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><A
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>8.11. Object Identifier Types</A
></H1
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><P
> Object identifiers (OIDs) are used internally by
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> as primary keys for various system
tables. Also, an OID system column is added to user-created tables
(unless <TT
CLASS="LITERAL"
>WITHOUT OIDS</TT
> is specified at table creation time).
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
><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,
one may write <TT
CLASS="LITERAL"
>'mytable'::regclass</TT
> to get the OID of table
<TT
CLASS="LITERAL"
>mytable</TT
>, rather than <TT
CLASS="LITERAL"
>SELECT oid FROM pg_class WHERE
relname = 'mytable'</TT
>. (In reality, a much more complicated <TT
CLASS="COMMAND"
>SELECT</TT
> would
be needed to deal with selecting the right OID when there are multiple
tables named <TT
CLASS="LITERAL"
>mytable</TT
> in different schemas.)
</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 5.2</A
>.)
</P
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