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Commit06c4f3a

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doc: Improve explanations when a table rewrite is needed
Further improvement for commit11bd831. That commit confusedidentity and generated columns; fix that. Also, virtual generatedcolumns have since been added; add more details about that. Also somesmall rewordings and reformattings to further improve clarity.Reviewed-by: Robert Treat <rob@xzilla.net>Discussion:https://postgr.es/m/00e6eb5f5c793b8ef722252c7a519c9a@oss.nttdata.com
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‎doc/src/sgml/ref/alter_table.sgml

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@@ -1436,22 +1436,31 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
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<para>
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Adding a column with a volatile <literal>DEFAULT</literal>
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(e.g., <function>clock_timestamp()</function>), a generated column
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(e.g., <literal>GENERATED BY DEFAULT AS IDENTITY</literal>), a domain
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data type with constraints will require the entire table and its
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indexes to be rewritten, as will changing the type of an existing
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column. As an exception, when changing the type of an existing column,
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(e.g., <function>clock_timestamp()</function>), a stored generated column,
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an identity column, or a column with a domain data type that has
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constraints will cause the entire table and its indexes to be rewritten.
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Adding a virtual generated column never requires a rewrite.
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</para>
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<para>
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Changing the type of an existing column will normally cause the entire table
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and its indexes to be rewritten.
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As an exception, when changing the type of an existing column,
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if the <literal>USING</literal> clause does not change the column
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contents and the old type is either binary coercible to the new type
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or an unconstrained domain over the new type, a table rewrite is not
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needed. However, indexesmust always be rebuilt unless the system
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needed. However, indexeswill still be rebuilt unless the system
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can verify that the new index would be logically equivalent to the
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existing one. For example, if the collation for a column has been
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changed, an index rebuild is required because the new sort
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order might be different. However, in the absence of a collation
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change, a column can be changed from <type>text</type> to
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<type>varchar</type> (or vice versa) without rebuilding the indexes
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because these data types sort identically. Table and/or index
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because these data types sort identically.
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</para>
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<para>
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Table and/or index
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rebuilds may take a significant amount of time for a large table,
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and will temporarily require as much as double the disk space.
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</para>

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