<!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.18 2001/01/05 06:34:16 tgl Exp $
-Postgres documentation
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.62 2003/09/22 00:16:57 petere Exp $
+PostgreSQL documentation
-->
<refentry id="SQL-ALTERTABLE">
<refmeta>
- <refentrytitle id="sql-altertable-title">
- ALTER TABLE
- </refentrytitle>
+ <refentrytitle id="sql-altertable-title">ALTER TABLE</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
+
<refnamediv>
- <refname>
- ALTER TABLE
- </refname>
- <refpurpose>
- Modifies table properties
- </refpurpose>
+ <refname>ALTER TABLE</refname>
+ <refpurpose>change the definition of a table</refpurpose>
</refnamediv>
+
+ <indexterm zone="sql-altertable">
+ <primary>ALTER TABLE</primary>
+ </indexterm>
+
<refsynopsisdiv>
- <refsynopsisdivinfo>
- <date>1999-07-20</date>
- </refsynopsisdivinfo>
- <synopsis>
-ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
- ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable
- class="PARAMETER">type</replaceable>
-ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
- ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET DEFAULT <replaceable
- class="PARAMETER">value</replaceable> | DROP DEFAULT }
-ALTER TABLE <replaceable class="PARAMETER">table</replaceable> [ * ]
+<synopsis>
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable class="PARAMETER">type</replaceable> [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ DROP [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ RESTRICT | CASCADE ]
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET DEFAULT <replaceable class="PARAMETER">expression</replaceable> | DROP DEFAULT }
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET | DROP } NOT NULL
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STATISTICS <replaceable class="PARAMETER">integer</replaceable>
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ SET WITHOUT OIDS
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
RENAME [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> TO <replaceable
- class="PARAMETER">newcolumn</replaceable>
-ALTER TABLE <replaceable class="PARAMETER">table</replaceable>
- RENAME TO <replaceable class="PARAMETER">newtable</replaceable>
-ALTER TABLE <replaceable class="PARAMETER">table</replaceable>
- ADD <replaceable class="PARAMETER">table constraint definition</replaceable>
-ALTER TABLE <replaceable class="PARAMETER">table</replaceable>
- OWNER TO <replaceable class="PARAMETER">new owner</replaceable>
- </synopsis>
-
- <refsect2 id="R2-SQL-ALTERTABLE-1">
- <refsect2info>
- <date>1998-04-15</date>
- </refsect2info>
- <title>
- Inputs
- </title>
- <para>
+ class="PARAMETER">new_column</replaceable>
+ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
+ RENAME TO <replaceable class="PARAMETER">new_name</replaceable>
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ ADD <replaceable class="PARAMETER">table_constraint</replaceable>
+ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
+ DROP CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
+ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
+ OWNER TO <replaceable class="PARAMETER">new_owner</replaceable>
+ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
+ CLUSTER ON <replaceable class="PARAMETER">index_name</replaceable>
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para>
+ <command>ALTER TABLE</command> changes the definition of an existing table.
+ There are several subforms:
+
+ <variablelist>
+ <varlistentry>
+ <term><literal>ADD COLUMN</literal></term>
+ <listitem>
+ <para>
+ This form adds a new column to the table using the same syntax as
+ <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-TITLE">.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>DROP COLUMN</literal></term>
+ <listitem>
+ <para>
+ This form drops a column from a table. Indexes and
+ table constraints involving the column will be automatically
+ dropped as well. You will need to say <literal>CASCADE</> if
+ anything outside the table depends on the column, for example,
+ foreign key references or views.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
+ <listitem>
+ <para>
+ These forms set or remove the default value for a column.
+ The default values only apply to subsequent <command>INSERT</command>
+ commands; they do not cause rows already in the table to change.
+ Defaults may also be created for views, in which case they are
+ inserted into <command>INSERT</> statements on the view before
+ the view's <literal>ON INSERT</literal> rule is applied.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>SET</literal>/<literal>DROP NOT NULL</literal></term>
+ <listitem>
+ <para>
+ These forms change whether a column is marked to allow null
+ values or to reject null values. You can only use <literal>SET
+ NOT NULL</> when the column contains no null values.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>SET STATISTICS</literal></term>
+ <listitem>
+ <para>
+ This form
+ sets the per-column statistics-gathering target for subsequent
+ <xref linkend="sql-analyze" endterm="sql-analyze-title"> operations.
