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Update discussion of ALTER TABLE ADD COLUMN, per Michael Fuhr.

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This commit is contained in:
Tom Lane 2005-01-09 17:47:30 +00:00
parent 0549de06ba
commit 7bbdb078bd
1 changed files with 17 additions and 13 deletions
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doc/src/sgml/ddl.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.36 2005/01/08 01:44:05 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.37 2005/01/09 17:47:30 tgl Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
@ -1279,23 +1279,22 @@ WHERE c.altitude &gt; 500 and c.tableoid = p.oid;
<programlisting>
ALTER TABLE products ADD COLUMN description text;
</programlisting>
The new column will initially be filled with null values in the
existing rows of the table.
The new column is initially filled with whatever default
value is given (null if you don't specify a <literal>DEFAULT</> clause).
</para>
<para>
You can also define a constraint on the column at the same time,
You can also define constraints on the column at the same time,
using the usual syntax:
<programlisting>
ALTER TABLE products ADD COLUMN description text CHECK (description &lt;&gt; '');
</programlisting>
A new column cannot have a not-null constraint since the column
initially has to contain null values. But you can add a not-null
constraint later. Also, you cannot define a default value on a
new column. According to the SQL standard, this would have to
fill the new columns in the existing rows with the default value,
which is not implemented yet. But you can adjust the column
default later on.
In fact all the options that can be applied to a column description
in <command>CREATE TABLE</> can be used here. Keep in mind however
that the default value must satisfy the given constraints, or the
<literal>ADD</> will fail. Alternatively, you can add
constraints later (see below) after you've filled in the new column
correctly.
</para>
</sect2>
@ -1390,12 +1389,17 @@ ALTER TABLE products ALTER COLUMN product_no DROP NOT NULL;
<programlisting>
ALTER TABLE products ALTER COLUMN price SET DEFAULT 7.77;
</programlisting>
Note that this doesn't affect any existing rows in the table, it
just changes the default for future <command>INSERT</> commands.
</para>
<para>
To remove any default value, use
<programlisting>
ALTER TABLE products ALTER COLUMN price DROP DEFAULT;
</programlisting>
This is equivalent to setting the default to null, at least in
<productname>PostgreSQL</>. As a consequence, it is not an error
This is equivalent to setting the default to null.
As a consequence, it is not an error
to drop a default where one hadn't been defined, because the
default is implicitly the null value.
</para>