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Commit12b9aff

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Tweak partitioning documentation wording
For clarity, precision, grammar.Author: Justin PryzbyReviewed-by: Amit Langote, Álvaro HerreraDiscussion:https://postgr.es/m/20180523213513.GM30060@telsasoft.com
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‎doc/src/sgml/ddl.sgml

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@@ -2979,15 +2979,16 @@ VALUES ('Albany', NULL, NULL, 'NY');
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<para>
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Individual partitions are linked to the partitioned table with inheritance
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behind-the-scenes; however, it is not possible to use some of the
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inheritance features discussed in the previous section with partitioned
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tables and partitions. For example, a partition cannot have any parents
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other than the partitioned table it is a partition of, nor can a regular
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table inherit from a partitioned table making the latter its parent.
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That means partitioned tables and partitions do not participate in
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inheritance with regular tables. Since a partition hierarchy consisting
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of the partitioned table and its partitions is still an inheritance
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hierarchy, all the normal rules of inheritance apply as described in
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<xref linkend="ddl-inherit"/> with some exceptions, most notably:
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generic features of inheritance (discussed below) with declaratively
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partitioned tables or their partitions. For example, a partition
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cannot have any parents other than the partitioned table it is a
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partition of, nor can a regular table inherit from a partitioned table
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making the latter its parent. That means partitioned tables and their
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partitions do not participate in inheritance with regular tables.
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Since a partition hierarchy consisting of the partitioned table and its
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partitions is still an inheritance hierarchy, all the normal rules of
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inheritance apply as described in <xref linkend="ddl-inherit"/> with
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some exceptions, most notably:
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<itemizedlist>
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<listitem>
@@ -3003,23 +3004,28 @@ VALUES ('Albany', NULL, NULL, 'NY');
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<listitem>
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<para>
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Using <literal>ONLY</literal> to add or drop a constraint on only the
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partitioned table is supportedwhen there are no partitions. Once
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partitioned table is supportedas long as there are no partitions. Once
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partitions exist, using <literal>ONLY</literal> will result in an error
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as adding or dropping constraints on only the partitioned table, when
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partitions exist, is not supported. Instead, constraints can be added
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or dropped, when they are not present in the parent table, directly on
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the partitions. As a partitioned table does not have any data
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directly, attempts to use <command>TRUNCATE</command>
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<literal>ONLY</literal> on a partitioned table will always return an
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error.
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partitions exist, is not supported. Instead, constraints on the
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partitions themselves can be added and (if they are not present in the
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parent table) dropped.
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</para>
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</listitem>
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<listitem>
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<para>
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As a partitioned table does not have any data directly, attempts to use
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<command>TRUNCATE</command> <literal>ONLY</literal> on a partitioned
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table will always return an error.
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</para>
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</listitem>
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<listitem>
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<para>
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Partitions cannot have columns that are not present in the parent. It
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isneither possible to specify columns when creating partitions with
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<command>CREATE TABLE</command> nor is it possible to add columns to
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isnot possible to specify columns when creating partitions with
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<command>CREATE TABLE</command>, nor is it possible to add columns to
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partitions after-the-fact using <command>ALTER TABLE</command>. Tables may be
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added as a partition with <command>ALTER TABLE ... ATTACH PARTITION</command>
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only if their columns exactly match the parent, including any
@@ -3044,7 +3050,7 @@ VALUES ('Albany', NULL, NULL, 'NY');
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<para>
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Updating the partition key of a row might cause it to be moved into a
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different partition where this row satisfiesits partitionconstraint.
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different partition where this row satisfiesthe partitionbounds.
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</para>
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<sect3 id="ddl-partitioning-declarative-example">
@@ -3358,15 +3364,15 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
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While the built-in declarative partitioning is suitable for most
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common use cases, there are some circumstances where a more flexible
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approach may be useful. Partitioning can be implemented using table
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inheritance, which allows for several featureswhich arenot supported
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inheritance, which allows for several features not supported
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by declarative partitioning, such as:
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<itemizedlist>
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<listitem>
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<para>
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Partitioning enforces a rule that allpartitions must have exactly
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the same setof columns as theparent, buttable inheritance allows
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children to have extra columns not present in the parent.
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For declarative partitioning,partitions must have exactly the same set
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of columns as thepartitioned table, whereas withtable inheritance,
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child tables may have extra columns not present in the parent.
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</para>
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</listitem>
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@@ -3768,7 +3774,8 @@ ANALYZE measurement;
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<para>
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<firstterm>Partition pruning</firstterm> is a query optimization technique
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that improves performance for partitioned tables. As an example:
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that improves performance for declaratively partitioned tables.
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As an example:
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<programlisting>
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SET enable_partition_pruning = on; -- the default
@@ -3786,12 +3793,11 @@ SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
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</para>
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<para>
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You can use the <command>EXPLAIN</command> command to show the
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difference between a plan whose partitions have been pruned from one
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whose partitions haven't, by using the
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<xref linkend="guc-enable-partition-pruning"/> configuration
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parameter. A typical unoptimized plan for this type of table setup
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is:
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By using the EXPLAIN command and the <xref
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linkend="guc-enable-partition-pruning"/> configuration parameter, it's
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possible to show the difference between a plan for which partitions have
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been pruned and one for which they have not. A typical unoptimized
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plan for this type of table setup is:
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<programlisting>
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SET enable_partition_pruning = off;
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EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
@@ -3892,9 +3898,9 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
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Currently, pruning of partitions during the planning of an
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<command>UPDATE</command> or <command>DELETE</command> command is
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implemented using the constraint exclusion method (however, it is
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still ruledby the <literal>enable_partition_pruning</literal>
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setting instead of<literal>constraint_exclusion</literal>) &mdash;
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see the next sectionfor details and caveats that apply.
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controlledby the <literal>enable_partition_pruning</literal> rather than
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<literal>constraint_exclusion</literal>) &mdash; see the following section
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for details and caveats that apply.
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</para>
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<para>
@@ -3927,10 +3933,10 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
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Constraint exclusion works in a very similar way to partition
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pruning, except that it uses each table's <literal>CHECK</literal>
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constraints &mdash; which gives it its name &mdash; whereas partition
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pruning uses the table'spartitioning constraint, which exists only in
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thecase of declarative partitioning. Another difference is that it
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is only applied at plan time; there is no attempt to remove
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partitions at execution time.
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pruning uses the table'spartition bounds, which exists only in the
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case of declarative partitioning. Another difference is that
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constraint exclusionis only applied at plan time; there is no attempt
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to removepartitions at execution time.
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</para>
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<para>
@@ -3959,8 +3965,8 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
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<itemizedlist>
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<listitem>
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<para>
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Constraint exclusion is only applied during query planning;it is
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not applied at execution time like partition pruning does.
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Constraint exclusion is only applied during query planning;unlike
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partition pruning, it cannot be applied during query execution.
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</para>
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</listitem>
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@@ -3970,7 +3976,7 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
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clause contains constants (or externally supplied parameters).
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For example, a comparison against a non-immutable function such as
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<function>CURRENT_TIMESTAMP</function> cannot be optimized, since the
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planner cannot know which partition the function value might fall
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planner cannot know which partition the function's value might fall
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into at run time.
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</para>
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</listitem>

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