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Postgres FD Implementation
Commit c6217210 authored by Neil Conway's avatar Neil Conway
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Improvements to the partitioning documentation. Original patch from Greg 

Stark, additional fixes and editorialization by Neil Conway. Greg's patch
description:

    Some more doc patches for partitioned tables. In particular replace
    the caveat that INCLUDING CONSTRAINTS doesn't exist and replace it
    with documentation of, well, INCLUDING CONSTRAINTS.

    Also, there was an instance of "LIKE WITH DEFAULTS" which is
    actually spelled "LIKE INCLUDING DEFAULTS".
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doc/src/sgml/ddl.sgml
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<!-- $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>
......@@ -2081,13 +2081,17 @@ VALUES ('New York', NULL, NULL, 'NY');
</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>
......@@ -2159,19 +2163,6 @@ VALUES ('New York', NULL, NULL, 'NY');
</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
......@@ -2577,56 +2568,86 @@ DO INSTEAD
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 &gt;= DATE '2006-02-01' AND logdate &lt 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>
......@@ -2651,11 +2672,12 @@ ALTER TABLE measurement_y2006m02 INHERIT MEASUREMENT;
<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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