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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>ALTER TABLE</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-altersystem.html" title="ALTER SYSTEM" /><link rel="next" href="sql-altertablespace.html" title="ALTER TABLESPACE" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">ALTER TABLE</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-altersystem.html" title="ALTER SYSTEM">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-altertablespace.html" title="ALTER TABLESPACE">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-ALTERTABLE"><div class="titlepage"></div><a id="id-1.9.3.35.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">ALTER TABLE</span></h2><p>ALTER TABLE — change the definition of a table</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
ALTER TABLE [ IF EXISTS ] [ ONLY ] <em class="replaceable"><code>name</code></em> [ * ]
<em class="replaceable"><code>action</code></em> [, ... ]
ALTER TABLE [ IF EXISTS ] [ ONLY ] <em class="replaceable"><code>name</code></em> [ * ]
RENAME [ COLUMN ] <em class="replaceable"><code>column_name</code></em> TO <em class="replaceable"><code>new_column_name</code></em>
ALTER TABLE [ IF EXISTS ] [ ONLY ] <em class="replaceable"><code>name</code></em> [ * ]
RENAME CONSTRAINT <em class="replaceable"><code>constraint_name</code></em> TO <em class="replaceable"><code>new_constraint_name</code></em>
ALTER TABLE [ IF EXISTS ] <em class="replaceable"><code>name</code></em>
RENAME TO <em class="replaceable"><code>new_name</code></em>
ALTER TABLE [ IF EXISTS ] <em class="replaceable"><code>name</code></em>
SET SCHEMA <em class="replaceable"><code>new_schema</code></em>
ALTER TABLE ALL IN TABLESPACE <em class="replaceable"><code>name</code></em> [ OWNED BY <em class="replaceable"><code>role_name</code></em> [, ... ] ]
SET TABLESPACE <em class="replaceable"><code>new_tablespace</code></em> [ NOWAIT ]
ALTER TABLE [ IF EXISTS ] <em class="replaceable"><code>name</code></em>
ATTACH PARTITION <em class="replaceable"><code>partition_name</code></em> { FOR VALUES <em class="replaceable"><code>partition_bound_spec</code></em> | DEFAULT }
ALTER TABLE [ IF EXISTS ] <em class="replaceable"><code>name</code></em>
DETACH PARTITION <em class="replaceable"><code>partition_name</code></em>
<span class="phrase">where <em class="replaceable"><code>action</code></em> is one of:</span>
ADD [ COLUMN ] [ IF NOT EXISTS ] <em class="replaceable"><code>column_name</code></em> <em class="replaceable"><code>data_type</code></em> [ COLLATE <em class="replaceable"><code>collation</code></em> ] [ <em class="replaceable"><code>column_constraint</code></em> [ ... ] ]
DROP [ COLUMN ] [ IF EXISTS ] <em class="replaceable"><code>column_name</code></em> [ RESTRICT | CASCADE ]
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> [ SET DATA ] TYPE <em class="replaceable"><code>data_type</code></em> [ COLLATE <em class="replaceable"><code>collation</code></em> ] [ USING <em class="replaceable"><code>expression</code></em> ]
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> SET DEFAULT <em class="replaceable"><code>expression</code></em>
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> DROP DEFAULT
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> { SET | DROP } NOT NULL
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <em class="replaceable"><code>sequence_options</code></em> ) ]
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> { SET GENERATED { ALWAYS | BY DEFAULT } | SET <em class="replaceable"><code>sequence_option</code></em> | RESTART [ [ WITH ] <em class="replaceable"><code>restart</code></em> ] } [...]
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> DROP IDENTITY [ IF EXISTS ]
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> SET STATISTICS <em class="replaceable"><code>integer</code></em>
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> SET ( <em class="replaceable"><code>attribute_option</code></em> = <em class="replaceable"><code>value</code></em> [, ... ] )
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> RESET ( <em class="replaceable"><code>attribute_option</code></em> [, ... ] )
ALTER [ COLUMN ] <em class="replaceable"><code>column_name</code></em> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ADD <em class="replaceable"><code>table_constraint</code></em> [ NOT VALID ]
ADD <em class="replaceable"><code>table_constraint_using_index</code></em>
ALTER CONSTRAINT <em class="replaceable"><code>constraint_name</code></em> [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
VALIDATE CONSTRAINT <em class="replaceable"><code>constraint_name</code></em>
DROP CONSTRAINT [ IF EXISTS ] <em class="replaceable"><code>constraint_name</code></em> [ RESTRICT | CASCADE ]
DISABLE TRIGGER [ <em class="replaceable"><code>trigger_name</code></em> | ALL | USER ]
ENABLE TRIGGER [ <em class="replaceable"><code>trigger_name</code></em> | ALL | USER ]
ENABLE REPLICA TRIGGER <em class="replaceable"><code>trigger_name</code></em>
ENABLE ALWAYS TRIGGER <em class="replaceable"><code>trigger_name</code></em>
DISABLE RULE <em class="replaceable"><code>rewrite_rule_name</code></em>
ENABLE RULE <em class="replaceable"><code>rewrite_rule_name</code></em>
ENABLE REPLICA RULE <em class="replaceable"><code>rewrite_rule_name</code></em>
ENABLE ALWAYS RULE <em class="replaceable"><code>rewrite_rule_name</code></em>
DISABLE ROW LEVEL SECURITY
