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>Chapter 28. Large Objects</H1
><DIV
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><DL
><DT
><B
>Table of Contents</B
></DT
><DT
>28.1. <A
HREF="largeobjects.html#LO-HISTORY"
>History</A
></DT
><DT
>28.2. <A
HREF="lo-implementation.html"
>Implementation Features</A
></DT
><DT
>28.3. <A
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>Client Interfaces</A
></DT
><DD
><DL
><DT
>28.3.1. <A
HREF="lo-interfaces.html#AEN24996"
>Creating a Large Object</A
></DT
><DT
>28.3.2. <A
HREF="lo-interfaces.html#AEN25008"
>Importing a Large Object</A
></DT
><DT
>28.3.3. <A
HREF="lo-interfaces.html#AEN25015"
>Exporting a Large Object</A
></DT
><DT
>28.3.4. <A
HREF="lo-interfaces.html#AEN25023"
>Opening an Existing Large Object</A
></DT
><DT
>28.3.5. <A
HREF="lo-interfaces.html#AEN25039"
>Writing Data to a Large Object</A
></DT
><DT
>28.3.6. <A
HREF="lo-interfaces.html#AEN25050"
>Reading Data from a Large Object</A
></DT
><DT
>28.3.7. <A
HREF="lo-interfaces.html#AEN25061"
>Seeking in a Large Object</A
></DT
><DT
>28.3.8. <A
HREF="lo-interfaces.html#AEN25073"
>Obtaining the Seek Position of a Large Object</A
></DT
><DT
>28.3.9. <A
HREF="lo-interfaces.html#AEN25079"
>Closing a Large Object Descriptor</A
></DT
><DT
>28.3.10. <A
HREF="lo-interfaces.html#AEN25089"
>Removing a Large Object</A
></DT
></DL
></DD
><DT
>28.4. <A
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>Server-Side Functions</A
></DT
><DT
>28.5. <A
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>Example Program</A
></DT
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><A
NAME="AEN24944"
></A
><A
NAME="AEN24946"
></A
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> has a <I
CLASS="FIRSTTERM"
>large object</I
>
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.
</P
><P
> This chapter describes the implementation and the programming and
query language interfaces to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
large object data. We use the <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> C
library for the examples in this chapter, but most programming
interfaces native to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> support
equivalent functionality. Other interfaces may use the large
object interface internally to provide generic support for large
values. This is not described here.
</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="LO-HISTORY"
>28.1. History</A
></H1
><P
> <SPAN
CLASS="PRODUCTNAME"
>POSTGRES 4.2</SPAN
>, the indirect predecessor
of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, supported three standard
implementations of large objects: as files external to the
<SPAN
CLASS="PRODUCTNAME"
>POSTGRES</SPAN
> server, as external files
managed by the <SPAN
CLASS="PRODUCTNAME"
>POSTGRES</SPAN
> server, and as
data stored within the <SPAN
CLASS="PRODUCTNAME"
>POSTGRES</SPAN
>
database. This caused considerable confusion among users. As a
result, only support for large objects as data stored within the
database is retained in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
Even though this is slower to access, it provides stricter data
integrity. For historical reasons, this storage scheme is
referred to as <I
CLASS="FIRSTTERM"
>Inversion large
objects</I
>. (You will see the term Inversion used
occasionally to mean the same thing as large object.) Since
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL 7.1</SPAN
>, all large objects are
placed in one system table called
<CODE
CLASS="CLASSNAME"
>pg_largeobject</CODE
>.
</P
><P
> <A
NAME="AEN24969"
></A
>
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 7.1 introduced a mechanism
(nicknamed <SPAN
CLASS="QUOTE"
>"<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>"</SPAN
>) 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
CLASS="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.
</P
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