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doc: Improve "Partition Maintenance" section

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This adds some reference links and clarifies the wording a bit.

Author: Robert Treat <rob@xzilla.net>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/CABV9wwNGn-pweak6_pvL5PJ1mivDNPKfg0Tck_1oTUETv5Y=dg@mail.gmail.com
This commit is contained in:
Alvaro Herrera
2024-03-30 14:08:13 +01:00
parent 0294df2f1f
commit 056c565742
1 changed files with 36 additions and 36 deletions
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doc/src/sgml/ddl.sgml
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@ -4283,18 +4283,20 @@ CREATE TABLE measurement_y2008m02 PARTITION OF measurement
TABLESPACE fasttablespace; TABLESPACE fasttablespace;
</programlisting> </programlisting>
As an alternative, it is sometimes more convenient to create the As an alternative to creating a new partition, it is sometimes more
new table outside the partition structure, and attach it as a convenient to create a new table separate from the partition structure
partition later. This allows new data to be loaded, checked, and and attach it as a partition later. This allows new data to be loaded,
transformed prior to it appearing in the partitioned table. checked, and transformed prior to it appearing in the partitioned table.
Moreover, the <literal>ATTACH PARTITION</literal> operation requires Moreover, the <literal>ATTACH PARTITION</literal> operation requires
only <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the only a <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the
partitioned table, as opposed to the <literal>ACCESS partitioned table rather than the <literal>ACCESS EXCLUSIVE</literal>
EXCLUSIVE</literal> lock that is required by <command>CREATE TABLE lock required by <command>CREATE TABLE ... PARTITION OF</command>,
... PARTITION OF</command>, so it is more friendly to concurrent so it is more friendly to concurrent operations on the partitioned table;
operations on the partitioned table. see <link linkend="sql-altertable-attach-partition"><literal>ALTER TABLE ... ATTACH PARTITION</literal></link>
The <literal>CREATE TABLE ... LIKE</literal> option is helpful for additional details. The
to avoid tediously repeating the parent table's definition: <link linkend="sql-createtable-parms-like"><literal>CREATE TABLE ... LIKE</literal></link>
option can be helpful to avoid tediously repeating the parent table's
definition; for example:
<programlisting> <programlisting>
CREATE TABLE measurement_y2008m02 CREATE TABLE measurement_y2008m02
@ -4313,17 +4315,15 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
</para> </para>
<para> <para>
Before running the <command>ATTACH PARTITION</command> command, it is Note that when running the <command>ATTACH PARTITION</command> command,
recommended to create a <literal>CHECK</literal> constraint on the table to
be attached that matches the expected partition constraint, as
illustrated above. That way, the system will be able to skip the scan
which is otherwise needed to validate the implicit
partition constraint. Without the <literal>CHECK</literal> constraint,
the table will be scanned to validate the partition constraint while the table will be scanned to validate the partition constraint while
holding an <literal>ACCESS EXCLUSIVE</literal> lock on that partition. holding an <literal>ACCESS EXCLUSIVE</literal> lock on that partition.
It is recommended to drop the now-redundant <literal>CHECK</literal> As shown above, it is recommended to avoid this scan by creating a
constraint after the <command>ATTACH PARTITION</command> is complete. If <literal>CHECK</literal> constraint matching the expected partition
the table being attached is itself a partitioned table, then each of its constraint on the table prior to attaching it. Once the
<command>ATTACH PARTITION</command> is complete, it is recommended to drop
the now-redundant <literal>CHECK</literal> constraint.
If the table being attached is itself a partitioned table, then each of its
sub-partitions will be recursively locked and scanned until either a sub-partitions will be recursively locked and scanned until either a
suitable <literal>CHECK</literal> constraint is encountered or the leaf suitable <literal>CHECK</literal> constraint is encountered or the leaf
partitions are reached. partitions are reached.
@ -4333,7 +4333,7 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
Similarly, if the partitioned table has a <literal>DEFAULT</literal> Similarly, if the partitioned table has a <literal>DEFAULT</literal>
partition, it is recommended to create a <literal>CHECK</literal> partition, it is recommended to create a <literal>CHECK</literal>
constraint which excludes the to-be-attached partition's constraint. If constraint which excludes the to-be-attached partition's constraint. If
this is not done then the <literal>DEFAULT</literal> partition will be this is not done, the <literal>DEFAULT</literal> partition will be
scanned to verify that it contains no records which should be located in scanned to verify that it contains no records which should be located in
the partition being attached. This operation will be performed whilst the partition being attached. This operation will be performed whilst
holding an <literal>ACCESS EXCLUSIVE</literal> lock on the <literal> holding an <literal>ACCESS EXCLUSIVE</literal> lock on the <literal>
@ -4344,21 +4344,21 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
</para> </para>
<para> <para>
As explained above, it is possible to create indexes on partitioned tables As mentioned earlier, it is possible to create indexes on partitioned
so that they are applied automatically to the entire hierarchy. tables so that they are applied automatically to the entire hierarchy.
This is very This can be very convenient as not only will all existing partitions be
convenient, as not only will the existing partitions become indexed, but indexed, but any future partitions will be as well. However, one
also any partitions that are created in the future will. One limitation is limitation when creating new indexes on partitioned tables is that it
that it's not possible to use the <literal>CONCURRENTLY</literal> is not possible to use the <literal>CONCURRENTLY</literal>
qualifier when creating such a partitioned index. To avoid long lock qualifier, which could lead to long lock times. To avoid this, you can
times, it is possible to use <command>CREATE INDEX ON ONLY</command> use <command>CREATE INDEX ON ONLY</command> the partitioned table, which
the partitioned table; such an index is marked invalid, and the partitions creates the new index marked as invalid, preventing automatic application
do not get the index applied automatically. The indexes on partitions can to existing partitions. Instead, indexes can then be created individually
be created individually using <literal>CONCURRENTLY</literal>, and then on each partition using <literal>CONCURRENTLY</literal> and
<firstterm>attached</firstterm> to the index on the parent using <firstterm>attached</firstterm> to the partitioned index on the parent
<command>ALTER INDEX .. ATTACH PARTITION</command>. Once indexes for all using <command>ALTER INDEX ... ATTACH PARTITION</command>. Once indexes for
partitions are attached to the parent index, the parent index is marked all the partitions are attached to the parent index, the parent index will
valid automatically. Example: be marked valid automatically. Example:
<programlisting> <programlisting>
CREATE INDEX measurement_usls_idx ON ONLY measurement (unitsales); CREATE INDEX measurement_usls_idx ON ONLY measurement (unitsales);