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Documentation for transaction-ID-wraparound changes. Add a chapter to

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the Admin Guide about routine maintenance tasks.  Currently this only
discusses the various reasons for doing VACUUM, but perhaps it could be
fleshed out with topics like log rotation.
This commit is contained in:
Tom Lane
2001-08-26 21:17:12 +00:00
parent 3d162c0d1a
commit 8e0d642c9b
4 changed files with 393 additions and 7 deletions
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23
doc/src/sgml/syntax.sgml
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@@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.44 2001/08/10 18:57:32 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.45 2001/08/26 21:17:12 tgl Exp $
-->
<chapter id="sql-syntax">
@@ -640,7 +640,7 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
<indexterm>
<primary>OID</primary>
</indexterm>
The unique identifier (object ID) of a row. This is a serial number
The object identifier (object ID) of a row. This is a serial number
that is automatically added by Postgres to all table rows (unless
the table was created WITHOUT OIDS, in which case this column is
not present).
@@ -688,7 +688,7 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
<para>
The identity (transaction ID) of the deleting transaction,
or zero for an undeleted tuple. It is possible for this field
to be nonzero in a visible tuple: that indicates that the
to be nonzero in a visible tuple: that usually indicates that the
deleting transaction hasn't committed yet, or that an attempted
deletion was rolled back.
</para>
@@ -712,7 +712,8 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
(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.
will change each time it is updated or moved by <command>VACUUM
FULL</>.
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.
@@ -737,7 +738,19 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
</para>
<para>
Transaction and command identifiers are 32-bit quantities.
Transaction identifiers are 32-bit quantities. In a long-lived
database it is possible for transaction IDs to wrap around. This
is not a fatal problem given appropriate maintenance procedures;
see the Administrator's Guide for details. However, it is unwise
to depend on uniqueness of transaction IDs over the long term
(more than one billion transactions).
</para>
<para>
Command identifiers are also 32-bit quantities. This creates a hard
limit of 2^32 (4 billion) SQL commands within a single transaction.
In practice this limit is not a problem --- note that the limit is on
number of SQL queries, not number of tuples processed.
</para>
<para>