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NAME="CATALOG-PG-LARGEOBJECT"
>47.29. <TT
CLASS="STRUCTNAME"
>pg_largeobject</TT
></A
></H1
><P
> The catalog <TT
CLASS="STRUCTNAME"
>pg_largeobject</TT
> holds the data making up
<SPAN
CLASS="QUOTE"
>"large objects"</SPAN
>. A large object is identified by an OID
assigned when it is created. Each large object is broken into
segments or <SPAN
CLASS="QUOTE"
>"pages"</SPAN
> small enough to be conveniently stored as rows
in <TT
CLASS="STRUCTNAME"
>pg_largeobject</TT
>.
The amount of data per page is defined to be <TT
CLASS="SYMBOL"
>LOBLKSIZE</TT
> (which is currently
<TT
CLASS="LITERAL"
>BLCKSZ/4</TT
>, or typically 2 kB).
</P
><P
> Prior to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.0, there was no permission structure
associated with large objects. As a result,
<TT
CLASS="STRUCTNAME"
>pg_largeobject</TT
> was publicly readable and could be
used to obtain the OIDs (and contents) of all large objects in the system.
This is no longer the case; use
<A
HREF="catalog-pg-largeobject-metadata.html"
><TT
CLASS="STRUCTNAME"
>pg_largeobject_metadata</TT
></A
>
to obtain a list of large object OIDs.
</P
><DIV
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><P
><B
>Table 47-29. <TT
CLASS="STRUCTNAME"
>pg_largeobject</TT
> Columns</B
></P
><TABLE
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><COL><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Type</TH
><TH
>References</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>loid</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-largeobject-metadata.html"
><TT
CLASS="STRUCTNAME"
>pg_largeobject_metadata</TT
></A
>.oid</TT
></TD
><TD
>Identifier of the large object that includes this page</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>pageno</TT
></TD
><TD
><TT
CLASS="TYPE"
>int4</TT
></TD
><TD
> </TD
><TD
>Page number of this page within its large object
(counting from zero)</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>data</TT
></TD
><TD
><TT
CLASS="TYPE"
>bytea</TT
></TD
><TD
> </TD
><TD
> Actual data stored in the large object.
This will never be more than <TT
CLASS="SYMBOL"
>LOBLKSIZE</TT
> bytes and might be less.
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> Each row of <TT
CLASS="STRUCTNAME"
>pg_largeobject</TT
> holds data
for one page of a large object, beginning at
byte offset (<TT
CLASS="LITERAL"
>pageno * LOBLKSIZE</TT
>) within the object. The implementation
allows sparse storage: pages might be missing, and might be shorter than
<TT
CLASS="LITERAL"
>LOBLKSIZE</TT
> bytes even if they are not the last page of the object.
Missing regions within a large object read as zeroes.
</P
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