ENABLE ROW LEVEL SECURITY
FORCE ROW LEVEL SECURITY
NO FORCE ROW LEVEL SECURITY
CLUSTER ON <em class="replaceable"><code>index_name</code></em>
SET WITHOUT CLUSTER
SET WITH OIDS
SET WITHOUT OIDS
SET TABLESPACE <em class="replaceable"><code>new_tablespace</code></em>
SET { LOGGED | UNLOGGED }
SET ( <em class="replaceable"><code>storage_parameter</code></em> = <em class="replaceable"><code>value</code></em> [, ... ] )
RESET ( <em class="replaceable"><code>storage_parameter</code></em> [, ... ] )
INHERIT <em class="replaceable"><code>parent_table</code></em>
NO INHERIT <em class="replaceable"><code>parent_table</code></em>
OF <em class="replaceable"><code>type_name</code></em>
NOT OF
OWNER TO { <em class="replaceable"><code>new_owner</code></em> | CURRENT_USER | SESSION_USER }
REPLICA IDENTITY { DEFAULT | USING INDEX <em class="replaceable"><code>index_name</code></em> | FULL | NOTHING }
<span class="phrase">and <em class="replaceable"><code>partition_bound_spec</code></em> is:</span>
IN ( { <em class="replaceable"><code>numeric_literal</code></em> | <em class="replaceable"><code>string_literal</code></em> | TRUE | FALSE | NULL } [, ...] ) |
FROM ( { <em class="replaceable"><code>numeric_literal</code></em> | <em class="replaceable"><code>string_literal</code></em> | TRUE | FALSE | MINVALUE | MAXVALUE } [, ...] )
TO ( { <em class="replaceable"><code>numeric_literal</code></em> | <em class="replaceable"><code>string_literal</code></em> | TRUE | FALSE | MINVALUE | MAXVALUE } [, ...] ) |
WITH ( MODULUS <em class="replaceable"><code>numeric_literal</code></em>, REMAINDER <em class="replaceable"><code>numeric_literal</code></em> )
<span class="phrase">and <em class="replaceable"><code>column_constraint</code></em> is:</span>
[ CONSTRAINT <em class="replaceable"><code>constraint_name</code></em> ]
{ NOT NULL |
NULL |
CHECK ( <em class="replaceable"><code>expression</code></em> ) [ NO INHERIT ] |
DEFAULT <em class="replaceable"><code>default_expr</code></em> |
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <em class="replaceable"><code>sequence_options</code></em> ) ] |
UNIQUE <em class="replaceable"><code>index_parameters</code></em> |
PRIMARY KEY <em class="replaceable"><code>index_parameters</code></em> |
REFERENCES <em class="replaceable"><code>reftable</code></em> [ ( <em class="replaceable"><code>refcolumn</code></em> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE <em class="replaceable"><code>action</code></em> ] [ ON UPDATE <em class="replaceable"><code>action</code></em> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
<span class="phrase">and <em class="replaceable"><code>table_constraint</code></em> is:</span>
[ CONSTRAINT <em class="replaceable"><code>constraint_name</code></em> ]
{ CHECK ( <em class="replaceable"><code>expression</code></em> ) [ NO INHERIT ] |
UNIQUE ( <em class="replaceable"><code>column_name</code></em> [, ... ] ) <em class="replaceable"><code>index_parameters</code></em> |
PRIMARY KEY ( <em class="replaceable"><code>column_name</code></em> [, ... ] ) <em class="replaceable"><code>index_parameters</code></em> |
EXCLUDE [ USING <em class="replaceable"><code>index_method</code></em> ] ( <em class="replaceable"><code>exclude_element</code></em> WITH <em class="replaceable"><code>operator</code></em> [, ... ] ) <em class="replaceable"><code>index_parameters</code></em> [ WHERE ( <em class="replaceable"><code>predicate</code></em> ) ] |
FOREIGN KEY ( <em class="replaceable"><code>column_name</code></em> [, ... ] ) REFERENCES <em class="replaceable"><code>reftable</code></em> [ ( <em class="replaceable"><code>refcolumn</code></em> [, ... ] ) ]
[ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE <em class="replaceable"><code>action</code></em> ] [ ON UPDATE <em class="replaceable"><code>action</code></em> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
<span class="phrase">and <em class="replaceable"><code>table_constraint_using_index</code></em> is:</span>
[ CONSTRAINT <em class="replaceable"><code>constraint_name</code></em> ]
{ UNIQUE | PRIMARY KEY } USING INDEX <em class="replaceable"><code>index_name</code></em>
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
<span class="phrase"><em class="replaceable"><code>index_parameters</code></em> in <code class="literal">UNIQUE</code>, <code class="literal">PRIMARY KEY</code>, and <code class="literal">EXCLUDE</code> constraints are:</span>
[ INCLUDE ( <em class="replaceable"><code>column_name</code></em> [, ... ] ) ]
[ WITH ( <em class="replaceable"><code>storage_parameter</code></em> [= <em class="replaceable"><code>value</code></em>] [, ... ] ) ]
[ USING INDEX TABLESPACE <em class="replaceable"><code>tablespace_name</code></em> ]
<span class="phrase"><em class="replaceable"><code>exclude_element</code></em> in an <code class="literal">EXCLUDE</code> constraint is:</span>
{ <em class="replaceable"><code>column_name</code></em> | ( <em class="replaceable"><code>expression</code></em> ) } [ <em class="replaceable"><code>opclass</code></em> ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]
</pre></div><div class="refsect1" id="id-1.9.3.35.5"><h2>Description</h2><p>
<code class="command">ALTER TABLE</code> 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 <code class="literal">ACCESS EXCLUSIVE</code> lock is held
unless explicitly noted. When multiple subcommands are listed, the lock
held will be the strictest one required from any subcommand.
