-<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.61 2006/09/05 21:08:33 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.62 2006/09/20 01:20:38 neilc Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
</para>
<para>
- One convenient way to create a compatible table to be a new child is using
- the <command>LIKE</command> option of <command>CREATE TABLE</command>. This
- creates a table with the same columns with the same type (however note the
- caveat below regarding constraints). Alternatively a compatible table can
+ One convenient way to create a compatible table to be a new child
+ is specify the <command>LIKE</command> clause in <command>CREATE
+ TABLE</command>. This creates a new table with the same columns as
+ the source table. If there are any <command>CHECK</command>
+ constraints defined on the parent table, the <command>INCLUDING
+ CONSTRAINTS</command> option to <command>LIKE</command> may be
+ useful, as the new child must have constraints matching the parent
+ to be considered compatible. Alternatively a compatible table can
be created by first creating a new child using <command>CREATE
- TABLE</command> then removing the inheritance link with <command>ALTER
- TABLE</command>.
+ TABLE</command> then removing the inheritance link with
+ <command>ALTER TABLE</command>.
</para>
<para>
</para>
</listitem>
- <listitem>
- <para>
- There is no convenient way to define a table compatible with a specific
- parent including columns and constraints. The <command>LIKE</command>
- option for <command>CREATE TABLE</command> does not copy constraints
- which makes the tables it creates ineligible for being added using
- <command>ALTER TABLE</command>. Matching check constraints must be added
- manually or the table must be created as a child immediately, then if
- needed removed from the inheritance structure temporarily to be added
- again later.
- </para>
- </listitem>
-
<listitem>
<para>
If a table is ever removed from the inheritance structure using
constraint for its partition.
</para>
</listitem>
+ </orderedlist>
+ </para>
- <listitem>
- <para>
- When the time comes to archive and remove the old data we first remove
- it from the production table using:
+ <para>
+ As we can see, a complex partitioning scheme could require a
+ substantial amount of DDL. In the above example we would be
+ creating a new partition each month, so it may be wise to write a
+ script that generates the required DDL automatically.
+ </para>
+
+ <sect2 id="ddl-partitioning-managing-partitions">
+ <title>Managing Partitions</title>
+
+ <para>
+ Normally the set of partitions established when initially
+ defining the table are not intended to remain static. It is
+ common to want to remove old partitions of data and periodically
+ add new partitions for new data. One of the most important
+ advantages of partitioning is precisely that it allows this
+ otherwise painful task to be executed nearly instantaneously by
+ manipulating the partition structure, rather than moving large
+ amounts of data around physically.
+ </para>
+ <para>
+ The simplest option for removing old data is to simply drop the partition
+ that is no longer necessary:
<programlisting>
-ALTER TABLE measurement_y2003mm02 NO INHERIT measurement
+DROP TABLE measurement_y2003mm02;
</programlisting>
+ This can very quickly delete millions of records because it doesn't have
+ to individually delete every record.
+ </para>
- Then we can perform any sort of data modification necessary prior to
- archiving without impacting the data viewed by the production system.
- This could include, for example, deleting or compressing out redundant
- data.
- </para>
- </listitem>
- <listitem>
- <para>
+ <para>
+ Another option that is often preferable is to remove the partition from
+ the partitioned table but retain access to it as a table in its own
+ right:
+<programlisting>
+ALTER TABLE measurement_y2003mm02 NO INHERIT measurement;
+</programlisting>
+ This allows further operations to be performed on the data before
+ it is dropped. For example, this is often a useful time to backup
+ the data using <command>COPY</>, <application>pg_dump</>, or
+ similar tools. It can also be a useful time to aggregate data
+ into smaller formats, perform other data manipulations, or run
+ reports.
+ </para>
- Similarly we can a new partition to handle new data. We can either
- create an empty partition as the original partitions were created
- above, or for some applications it's necessary to bulk load and clean
- data for the new partition. If that operation involves multiple steps
- by different processes it can be helpful to work with it in a fresh
- table outside of the master partitioned table until it's ready to be
- loaded:
+ <para>
+ Similarly we can add a new partition to handle new data. We can create an
+ empty partition in the partitioned table just as the original partitions
+ were created above.
<programlisting>
-CREATE TABLE measurement_y2006m02 (LIKE measurement WITH DEFAULTS);
-\COPY measurement_y2006m02 FROM 'measurement_y2006m02'
-UPDATE ...
-ALTER TABLE measurement_y2006m02 ADD CONSTRAINT y2006m02 CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' );
-ALTER TABLE measurement_y2006m02 INHERIT MEASUREMENT;
+CREATE TABLE measurement_y2006m02 (
+ CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' )
+) INHERITS (measurement);
</programlisting>
- </para>
- </listitem>
-
- </orderedlist>
- </para>
+ As an alternative, it is sometimes more convenient to create the
+ new table outside the partition structure, and make it a proper
+ partition later. This allows the data to be loaded, checked, and
+ transformed prior to it appearing in the partitioned table.
- <para>
- As we can see, a complex partitioning scheme could require a
- substantial amount of DDL. In the above example we would be
- creating a new partition each month, so it may be wise to write a
- script that generates the required DDL automatically.
+<programlisting>
+CREATE TABLE measurement_y2006m02 (LIKE measurement INCLUDING DEFAULTS INCLUDING CONSTRAINTS);
+\COPY measurement_y2006m02 FROM 'measurement_y2006m02'
+UPDATE ... ;
+ALTER TABLE measurement_y2006m02 ADD CONSTRAINT y2006m02 CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' );
+ALTER TABLE measurement_y2006m02 INHERIT measurement;
+</programlisting>
</para>
+ </sect2>
+ <sect2 id="ddl-partitioning-caveats">
+ <title>Caveats</title>
+
<para>
- The following caveats apply:
+ The following caveats apply to partitioned tables:
<itemizedlist>
<listitem>
<para>
<listitem>
<para>
- When using the <literal>LIKE</> option above to create new partitions
- check constraints are not copied from the parent. If there are any check
- constraints defined for the parent they must be manually created in new
- partitions before <command>ALTER TABLE</command> will allow them to be
- added.
+ When using the <literal>LIKE</> option above to create new
+ partitions, <literal>CHECK</> constraints are not copied from
+ the parent. If there are any <literal>CHECK</> constraints
+ defined for the parent, they must be manually created in new
+ partitions before <command>ALTER TABLE</command> will allow them
+ to be added.
</para>
</listitem>
</itemizedlist>
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