+ The target can be set in the range 0 to 1000; alternatively, set it
+ to -1 to revert to using the system default statistics target.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>SET STORAGE</literal></term>
+ <listitem>
+ <para>
+ This form sets the storage mode for a column. This controls whether this
+ column is held inline or in a supplementary table, and whether the data
+ should be compressed or not. <literal>PLAIN</literal> must be used
+ for fixed-length values such as <type>integer</type> and is
+ inline, uncompressed. <literal>MAIN</literal> is for inline,
+ compressible data. <literal>EXTERNAL</literal> is for external,
+ uncompressed data, and <literal>EXTENDED</literal> is for external,
+ compressed data. <literal>EXTENDED</literal> is the default for all
+ data types that support it. The use of <literal>EXTERNAL</literal> will, for example,
+ make substring operations on a <type>text</type> column faster, at the penalty of
+ increased storage space.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>SET WITHOUT OIDS</literal></term>
+ <listitem>
+ <para>
+ This form removes the <literal>oid</literal> column from the
+ table. Removing OIDs from a table does not occur immediately.
+ The space that the OID uses will be reclaimed when the row is
+ updated. Without updating the row, both the space and the value
+ of the OID are kept indefinitely. This is semantically similar
+ to the <literal>DROP COLUMN</literal> process.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>RENAME</literal></term>
+ <listitem>
+ <para>
+ The <literal>RENAME</literal> forms change the name of a table
+ (or an index, sequence, or view) or the name of an individual column in
+ a table. There is no effect on the stored data.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>ADD <replaceable class="PARAMETER">table_constraint</replaceable></literal></term>
+ <listitem>
+ <para>
+ This form adds a new constraint to a table using the same syntax as
+ <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-TITLE">.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>DROP CONSTRAINT</literal></term>
+ <listitem>
+ <para>
+ This form drops constraints on a table.
+ Currently, constraints on tables are not required to have unique
+ names, so there may be more than one constraint matching the specified
+ name. All such constraints will be dropped.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>OWNER</literal></term>
+ <listitem>
+ <para>
+ This form changes the owner of the table, index, sequence, or view to the
+ specified user.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><literal>CLUSTER</literal></term>
+ <listitem>
+ <para>
+ This form marks a table for future <xref linkend="SQL-CLUSTER" endterm="sql-cluster-title">
+ operations.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ </variablelist>
+ </para>
+
+ <para>
+ You must own the table to use <command>ALTER TABLE</>; except for
+ <command>ALTER TABLE OWNER</>, which may only be executed by a superuser.
+ </para>
+ </refsect1>
+
+ <refsect1>
+ <title>Parameters</title>
+
<variablelist>
+
<varlistentry>
- <term><replaceable class="PARAMETER"> table </replaceable></term>
+ <term><replaceable class="PARAMETER">name</replaceable></term>
<listitem>
<para>
- The name of an existing table to alter.
+ The name (possibly schema-qualified) of an existing table to
+ alter. If <literal>ONLY</> is specified, only that table is
+ altered. If <literal>ONLY</> is not specified, the table and all
+ its descendant tables (if any) are updated. <literal>*</> can be
+ appended to the table name to indicate that descendant tables are
+ to be altered, but in the current version, this is the default
+ behavior. (In releases before 7.1, <literal>ONLY</> was the
+ default behavior. The default can be altered by changing the
+ configuration parameter <varname>sql_inheritance</varname>.)
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><replaceable class="PARAMETER"> column </replaceable></term>
+ <term><replaceable class="PARAMETER">column</replaceable></term>
<listitem>
<para>
Name of a new or existing column.
</varlistentry>
<varlistentry>
- <term><replaceable class="PARAMETER"> type </replaceable></term>
+ <term><replaceable class="PARAMETER">type</replaceable></term>
<listitem>
<para>
- Type of the new column.
+ Data type of the new column.