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">ADD COLUMN [ IF NOT EXISTS ]</code></span></dt><dd><p>
This form adds a new column to the table, using the same syntax as
<a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>. If <code class="literal">IF NOT EXISTS</code>
is specified and a column already exists with this name,
no error is thrown.
</p></dd><dt><span class="term"><code class="literal">DROP COLUMN [ IF EXISTS ]</code></span></dt><dd><p>
This form drops a column from a table. Indexes and
table constraints involving the column will be automatically
dropped as well.
Multivariate statistics referencing the dropped column will also be
removed if the removal of the column would cause the statistics to
contain data for only a single column.
You will need to say <code class="literal">CASCADE</code> if anything outside the table
depends on the column, for example, foreign key references or views.
If <code class="literal">IF EXISTS</code> is specified and the column
does not exist, no error is thrown. In this case a notice
is issued instead.
</p></dd><dt><span class="term"><code class="literal">SET DATA TYPE</code></span></dt><dd><p>
This form changes the type of a column of a table. Indexes and
simple table constraints involving the column will be automatically
converted to use the new column type by reparsing the originally
supplied expression.
The optional <code class="literal">COLLATE</code> clause specifies a collation
for the new column; if omitted, the collation is the default for the
new column type.
The optional <code class="literal">USING</code>
clause specifies how to compute the new column value from the old;
if omitted, the default conversion is the same as an assignment
cast from old data type to new. A <code class="literal">USING</code>
clause must be provided if there is no implicit or assignment
cast from old to new type.
</p></dd><dt><span class="term"><code class="literal">SET</code>/<code class="literal">DROP DEFAULT</code></span></dt><dd><p>
These forms set or remove the default value for a column.
Default values only apply in subsequent <code class="command">INSERT</code>
or <code class="command">UPDATE</code> commands; they do not cause rows already in the
table to change.
</p></dd><dt><span class="term"><code class="literal">SET</code>/<code class="literal">DROP NOT NULL</code></span></dt><dd><p>
These forms change whether a column is marked to allow null
values or to reject null values. You can only use <code class="literal">SET
NOT NULL</code> when the column contains no null values.
</p><p>
If this table is a partition, one cannot perform <code class="literal">DROP NOT NULL</code>
on a column if it is marked <code class="literal">NOT NULL</code> in the parent
table. To drop the <code class="literal">NOT NULL</code> constraint from all the
partitions, perform <code class="literal">DROP NOT NULL</code> on the parent
table. Even if there is no <code class="literal">NOT NULL</code> constraint on the
parent, such a constraint can still be added to individual partitions,
if desired; that is, the children can disallow nulls even if the parent
allows them, but not the other way around.
</p></dd><dt><span class="term"><code class="literal">ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY</code><br /></span><span class="term"><code class="literal">SET GENERATED { ALWAYS | BY DEFAULT }</code><br /></span><span class="term"><code class="literal">DROP IDENTITY [ IF EXISTS ]</code></span></dt><dd><p>
These forms change whether a column is an identity column or change the
generation attribute of an existing identity column.
See <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> for details.
</p><p>
If <code class="literal">DROP IDENTITY IF EXISTS</code> is specified and the
column is not an identity column, no error is thrown. In this case a
notice is issued instead.
</p></dd><dt><span class="term"><code class="literal">SET <em class="replaceable"><code>sequence_option</code></em></code><br /></span><span class="term"><code class="literal">RESTART</code></span></dt><dd><p>
These forms alter the sequence that underlies an existing identity
column. <em class="replaceable"><code>sequence_option</code></em> is an option
supported by <a class="xref" href="sql-altersequence.html" title="ALTER SEQUENCE"><span class="refentrytitle">ALTER SEQUENCE</span></a> such
as <code class="literal">INCREMENT BY</code>.
</p></dd><dt><span class="term"><code class="literal">SET STATISTICS</code></span></dt><dd><p>
This form
sets the per-column statistics-gathering target for subsequent
<a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a> operations.
The target can be set in the range 0 to 10000; alternatively, set it
to -1 to revert to using the system default statistics
target (<a class="xref" href="runtime-config-query.html#GUC-DEFAULT-STATISTICS-TARGET">default_statistics_target</a>).
For more information on the use of statistics by the
<span class="productname">PostgreSQL</span> query planner, refer to
<a class="xref" href="planner-stats.html" title="14.2. Statistics Used by the Planner">Section 14.2</a>.
</p><p>
<code class="literal">SET STATISTICS</code> acquires a
<code class="literal">SHARE UPDATE EXCLUSIVE</code> lock.
