<para>

<command>ALTER TABLE</command> changes the definition of an existing table.

There are several subforms described below. Note that the lock level required

may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is held

unless explicitly noted. When multiple subcommands arelisted, the lock

heldwill be the strictest one requiredfrom any subcommand.

may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is

acquiredunless explicitly noted. When multiple subcommands aregiven, the

lock acquiredwill be the strictest one requiredby any subcommand.

<variablelist>

<varlistentry>

<term><literal>ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]</literal></term>

<listitem>

<para>

This form adds a new constraint to a table using the samesyntax as

<xref linkend="sql-createtable"/>, plus the option <literal>NOT

This form adds a new constraint to a table using the sameconstraint

syntax as<xref linkend="sql-createtable"/>, plus the option <literal>NOT

VALID</literal>, which is currently only allowed for foreign key

and CHECK constraints.

</para>

</para>

<para>

Addition of a foreign key constraint requires a

<literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table,

in addition to the lock on the table receiving the constraint.

Although most forms of <literal>ADD

<replaceable class="parameter">table_constraint</replaceable></literal>

require an <literal>ACCESS EXCLUSIVE</literal> lock, <literal>ADD

FOREIGN KEY</literal> requires only a <literal>SHARE ROW

EXCLUSIVE</literal> lock. Note that <literal>ADD FOREIGN KEY</literal>

also acquires a <literal>SHARE ROW EXCLUSIVE</literal> lock on the

referenced table, in addition to the lock on the table on which the

constraint is declared.

</para>

<para>