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From: Amit Langote <amitlangote09@gmail.com>
Date: Fri, 4 Apr 2014 04:34:49 +0900
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Subject: Re: Fwd: [GENERAL] Table Rewrite During ALTER TABLE ... ADD
 COLUMN ... DEFAULT NULL
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-docs@postgresql.org
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On Fri, Apr 4, 2014 at 1:46 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Amit Langote <amitlangote09@gmail.com> writes:
>> On Thu, Apr 3, 2014 at 1:19 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> We could just rephrase the ALTER TABLE docs to say that the table
>>> rewrite is avoided if you omit the DEFAULT clause, rather than
>>> saying that a null default works.
>
>> How does the attached sound?
>> Wonder if a rewrite-warning is necessary?
>
> I had in mind more like the attached.
>
> This is still not the full truth, as for example this case must do
> a rewrite:
>
> regression=# create domain dnn as int check(value is not null);
> CREATE DOMAIN
> regression=#  create table foo1 (f1 int);
> CREATE TABLE
> regression=# insert into foo1 values(42);
> INSERT 0 1
> regression=# alter table foo1 add column ff dnn;
> ERROR:  value for domain dnn violates check constraint "dnn_check"
>
> But I think we can avoid getting into such complexities here.
>
>                         regards, tom lane
>
>
> diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
> index 4847d66..f0a8b86 100644
> *** a/doc/src/sgml/ref/alter_table.sgml
> --- b/doc/src/sgml/ref/alter_table.sgml
> *************** ALTER TABLE [ IF EXISTS ] <replaceable c
> *** 854,867 ****
>       When a column is added with <literal>ADD COLUMN</literal>, all existing
>       rows in the table are initialized with the column's default value
>       (NULL if no <literal>DEFAULT</> clause is specified).
>      </para>
>
>      <para>
> !     Adding a column with a non-null default or changing the type of an
> !     existing column will require the entire table and indexes to be rewritten.
> !     As an exception, if the <literal>USING</> clause does not change the column
>       contents and the old type is either binary coercible to the new type or
> !     an unconstrained domain over the new type, a table rewrite is not needed,
>       but any indexes on the affected columns must still be rebuilt.  Adding or
>       removing a system <literal>oid</> column also requires rewriting the entire
>       table.  Table and/or index rebuilds may take a significant amount of time
> --- 854,871 ----
>       When a column is added with <literal>ADD COLUMN</literal>, all existing
>       rows in the table are initialized with the column's default value
>       (NULL if no <literal>DEFAULT</> clause is specified).
> +     If there is no <literal>DEFAULT</> clause, this is merely a metadata
> +     change and does not require any immediate update of the table's data;
> +     the added NULL values are supplied on readout, instead.
>      </para>
>
>      <para>
> !     Adding a column with a <literal>DEFAULT</> clause or changing the type of
> !     an existing column will require the entire table and its indexes to be
> !     rewritten.  As an exception when changing the type of an existing column,
> !     if the <literal>USING</> clause does not change the column
>       contents and the old type is either binary coercible to the new type or
> !     an unconstrained domain over the new type, a table rewrite is not needed;
>       but any indexes on the affected columns must still be rebuilt.  Adding or
>       removing a system <literal>oid</> column also requires rewriting the entire
>       table.  Table and/or index rebuilds may take a significant amount of time

Thanks for the fix.

--
Amit


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