</p></dd><dt><span class="term"><code class="literal">SET ( <em class="replaceable"><code>attribute_option</code></em> = <em class="replaceable"><code>value</code></em> [, ... ] )</code><br /></span><span class="term"><code class="literal">RESET ( <em class="replaceable"><code>attribute_option</code></em> [, ... ] )</code></span></dt><dd><p>
This form sets or resets per-attribute options. Currently, the only
defined per-attribute options are <code class="literal">n_distinct</code> and
<code class="literal">n_distinct_inherited</code>, which override the
number-of-distinct-values estimates made by subsequent
<a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a>
operations. <code class="literal">n_distinct</code> affects the statistics for the table
itself, while <code class="literal">n_distinct_inherited</code> affects the statistics
gathered for the table plus its inheritance children. When set to a
positive value, <code class="command">ANALYZE</code> will assume that the column contains
exactly the specified number of distinct nonnull values. When set to a
negative value, which must be greater
than or equal to -1, <code class="command">ANALYZE</code> will assume that the number of
distinct nonnull values in the column is linear in the size of the
table; the exact count is to be computed by multiplying the estimated
table size by the absolute value of the given number. For example,
a value of -1 implies that all values in the column are distinct, while
a value of -0.5 implies that each value appears twice on the average.
This can be useful when the size of the table changes over time, since
the multiplication by the number of rows in the table is not performed
until query planning time. Specify a value of 0 to revert to estimating
the number of distinct values normally. For more information on the use
of statistics by the <span class="productname">PostgreSQL</span> query
planner, refer to <a class="xref" href="planner-stats.html" title="14.2. Statistics Used by the Planner">Section 14.2</a>.
</p><p>
Changing per-attribute options acquires a
<code class="literal">SHARE UPDATE EXCLUSIVE</code> lock.
</p></dd><dt><span class="term">
<code class="literal">SET STORAGE</code>
<a id="id-1.9.3.35.5.2.3.10.1.2" class="indexterm"></a>
</span></dt><dd><p>
This form sets the storage mode for a column. This controls whether this
column is held inline or in a secondary <acronym class="acronym">TOAST</acronym> table, and
whether the data
should be compressed or not. <code class="literal">PLAIN</code> must be used
for fixed-length values such as <code class="type">integer</code> and is
inline, uncompressed. <code class="literal">MAIN</code> is for inline,
compressible data. <code class="literal">EXTERNAL</code> is for external,
uncompressed data, and <code class="literal">EXTENDED</code> is for external,
compressed data. <code class="literal">EXTENDED</code> is the default for most
data types that support non-<code class="literal">PLAIN</code> storage.
Use of <code class="literal">EXTERNAL</code> will make substring operations on
very large <code class="type">text</code> and <code class="type">bytea</code> values run faster,
at the penalty of increased storage space. Note that
<code class="literal">SET STORAGE</code> doesn't itself change anything in the table,
it just sets the strategy to be pursued during future table updates.
See <a class="xref" href="storage-toast.html" title="68.2. TOAST">Section 68.2</a> for more information.
</p></dd><dt><span class="term"><code class="literal">ADD <em class="replaceable"><code>table_constraint</code></em> [ NOT VALID ]</code></span></dt><dd><p>
This form adds a new constraint to a table using the same syntax as
<a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>, plus the option <code class="literal">NOT
VALID</code>, which is currently only allowed for foreign key
and CHECK constraints.
If the constraint is marked <code class="literal">NOT VALID</code>, the
potentially-lengthy initial check to verify that all rows in the table
satisfy the constraint is skipped. The constraint will still be
enforced against subsequent inserts or updates (that is, they'll fail
unless there is a matching row in the referenced table, in the case
of foreign keys; and they'll fail unless the new row matches the
specified check constraints). But the
database will not assume that the constraint holds for all rows in
the table, until it is validated by using the <code class="literal">VALIDATE
CONSTRAINT</code> option. Foreign key constraints on partitioned
tables may not be declared <code class="literal">NOT VALID</code> at present.
</p><p>
The addition of a foreign key constraint requires a
<code class="literal">SHARE ROW EXCLUSIVE</code> lock on the referenced table.
</p><p>
Additional restrictions apply when unique or primary key constraints
are added to partitioned tables; see <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>.
</p></dd><dt><span class="term"><code class="literal">ADD <em class="replaceable"><code>table_constraint_using_index</code></em></code></span></dt><dd><p>
This form adds a new <code class="literal">PRIMARY KEY</code> or <code class="literal">UNIQUE</code>
constraint to a table based on an existing unique index. All the
columns of the index will be included in the constraint.
</p><p>
The index cannot have expression columns nor be a partial index.
Also, it must be a b-tree index with default sort ordering. These
restrictions ensure that the index is equivalent to one that would be
built by a regular <code class="literal">ADD PRIMARY KEY</code> or <code class="literal">ADD UNIQUE</code>
command.
</p><p>
If <code class="literal">PRIMARY KEY</code> is specified, and the index's columns are not
already marked <code class="literal">NOT NULL</code>, then this command will attempt to
do <code class="literal">ALTER COLUMN SET NOT NULL</code> against each such column.
That requires a full table scan to verify the column(s) contain no
nulls. In all other cases, this is a fast operation.
