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Commitf5efc93

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Doc: Improve description around ALTER TABLE ATTACH PARTITION
This clarifies more how to use and how to take advantage of constraintswhen attaching a new partition.Author: Justin PryzbyReviewed-by: Amit Langote, Álvaro Herrera, Michael PaquierDiscussion:https://postgr.es/m/20191028001207.GB23808@telsasoft.comBackpatch-through: 10
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‎doc/src/sgml/ddl.sgml

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@@ -3308,13 +3308,13 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
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<para>
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Before running the <command>ATTACH PARTITION</> command, it is
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recommended to create a <literal>CHECK</> constraint on the table to
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be attacheddescribing the desired partition constraint. That way,
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be attachedmatching the desired partition constraint. That way,
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the system will be able to skip the scan to validate the implicit
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partition constraint. Withoutsuch a constraint, the table will be
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scanned to validate the partition constraint while holding an
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<literal>ACCESS EXCLUSIVE</literal> lock on the parent table.
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One maythendrop theconstraint after <command>ATTACH PARTITION</>
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is finished, because itisno longer necessary.
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partition constraint. Withoutthe <literal>CHECK</> constraint,
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the table will bescanned to validate the partition constraint while
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holding an<literal>ACCESS EXCLUSIVE</literal> lock on the parent table.
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It maybe desired todrop theredundant <literal>CHECK</> constraint
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after <command>ATTACH PARTITION</>isfinished.
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</para>
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</sect3>
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‎doc/src/sgml/ref/alter_table.sgml

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@@ -781,23 +781,24 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
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<literal>FOREIGN KEY</literal> constraints are not considered.
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If any of the <literal>CHECK</literal> constraints of the table being
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attached is marked <literal>NO INHERIT</literal>, the command will fail;
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sucha constraintmust be recreated without the <literal>NO INHERIT</literal>
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clause.
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suchconstraintsmust be recreated without the
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<literal>NO INHERIT</literal>clause.
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</para>
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<para>
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If the new partition is a regular table, a full table scan is performed
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to check that no existing row in the table violates the partition
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constraint. It is possible to avoid this scan by adding a valid
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<literal>CHECK</literal> constraint to the table that would allow only
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the rows satisfying the desired partition constraint before running this
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command. It will be determined using such a constraint that the table
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need not be scanned to validate the partition constraint. This does not
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work, however, if any of the partition keys is an expression and the
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partition does not accept <literal>NULL</literal> values. If attaching
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a list partition that will not accept <literal>NULL</literal> values,
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also add <literal>NOT NULL</literal> constraint to the partition key
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column, unless it's an expression.
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to check that existing rows in the table do not violate the partition
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constraint. It is possible to avoid this scan by adding a valid
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<literal>CHECK</literal> constraint to the table that allows only
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rows satisfying the desired partition constraint before running this
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command. The <literal>CHECK</literal> constraint will be used to
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determine that the table need not be scanned to validate the partition
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constraint. This does not work, however, if any of the partition keys
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is an expression and the partition does not accept
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<literal>NULL</literal> values. If attaching a list partition that will
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not accept <literal>NULL</literal> values, also add
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<literal>NOT NULL</literal> constraint to the partition key column,
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unless it's an expression.
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</para>
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<para>

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