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><replaceable class="PARAMETER"> newcolumn </replaceable></term>
+ <term><replaceable class="PARAMETER">new_column</replaceable></term>
<listitem>
<para>
New name for an existing column.
</varlistentry>
<varlistentry>
- <term><replaceable class="PARAMETER"> newtable </replaceable></term>
+ <term><replaceable class="PARAMETER">new_name</replaceable></term>
<listitem>
<para>
New name for the table.
</varlistentry>
<varlistentry>
- <term><replaceable class="PARAMETER"> table constraint definition </replaceable></term>
+ <term><replaceable class="PARAMETER">table_constraint</replaceable></term>
<listitem>
<para>
- New table constraint for the table
+ New table constraint for the table.
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><replaceable class="PARAMETER">New user </replaceable></term>
+ <term><replaceable class="PARAMETER">constraint_name</replaceable></term>
<listitem>
<para>
- The userid of the new owner of the table.
+ Name of an existing constraint to drop.
</para>
</listitem>
</varlistentry>
- </variablelist>
- </para>
- </refsect2>
-
- <refsect2 id="R2-SQL-ALTERTABLE-2">
- <refsect2info>
- <date>1998-04-15</date>
- </refsect2info>
- <title>
- Outputs
- </title>
- <para>
+ <varlistentry>
+ <term><replaceable class="PARAMETER">new_owner</replaceable></term>
+ <listitem>
+ <para>
+ The user name of the new owner of the table.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><replaceable class="PARAMETER">index_name</replaceable></term>
+ <listitem>
+ <para>
+ The index name on which the table should be marked for clustering.
+ </para>
+ </listitem>
+ </varlistentry>
- <variablelist>
<varlistentry>
- <term><computeroutput>ALTER</computeroutput></term>
+ <term><literal>CASCADE</literal></term>
<listitem>
<para>
- Message returned from column or table renaming.
+ Automatically drop objects that depend on the dropped column
+ or constraint (for example, views referencing the column).
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><computeroutput>ERROR</computeroutput></term>
+ <term><literal>RESTRICT</literal></term>
<listitem>
<para>
- Message returned if table or column is not available.
+ Refuse to drop the column or constraint if there are any dependent
+ objects. This is the default behavior.
</para>
</listitem>
</varlistentry>
+
</variablelist>
- </para>
- </refsect2>
- </refsynopsisdiv>
+ </refsect1>
- <refsect1 id="R1-SQL-ALTERTABLE-1">
- <refsect1info>
- <date>1998-04-15</date>
- </refsect1info>
- <title>
- Description
- </title>
- <para>
- <command>ALTER TABLE</command> changes the definition of an existing table.
- The <literal>ADD COLUMN</literal> form adds a new column to the table
- using the same syntax as <xref linkend="SQL-CREATETABLE"
- endterm="SQL-CREATETABLE-title">. The <literal>ALTER COLUMN</literal> form
- allows you to set or remove the default for the column. Note that defaults
- only apply to newly inserted rows.
- The <literal>RENAME</literal> clause causes the name of a table or column
- to change without changing any of the data contained in
- the affected table. Thus, the table or column will
- remain of the same type and size after this command is
- executed.
- The ADD <replaceable class="PARAMETER">table constraint definition</replaceable> clause
- adds a new constraint to the table using the same syntax as <xref
- linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-title">.
- The OWNER clause chnages the owner of the table to the user <replaceable class="PARAMETER">
- new user</replaceable>.
- </para>
+ <refsect1>
+ <title>Notes</title>
- <para>
- You must own the table in order to change its schema.
- </para>
+ The key word <literal>COLUMN</literal> is noise and can be omitted.
+ </para>
- <refsect2 id="R2-SQL-ALTERTABLE-3">
- <refsect2info>
- <date>1998-04-15</date>
- </refsect2info>
- <title>
- Notes
-</title>
<para>
- The keyword <literal>COLUMN</literal> is noise and can be omitted.
+ In the current implementation of <literal>ADD COLUMN</literal>,
+ default and <literal>NOT NULL</> clauses for the new column are not supported.
+ The new column always comes into being with all values null.
+ You can use the <literal>SET DEFAULT</literal> form
+ of <command>ALTER TABLE</command> to set the default afterward.