</p><p>
If a constraint name is provided then the index will be renamed to match
the constraint name. Otherwise the constraint will be named the same as
the index.
</p><p>
After this command is executed, the index is <span class="quote">“<span class="quote">owned</span>”</span> by the
constraint, in the same way as if the index had been built by
a regular <code class="literal">ADD PRIMARY KEY</code> or <code class="literal">ADD UNIQUE</code>
command. In particular, dropping the constraint will make the index
disappear too.
</p><p>
This form is not currently supported on partitioned tables.
</p><div class="note"><h3 class="title">Note</h3><p>
Adding a constraint using an existing index can be helpful in
situations where a new constraint needs to be added without blocking
table updates for a long time. To do that, create the index using
<code class="command">CREATE INDEX CONCURRENTLY</code>, and then install it as an
official constraint using this syntax. See the example below.
</p></div></dd><dt><span class="term"><code class="literal">ALTER CONSTRAINT</code></span></dt><dd><p>
This form alters the attributes of a constraint that was previously
created. Currently only foreign key constraints may be altered.
</p></dd><dt><span class="term"><code class="literal">VALIDATE CONSTRAINT</code></span></dt><dd><p>
This form validates a foreign key or check constraint that was previously created
as <code class="literal">NOT VALID</code>, by scanning the table to ensure there
are no rows for which the constraint is not satisfied.
Nothing happens if the constraint is already marked valid.
</p><p>
Validation can be a long process on larger tables. The value of separating
validation from initial creation is that you can defer validation to less
busy times, or can be used to give additional time to correct pre-existing
errors while preventing new errors. Note also that validation on its own
does not prevent normal write commands against the table while it runs.
</p><p>
Validation acquires only a <code class="literal">SHARE UPDATE EXCLUSIVE</code> lock
on the table being altered. If the constraint is a foreign key then
a <code class="literal">ROW SHARE</code> lock is also required on
the table referenced by the constraint.
</p></dd><dt><span class="term"><code class="literal">DROP CONSTRAINT [ IF EXISTS ]</code></span></dt><dd><p>
This form drops the specified constraint on a table, along with
any index underlying the constraint.
If <code class="literal">IF EXISTS</code> is specified and the constraint
does not exist, no error is thrown. In this case a notice is issued instead.
</p></dd><dt><span class="term"><code class="literal">DISABLE</code>/<code class="literal">ENABLE [ REPLICA | ALWAYS ] TRIGGER</code></span></dt><dd><p>
These forms configure the firing of trigger(s) belonging to the table.
A disabled trigger is still known to the system, but is not executed
when its triggering event occurs. For a deferred trigger, the enable
status is checked when the event occurs, not when the trigger function
is actually executed. One can disable or enable a single
trigger specified by name, or all triggers on the table, or only
user triggers (this option excludes internally generated constraint
triggers such as those that are used to implement foreign key
constraints or deferrable uniqueness and exclusion constraints).
Disabling or enabling internally generated constraint triggers
requires superuser privileges; it should be done with caution since
of course the integrity of the constraint cannot be guaranteed if the
triggers are not executed.
</p><p>
The trigger firing mechanism is also affected by the configuration
variable <a class="xref" href="runtime-config-client.html#GUC-SESSION-REPLICATION-ROLE">session_replication_role</a>. Simply enabled
triggers (the default) will fire when the replication role is <span class="quote">“<span class="quote">origin</span>”</span>
(the default) or <span class="quote">“<span class="quote">local</span>”</span>. Triggers configured as <code class="literal">ENABLE
REPLICA</code> will only fire if the session is in <span class="quote">“<span class="quote">replica</span>”</span>
mode, and triggers configured as <code class="literal">ENABLE ALWAYS</code> will
fire regardless of the current replication role.
</p><p>
The effect of this mechanism is that in the default configuration,
triggers do not fire on replicas. This is useful because if a trigger
is used on the origin to propagate data between tables, then the
replication system will also replicate the propagated data, and the
trigger should not fire a second time on the replica, because that would
lead to duplication. However, if a trigger is used for another purpose
such as creating external alerts, then it might be appropriate to set it
to <code class="literal">ENABLE ALWAYS</code> so that it is also fired on
replicas.
</p><p>
This command acquires a <code class="literal">SHARE ROW EXCLUSIVE</code> lock.
</p></dd><dt><span class="term"><code class="literal">DISABLE</code>/<code class="literal">ENABLE [ REPLICA | ALWAYS ] RULE</code></span></dt><dd><p>
These forms configure the firing of rewrite rules belonging to the table.
A disabled rule is still known to the system, but is not applied
during query rewriting. The semantics are as for disabled/enabled
triggers. This configuration is ignored for <code class="literal">ON SELECT</code> rules, which
are always applied in order to keep views working even if the current
session is in a non-default replication role.
</p><p>
The rule firing mechanism is also affected by the configuration variable
<a class="xref" href="runtime-config-client.html#GUC-SESSION-REPLICATION-ROLE">session_replication_role</a>, analogous to triggers as
described above.
</p></dd><dt><span class="term"><code class="literal">DISABLE</code>/<code class="literal">ENABLE ROW LEVEL SECURITY</code></span></dt><dd><p>
These forms control the application of row security policies belonging
to the table. If enabled and no policies exist for the table, then a
default-deny policy is applied. Note that policies can exist for a table
even if row level security is disabled - in this case, the policies will
NOT be applied and the policies will be ignored.
