para>
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</note>
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</para>
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<para>
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The <productname>Postgres</productname> large object interface is modeled after
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The <productname>PostgreSQL</productname> large object interface is modeled after
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the <acronym>Unix</acronym> file system interface, with analogues of
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<function>open(2)</function>, <function>read(2)</function>,
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<function>write(2)</function>,
@@ -96,7 +116,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/lobj.sgml,v 1.21 2001/09/15 16:08:59 petere
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examined, by the beard function.
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Large objects may be accessed from dynamically-loaded <acronym>C</acronym>
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functions or database client programs that link the
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library. <productname>Postgres</productname> provides a set of routines that
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library. <productname>PostgreSQL</productname> provides a set of routines that
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support opening, reading, writing, closing, and seeking on
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large objects.
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</para>
@@ -113,18 +133,17 @@ Oid lo_creat(PGconn *<replaceable class="parameter">conn</replaceable>, int <rep
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<replaceable class="parameter">mode</replaceable> is a bit mask
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describing several different attributes of the new
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object. The symbolic constants listed here are defined
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in
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<filename>$<envar>PGROOT</envar>/src/backend/libpq/libpq-fs.h</filename>
136+
in the header file <filename>libpq/libpq-fs.h</filename>.
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The access type (read, write, or both) is controlled by
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OR'ing together the bits <acronym>INV_READ</acronym> and
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<acronym>INV_WRITE</acronym>. The low-order sixteen bits of mask are
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the storage manager number on which the large object
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should reside.For sites other than Berkeley, these
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bits should always be zero.
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The commands below createan (Inversion) large object:
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<programlisting>
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OR'ing together the bits <symbol>INV_READ</symbol> and
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<symbol>INV_WRITE</symbol>. The low-order sixteen bits ofthemaskhave
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historically been used at Berkeley to designate the storage manager number on which the large object
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should reside.These
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bits should always be zero now.
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The commands below createa large object:
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<programlisting>
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inv_oid = lo_creat(INV_READ|INV_WRITE);
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</programlisting>
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</programlisting>
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</para>
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</sect2>
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