+ (You may also want to update the already existing rows to the
+ new default value, using
+ <xref linkend="sql-update" endterm="sql-update-title">.)
+ If you want to mark the column non-null, use the <literal>SET NOT NULL</>
+ form after you've entered non-null values for the column in all rows.
</para>
<para>
- In the current implementation, default and constraint clauses for the
- new column will be ignored. You can use the <literal>SET DEFAULT</literal>
- form of <command>ALTER TABLE</command> to set the default later.
- (You will also have to update the already existing rows to the
- new default value, using <xref linkend="sql-update"
- endterm="sql-update-title">.)
+ The <literal>DROP COLUMN</literal> form does not physically remove
+ the column, but simply makes it invisible to SQL operations. Subsequent
+ insert and update operations in the table will store a null value for the column.
+ Thus, dropping a column is quick but it will not immediately reduce the
+ on-disk size of your table, as the space occupied
+ by the dropped column is not reclaimed. The space will be
+ reclaimed over time as existing rows are updated.
+ To reclaim the space at once, do a dummy <command>UPDATE</> of all rows
+ and then vacuum, as in:
+<programlisting>
+UPDATE table SET col = col;
+VACUUM FULL table;
+</programlisting>
</para>
<para>
- In the current implementation, only FOREIGN KEY constraints can
- be added to a table. To create or remove a unique constraint, create
- a unique index (see <xref linkend="SQL-CREATEINDEX"
- endterm="SQL-CREATEINDEX-title">). To add check constraints
- you need to recreate and reload the table, using other
- parameters to the <xref linkend="SQL-CREATETABLE"
- endterm="SQL-CREATETABLE-title"> command.
+ If a table has any descendant tables, it is not permitted to add
+ or rename a column in the parent table without doing the same to
+ the descendants. That is, <command>ALTER TABLE ONLY</command>
+ will be rejected. This ensures that the descendants always have
+ columns matching the parent.
</para>
<para>
- You must own the class in order to change its schema.
- Renaming any part of the schema of a system
- catalog is not permitted.
- The <citetitle>PostgreSQL User's Guide</citetitle> has further
- information on inheritance.
+ A recursive <literal>DROP COLUMN</literal> operation will remove a
+ descendant table's column only if the descendant does not inherit
+ that column from any other parents and never had an independent
+ definition of the column. A nonrecursive <literal>DROP
+ COLUMN</literal> (i.e., <command>ALTER TABLE ONLY ... DROP
+ COLUMN</command>) never removes any descendant columns, but
+ instead marks them as independently defined rather than inherited.
+ </para>
+
+ <para>
+ Changing any part of a system catalog table is not permitted.
</para>
<para>
Refer to <command>CREATE TABLE</command> for a further description
- of valid arguments.
+ of valid parameters. <xref linkend="ddl"> has further information on
+ inheritance.
</para>
- </refsect2>
</refsect1>
- <refsect1 id="R1-SQL-ALTERTABLE-2">
- <title>
- Usage
- </title>
+ <refsect1>
+ <title>Examples</title>
+
<para>
- To add a column of type VARCHAR to a table:
- <programlisting>
-ALTER TABLE distributors ADD COLUMN address VARCHAR(30);
- </programlisting>
+ To add a column of type <type>varchar</type> to a table:
+<programlisting>
+ALTER TABLE distributors ADD COLUMN address varchar(30);
+</programlisting>
+ </para>
+
+ <para>
+ To drop a column from a table:
+<programlisting>
+ALTER TABLE distributors DROP COLUMN address RESTRICT;
+</programlisting>
</para>
<para>
To rename an existing column:
+<programlisting>
ALTER TABLE distributors RENAME COLUMN address TO city;
- </programlisting>
+</programlisting>
</para>
<para>
To rename an existing table:
+<programlisting>
ALTER TABLE distributors RENAME TO suppliers;
- </programlisting>
+</programlisting>
+ </para>
+
+ <para>
+ To add a not-null constraint to a column:
+<programlisting>
+ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;
+</programlisting>
+ To remove a not-null constraint from a column:
+<programlisting>
+ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;