See also
<a class="xref" href="sql-createpolicy.html" title="CREATE POLICY"><span class="refentrytitle">CREATE POLICY</span></a>.
</p></dd><dt><span class="term"><code class="literal">NO FORCE</code>/<code class="literal">FORCE ROW LEVEL SECURITY</code></span></dt><dd><p>
These forms control the application of row security policies belonging
to the table when the user is the table owner. If enabled, row level
security policies will be applied when the user is the table owner. If
disabled (the default) then row level security will not be applied when
the user is the table owner.
See also
<a class="xref" href="sql-createpolicy.html" title="CREATE POLICY"><span class="refentrytitle">CREATE POLICY</span></a>.
</p></dd><dt><span class="term"><code class="literal">CLUSTER ON</code></span></dt><dd><p>
This form selects the default index for future
<a class="xref" href="sql-cluster.html" title="CLUSTER"><span class="refentrytitle">CLUSTER</span></a>
operations. It does not actually re-cluster the table.
</p><p>
Changing cluster options acquires a <code class="literal">SHARE UPDATE EXCLUSIVE</code> lock.
</p></dd><dt><span class="term"><code class="literal">SET WITHOUT CLUSTER</code></span></dt><dd><p>
This form removes the most recently used
<a class="xref" href="sql-cluster.html" title="CLUSTER"><span class="refentrytitle">CLUSTER</span></a>
index specification from the table. This affects
future cluster operations that don't specify an index.
</p><p>
Changing cluster options acquires a <code class="literal">SHARE UPDATE EXCLUSIVE</code> lock.
</p></dd><dt><span class="term"><code class="literal">SET WITH OIDS</code></span></dt><dd><p>
This form adds an <code class="literal">oid</code> system column to the
table (see <a class="xref" href="ddl-system-columns.html" title="5.4. System Columns">Section 5.4</a>).
It does nothing if the table already has OIDs.
</p><p>
Note that this is not equivalent to <code class="literal">ADD COLUMN oid oid</code>;
that would add a normal column that happened to be named
<code class="literal">oid</code>, not a system column.
</p></dd><dt><span class="term"><code class="literal">SET WITHOUT OIDS</code></span></dt><dd><p>
This form removes the <code class="literal">oid</code> system column from the
table. This is exactly equivalent to
<code class="literal">DROP COLUMN oid RESTRICT</code>,
except that it will not complain if there is already no
<code class="literal">oid</code> column.
</p></dd><dt><span class="term"><code class="literal">SET TABLESPACE</code></span></dt><dd><p>
This form changes the table's tablespace to the specified tablespace and
moves the data file(s) associated with the table to the new tablespace.
Indexes on the table, if any, are not moved; but they can be moved
separately with additional <code class="literal">SET TABLESPACE</code> commands.
All tables in the current database in a tablespace can be moved by using
the <code class="literal">ALL IN TABLESPACE</code> form, which will lock all tables
to be moved first and then move each one. This form also supports
<code class="literal">OWNED BY</code>, which will only move tables owned by the
roles specified. If the <code class="literal">NOWAIT</code> option is specified
then the command will fail if it is unable to acquire all of the locks
required immediately. Note that system catalogs are not moved by this
command, use <code class="command">ALTER DATABASE</code> or explicit
<code class="command">ALTER TABLE</code> invocations instead if desired. The
<code class="literal">information_schema</code> relations are not considered part
of the system catalogs and will be moved.
See also
<a class="xref" href="sql-createtablespace.html" title="CREATE TABLESPACE"><span class="refentrytitle">CREATE TABLESPACE</span></a>.
</p></dd><dt><span class="term"><code class="literal">SET { LOGGED | UNLOGGED }</code></span></dt><dd><p>
This form changes the table from unlogged to logged or vice-versa
(see <a class="xref" href="sql-createtable.html#SQL-CREATETABLE-UNLOGGED"><code class="literal">UNLOGGED</code></a>). It cannot be applied
to a temporary table.
</p></dd><dt><span class="term"><code class="literal">SET ( <em class="replaceable"><code>storage_parameter</code></em> = <em class="replaceable"><code>value</code></em> [, ... ] )</code></span></dt><dd><p>
This form changes one or more storage parameters for the table. See
<a class="xref" href="sql-createtable.html#SQL-CREATETABLE-STORAGE-PARAMETERS" title="Storage Parameters">Storage Parameters</a>
for details on the available parameters. Note that the table contents
will not be modified immediately by this command; depending on the
parameter you might need to rewrite the table to get the desired effects.
That can be done with <a class="link" href="sql-vacuum.html" title="VACUUM">VACUUM
FULL</a>, <a class="xref" href="sql-cluster.html" title="CLUSTER"><span class="refentrytitle">CLUSTER</span></a> or one of the forms
of <code class="command">ALTER TABLE</code> that forces a table rewrite.
For planner related parameters, changes will take effect from the next
time the table is locked so currently executing queries will not be
affected.
