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><H1
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><A
NAME="DATATYPE-OID"
>8.12. Object Identifier Types</A
></H1
><A
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><P
> Object identifiers (OIDs) are used internally by
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> as primary keys for various
system tables. An OID system column is also added to user-created
tables, unless <TT
CLASS="LITERAL"
>WITHOUT OIDS</TT
> is specified when
the table is created, or the <A
HREF="runtime-config.html#GUC-DEFAULT-WITH-OIDS"
>default_with_oids</A
>
configuration variable is set to false. Type <TT
CLASS="TYPE"
>oid</TT
>
represents an object identifier. There are also several alias
types for <TT
CLASS="TYPE"
>oid</TT
>: <TT
CLASS="TYPE"
>regproc</TT
>, <TT
CLASS="TYPE"
>regprocedure</TT
>,
<TT
CLASS="TYPE"
>regoper</TT
>, <TT
CLASS="TYPE"
>regoperator</TT
>, <TT
CLASS="TYPE"
>regclass</TT
>, and
<TT
CLASS="TYPE"
>regtype</TT
>. <A
HREF="datatype-oid.html#DATATYPE-OID-TABLE"
>Table 8-19</A
> shows an
overview.
</P
><P
> The <TT
CLASS="TYPE"
>oid</TT
> type is currently implemented as an unsigned
four-byte integer. Therefore, it is not large enough to provide
database-wide uniqueness in large databases, or even in large
individual tables. So, using a user-created table's OID column as
a primary key is discouraged. OIDs are best used only for
references to system tables.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> OIDs are included by default in user-created tables in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.0.11. However, this
behavior is likely to change in a future version of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. Eventually, user-created
tables will not include an OID system column unless <TT
CLASS="LITERAL"
>WITH
OIDS</TT
> is specified when the table is created, or the
<TT
CLASS="VARNAME"
>default_with_oids</TT
> configuration variable is set
to true. If your application requires the presence of an OID
system column in a table, it should specify <TT
CLASS="LITERAL"
>WITH
OIDS</TT
> when that table is created to ensure compatibility
with future releases of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</P
></BLOCKQUOTE
></DIV
><P
> The <TT
CLASS="TYPE"
>oid</TT
> type itself has few operations beyond comparison.
It can be cast to
integer, however, and then manipulated using the standard integer
operators. (Beware of possible signed-versus-unsigned confusion
if you do this.)
</P
><P
> The OID alias types have no operations of their own except
for specialized input and output routines. These routines are able
to accept and display symbolic names for system objects, rather than
the raw numeric value that type <TT
CLASS="TYPE"
>oid</TT
> would use. The alias
types allow simplified lookup of OID values for objects. For example,
to examine the <TT
CLASS="STRUCTNAME"
>pg_attribute</TT
> rows related to a table
<TT
CLASS="LITERAL"
>mytable</TT
>, one could write
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM pg_attribute WHERE attrelid = 'mytable'::regclass;</PRE
><P>
rather than
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM pg_attribute
WHERE attrelid = (SELECT oid FROM pg_class WHERE relname = 'mytable');</PRE
><P>
While that doesn't look all that bad by itself, it's still oversimplified.
A far more complicated sub-select would be needed to
select the right OID if there are multiple tables named
<TT
CLASS="LITERAL"
>mytable</TT
> in different schemas.
The <TT
CLASS="TYPE"
>regclass</TT
> input converter handles the table lookup according
to the schema path setting, and so it does the <SPAN
CLASS="QUOTE"
>"right thing"</SPAN
>
automatically. Similarly, casting a table's OID to
<TT
CLASS="TYPE"
>regclass</TT
> is handy for symbolic display of a numeric OID.
</P
><DIV
CLASS="TABLE"
><A
NAME="DATATYPE-OID-TABLE"
></A
><P
><B
>Table 8-19. Object Identifier Types</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>References</TH
><TH
>Description</TH
><TH
>Value Example</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
>any</TD
><TD
>numeric object identifier</TD
><TD
><TT
CLASS="LITERAL"
>564182</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></TD
><TD
>function name</TD
><TD
><TT
CLASS="LITERAL"
>sum</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>regprocedure</TT
></TD
><TD
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></TD
><TD
>function with argument types</TD
><TD
><TT
CLASS="LITERAL"
>sum(int4)</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>regoper</TT
></TD
><TD
><TT
CLASS="STRUCTNAME"
>pg_operator</TT
></TD
><TD
>operator name</TD
><TD
><TT
CLASS="LITERAL"
>+</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>regoperator</TT
></TD
><TD
><TT
CLASS="STRUCTNAME"
>pg_operator</TT
></TD
><TD
>operator with argument types</TD
><TD
><TT
CLASS="LITERAL"
>*(integer,integer)</TT
> or <TT
CLASS="LITERAL"
>-(NONE,integer)</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>regclass</TT
></TD
><TD
><TT
CLASS="STRUCTNAME"
>pg_class</TT
></TD
><TD
>relation name</TD
><TD
><TT
CLASS="LITERAL"
>pg_type</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>regtype</TT
></TD
><TD
><TT
CLASS="STRUCTNAME"
>pg_type</TT
></TD
><TD
>data type name</TD
><TD
><TT
CLASS="LITERAL"
>integer</TT
></TD
></TR
></TBODY
></TABLE
></DIV
><P
> All of the OID alias types accept schema-qualified names, and will
display schema-qualified names on output if the object would not
be found in the current search path without being qualified.
The <TT
CLASS="TYPE"
>regproc</TT
> and <TT
CLASS="TYPE"
>regoper</TT
> alias types will only
accept input names that are unique (not overloaded), so they are
of limited use; for most uses <TT
CLASS="TYPE"
>regprocedure</TT
> or
<TT
CLASS="TYPE"
>regoperator</TT
> is more appropriate. For <TT
CLASS="TYPE"
>regoperator</TT
>,
unary operators are identified by writing <TT
CLASS="LITERAL"
>NONE</TT
> for the unused
operand.
</P
><P
> Another identifier type used by the system is <TT
CLASS="TYPE"
>xid</TT
>, or transaction
(abbreviated <ABBR
CLASS="ABBREV"
>xact</ABBR
>) identifier. This is the data type of the system columns
<TT
CLASS="STRUCTFIELD"
>xmin</TT
> and <TT
CLASS="STRUCTFIELD"
>xmax</TT
>. Transaction identifiers are 32-bit quantities.
</P
><P
> A third identifier type used by the system is <TT
CLASS="TYPE"
>cid</TT
>, or
command identifier. This is the data type of the system columns
<TT
CLASS="STRUCTFIELD"
>cmin</TT
> and <TT
CLASS="STRUCTFIELD"
>cmax</TT
>. Command identifiers are also 32-bit quantities.
</P
><P
> A final identifier type used by the system is <TT
CLASS="TYPE"
>tid</TT
>, or tuple
identifier (row identifier). This is the data type of the system column
<TT
CLASS="STRUCTFIELD"
>ctid</TT
>. A tuple ID is a pair
(block number, tuple index within block) that identifies the
physical location of the row within its table.
</P
><P
> (The system columns are further explained in <A
HREF="ddl-system-columns.html"
>Section 5.4</A
>.)
</P
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