+</programlisting>
+ </para>
+
+ <para>
+ To add a check constraint to a table:
+<programlisting>
+ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
+</programlisting>
+ </para>
+
+ <para>
+ To remove a check constraint from a table and all its children:
+<programlisting>
+ALTER TABLE distributors DROP CONSTRAINT zipchk;
+</programlisting>
</para>
<para>
To add a foreign key constraint to a table:
- <programlisting>
-ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses(address) MATCH FULL
- </programlisting>
+<programlisting>
+ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) MATCH FULL;
+</programlisting>
+ </para>
+
+ <para>
+ To add a (multicolumn) unique constraint to a table:
+<programlisting>
+ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);
+</programlisting>
+ </para>
+
+ <para>
+ To add an automatically named primary key constraint to a table, noting
+ that a table can only ever have one primary key:
+<programlisting>
+ALTER TABLE distributors ADD PRIMARY KEY (dist_id);
+</programlisting>
</para>
</refsect1>
- <refsect1 id="R1-SQL-ALTERTABLE-3">
- <title>
- Compatibility
- </title>
-
- <refsect2 id="R2-SQL-ALTERTABLE-4">
- <refsect2info>
- <date>1998-04-15</date>
- </refsect2info>
- <title>SQL92</title>
- <para>
- The <literal>ADD COLUMN</literal> form is compliant with the exception that
- it does not support defaults and constraints, as explained above.
- The <literal>ALTER COLUMN</literal> form is in full compliance.
- </para>
-
- <para>
- SQL92 specifies some additional capabilities for <command>ALTER TABLE</command>
- statement which are not yet directly supported by <productname>Postgres</productname>:
+ <refsect1>
+ <title>Compatibility</title>
- <variablelist>
- <varlistentry>
- <term>
- <synopsis>
-ALTER TABLE <replaceable class="PARAMETER">table</replaceable> DROP CONSTRAINT <replaceable class="PARAMETER">constraint</replaceable> { RESTRICT | CASCADE }
- </synopsis>
- </term>
- <listitem>
- <para>
- Removes a table constraint (such as a check constraint,
- unique constraint, or foreign key constraint). To
- remove a unique constraint, drop a unique index.
- To remove other kinds of constraints you need to recreate
- and reload the table, using other parameters to the
- <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-title">
- command.
- </para>
- <para>
- For example, to drop any constraints on a table <literal>distributors</literal>:
- <programlisting>
-CREATE TABLE temp AS SELECT * FROM distributors;
-DROP TABLE distributors;
-CREATE TABLE distributors AS SELECT * FROM temp;
-DROP TABLE temp;
- </programlisting>
- </para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term>
- <synopsis>
-ALTER TABLE <replaceable class="PARAMETER">table</replaceable> DROP [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { RESTRICT | CASCADE }
- </synopsis>
- </term>
- <listitem>
- <para>
- Removes a column from a table.
- Currently, to remove an existing column the table must be
- recreated and reloaded:
- <programlisting>
-CREATE TABLE temp AS SELECT did, city FROM distributors;
-DROP TABLE distributors;
-CREATE TABLE distributors (
- did DECIMAL(3) DEFAULT 1,
- name VARCHAR(40) NOT NULL
-);
-INSERT INTO distributors SELECT * FROM temp;
-DROP TABLE temp;
- </programlisting>
- </para>
- </listitem>
- </varlistentry>
- </variablelist>
- </para>
+ <para>
+ The <literal>ADD COLUMN</literal> form conforms with the SQL
+ standard, with the exception that it does not support defaults and
+ not-null constraints, as explained above. The <literal>ALTER
+ COLUMN</literal> form is in full conformance.
+ </para>
- <para>
- The clauses to rename columns and tables are <productname>Postgres</productname>
- extensions from SQL92.
- </para>
-
- </refsect2>
+ <para>
+ The clauses to rename tables, columns, indexes, views, and sequences are
+ <productname>PostgreSQL</productname> extensions of the SQL standard.
+ </para>
+
+ <para>
+ <command>ALTER TABLE DROP COLUMN</> can be used to drop the only
+ column of a table, leaving a zero-column table. This is an
+ extension of SQL, which disallows zero-column tables.
+ </para>
</refsect1>
</refentry>
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