</p><p>
<code class="literal">SHARE UPDATE EXCLUSIVE</code> lock will be taken for
fillfactor, toast and autovacuum storage parameters, as well as the
following planner related parameters:
<code class="varname">effective_io_concurrency</code>, <code class="varname">parallel_workers</code>, <code class="varname">seq_page_cost</code>,
<code class="varname">random_page_cost</code>, <code class="varname">n_distinct</code> and <code class="varname">n_distinct_inherited</code>.
</p><div class="note"><h3 class="title">Note</h3><p>
While <code class="command">CREATE TABLE</code> allows <code class="literal">OIDS</code> to be specified
in the <code class="literal">WITH (<em class="replaceable"><code>storage_parameter</code></em>)</code> syntax,
<code class="command">ALTER TABLE</code> does not treat <code class="literal">OIDS</code> as a
storage parameter. Instead use the <code class="literal">SET WITH OIDS</code>
and <code class="literal">SET WITHOUT OIDS</code> forms to change OID status.
</p></div></dd><dt><span class="term"><code class="literal">RESET ( <em class="replaceable"><code>storage_parameter</code></em> [, ... ] )</code></span></dt><dd><p>
This form resets one or more storage parameters to their
defaults. As with <code class="literal">SET</code>, a table rewrite might be
needed to update the table entirely.
</p></dd><dt><span class="term"><code class="literal">INHERIT <em class="replaceable"><code>parent_table</code></em></code></span></dt><dd><p>
This form adds the target table as a new child of the specified parent
table. Subsequently, queries against the parent will include records
of the target table. To be added as a child, the target table must
already contain all the same columns as the parent (it could have
additional columns, too). The columns must have matching data types,
and if they have <code class="literal">NOT NULL</code> constraints in the parent
then they must also have <code class="literal">NOT NULL</code> constraints in the
child.
</p><p>
There must also be matching child-table constraints for all
<code class="literal">CHECK</code> constraints of the parent, except those
marked non-inheritable (that is, created with <code class="literal">ALTER TABLE ... ADD CONSTRAINT ... NO INHERIT</code>)
in the parent, which are ignored; all child-table constraints matched
must not be marked non-inheritable.
Currently
<code class="literal">UNIQUE</code>, <code class="literal">PRIMARY KEY</code>, and
<code class="literal">FOREIGN KEY</code> constraints are not considered, but
this might change in the future.
</p></dd><dt><span class="term"><code class="literal">NO INHERIT <em class="replaceable"><code>parent_table</code></em></code></span></dt><dd><p>
This form removes the target table from the list of children of the
specified parent table.
Queries against the parent table will no longer include records drawn
from the target table.
</p></dd><dt><span class="term"><code class="literal">OF <em class="replaceable"><code>type_name</code></em></code></span></dt><dd><p>
This form links the table to a composite type as though <code class="command">CREATE
TABLE OF</code> had formed it. The table's list of column names and types
must precisely match that of the composite type; the presence of
an <code class="literal">oid</code> system column is permitted to differ. The table must
not inherit from any other table. These restrictions ensure
that <code class="command">CREATE TABLE OF</code> would permit an equivalent table
definition.
</p></dd><dt><span class="term"><code class="literal">NOT OF</code></span></dt><dd><p>
This form dissociates a typed table from its type.
</p></dd><dt><span class="term"><code class="literal">OWNER TO</code></span></dt><dd><p>
This form changes the owner of the table, sequence, view, materialized view,
or foreign table to the specified user.
</p></dd><dt id="SQL-CREATETABLE-REPLICA-IDENTITY"><span class="term"><code class="literal">REPLICA IDENTITY</code></span></dt><dd><p>
This form changes the information which is written to the write-ahead log
to identify rows which are updated or deleted. This option has no effect
except when logical replication is in use. <code class="literal">DEFAULT</code>
(the default for non-system tables) records the
old values of the columns of the primary key, if any. <code class="literal">USING INDEX</code>
records the old values of the columns covered by the named index, which
must be unique, not partial, not deferrable, and include only columns marked
<code class="literal">NOT NULL</code>. <code class="literal">FULL</code> records the old values of all columns
in the row. <code class="literal">NOTHING</code> records no information about the old row.
(This is the default for system tables.)
In all cases, no old values are logged unless at least one of the columns
that would be logged differs between the old and new versions of the row.
</p></dd><dt><span class="term"><code class="literal">RENAME</code></span></dt><dd><p>
The <code class="literal">RENAME</code> forms change the name of a table
(or an index, sequence, view, materialized view, or foreign table), the
name of an individual column in a table, or the name of a constraint of
the table. When renaming a constraint that has an underlying index,
the index is renamed as well.
There is no effect on the stored data.
</p></dd><dt><span class="term"><code class="literal">SET SCHEMA</code></span></dt><dd><p>
This form moves the table into another schema. Associated indexes,
constraints, and sequences owned by table columns are moved as well.
</p></dd><dt><span class="term"><code class="literal">ATTACH PARTITION <em class="replaceable"><code>partition_name</code></em> { FOR VALUES <em class="replaceable"><code>partition_bound_spec</code></em> | DEFAULT }</code></span></dt><dd><p>
This form attaches an existing table (which might itself be partitioned)
as a partition of the target table. The table can be attached
as a partition for specific values using <code class="literal">FOR VALUES</code>
or as a default partition by using <code class="literal">DEFAULT</code>.
