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Document the system attributes ctid and tableoid, which for some reason

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were never yet mentioned anywhere in our documentation.  Improve
explanations of the other system attributes, too.
This commit is contained in:
Tom Lane 2001-01-08 22:07:47 +00:00
parent 6334ef86a7
commit 10fb290aca
1 changed files with 51 additions and 16 deletions
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67
doc/src/sgml/syntax.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.32 2001/01/06 11:58:56 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.33 2001/01/08 22:07:47 tgl Exp $
-->
<chapter id="sql-syntax">
@ -568,9 +568,22 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
<term>oid</term>
<listitem>
<para>
stands for the unique identifier of an instance which is added by
Postgres to all instances automatically. OIDs are not reused and are
32-bit quantities.
The unique identifier (object ID) of a row. This is a serial number
that is added by Postgres to all rows automatically. OIDs are not
reused and are 32-bit quantities.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>tableoid</term>
<listitem>
<para>
The OID of the table containing this row. This attribute is
particularly handy for queries that select from inheritance
hierarchies, since without it, it's difficult to tell which
individual table a row came from. The tableoid can be joined
against the OID attribute of pg_class to obtain the table name.
</para>
</listitem>
</varlistentry>
@ -579,16 +592,9 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
<term>xmin</term>
<listitem>
<para>
The identity of the inserting transaction.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>xmax</term>
<listitem>
<para>
The identity of the deleting transaction.
The identity (transaction ID) of the inserting transaction for
this tuple. (Note: a tuple is an individual state of a row;
each UPDATE of a row creates a new tuple for the same logical row.)
</para>
</listitem>
</varlistentry>
@ -597,7 +603,19 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
<term>cmin</term>
<listitem>
<para>
The command identifier within the inserting transaction.
The command identifier (starting at zero) within the inserting
transaction.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>xmax</term>
<listitem>
<para>
The identity (transaction ID) of the deleting transaction,
or zero for an undeleted tuple. In practice, this is never nonzero
for a visible tuple.
</para>
</listitem>
</varlistentry>
@ -606,7 +624,24 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
<term>cmax</term>
<listitem>
<para>
The command identifier within the deleting transaction.
The command identifier within the deleting transaction, or zero.
Again, this is never nonzero for a visible tuple.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ctid</term>
<listitem>
<para>
The tuple ID of the tuple within its table. This is a pair
(block number, tuple index within block) that identifies the
physical location of the tuple. Note that although the ctid
can be used to locate the tuple very quickly, a row's ctid
will change each time it is updated or moved by VACUUM.
Therefore ctid is useless as a long-term row identifier.
The OID, or even better a user-defined serial number, should
be used to identify logical rows.
</para>
</listitem>
</varlistentry>