For each index in the target table, a corresponding
one will be created in the attached table; or, if an equivalent
index already exists, will be attached to the target table's index,
as if <code class="command">ALTER INDEX ATTACH PARTITION</code> had been executed.
Note that if the existing table is a foreign table, it is currently not
allowed to attach the table as a partition of the target table if there
are <code class="literal">UNIQUE</code> indexes on the target table. (See also
<a class="xref" href="sql-createforeigntable.html" title="CREATE FOREIGN TABLE"><span class="refentrytitle">CREATE FOREIGN TABLE</span></a>.)
</p><p>
A partition using <code class="literal">FOR VALUES</code> uses same syntax for
<em class="replaceable"><code>partition_bound_spec</code></em> as
<a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>. The partition bound specification
must correspond to the partitioning strategy and partition key of the
target table. The table to be attached must have all the same columns
as the target table and no more; moreover, the column types must also
match. Also, it must have all the <code class="literal">NOT NULL</code> and
<code class="literal">CHECK</code> constraints of the target table. Currently
<code class="literal">FOREIGN KEY</code> constraints are not considered.
<code class="literal">UNIQUE</code> and <code class="literal">PRIMARY KEY</code> constraints
from the parent table will be created in the partition, if they don't
already exist.
If any of the <code class="literal">CHECK</code> constraints of the table being
attached is marked <code class="literal">NO INHERIT</code>, the command will fail;
such a constraint must be recreated without the <code class="literal">NO INHERIT</code>
clause.
</p><p>
If the new partition is a regular table, a full table scan is performed
to check that no existing row in the table violates the partition
constraint. It is possible to avoid this scan by adding a valid
<code class="literal">CHECK</code> constraint to the table that would allow only
the rows satisfying the desired partition constraint before running this
command. It will be determined using such a constraint that the table
need not be scanned to validate the partition constraint. This does not
work, however, if any of the partition keys is an expression and the
partition does not accept <code class="literal">NULL</code> values. If attaching
a list partition that will not accept <code class="literal">NULL</code> values,
also add <code class="literal">NOT NULL</code> constraint to the partition key
column, unless it's an expression.
</p><p>
If the new partition is a foreign table, nothing is done to verify
that all the rows in the foreign table obey the partition constraint.
(See the discussion in <a class="xref" href="sql-createforeigntable.html" title="CREATE FOREIGN TABLE"><span class="refentrytitle">CREATE FOREIGN TABLE</span></a> about
constraints on the foreign table.)
</p><p>
When a table has a default partition, defining a new partition changes
the partition constraint for the default partition. The default
partition can't contain any rows that would need to be moved to the new
partition, and will be scanned to verify that none are present. This
scan, like the scan of the new partition, can be avoided if an
appropriate <code class="literal">CHECK</code> constraint is present. Also like
the scan of the new partition, it is always skipped when the default
partition is a foreign table.
</p></dd><dt><span class="term"><code class="literal">DETACH PARTITION</code> <em class="replaceable"><code>partition_name</code></em></span></dt><dd><p>
This form detaches specified partition of the target table. The detached
partition continues to exist as a standalone table, but no longer has any
ties to the table from which it was detached. Any indexes that were
attached to the target table's indexes are detached.
</p></dd></dl></div><p>
</p><p>
All the forms of ALTER TABLE that act on a single table, except
<code class="literal">RENAME</code>, <code class="literal">SET SCHEMA</code>,
<code class="literal">ATTACH PARTITION</code>, and
<code class="literal">DETACH PARTITION</code> can be combined into
a list of multiple alterations to be applied together. For example, it
is possible to add several columns and/or alter the type of several
columns in a single command. This is particularly useful with large
tables, since only one pass over the table need be made.
</p><p>
You must own the table to use <code class="command">ALTER TABLE</code>.
To change the schema or tablespace of a table, you must also have
<code class="literal">CREATE</code> privilege on the new schema or tablespace.
To add the table as a new child of a parent table, you must own the parent
table as well. Also, to attach a table as a new partition of the table,
you must own the table being attached.
To alter the owner, you must also be a direct or indirect member of the new
owning role, and that role must have <code class="literal">CREATE</code> privilege on
the table's schema. (These restrictions enforce that altering the owner
doesn't do anything you couldn't do by dropping and recreating the table.
However, a superuser can alter ownership of any table anyway.)
To add a column or alter a column type or use the <code class="literal">OF</code>
clause, you must also have <code class="literal">USAGE</code> privilege on the data
type.
</p></div><div class="refsect1" id="id-1.9.3.35.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">IF EXISTS</code></span></dt><dd><p>
Do not throw an error if the table does not exist. A notice is issued
in this case.
</p></dd><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p>
The name (optionally schema-qualified) of an existing table to
alter. If <code class="literal">ONLY</code> is specified before the table name, only
that table is altered. If <code class="literal">ONLY</code> is not specified, the table
and all its descendant tables (if any) are altered. Optionally,
<code class="literal">*</code> can be specified after the table name to explicitly
indicate that descendant tables are included.
</p></dd><dt><span class="term"><em class="replaceable"><code>column_name</code></em></span></dt><dd><p>
Name of a new or existing column.
</p></dd><dt><span class="term"><em class="replaceable"><code>new_column_name</code></em></span></dt><dd><p>
New name for an existing column.
</p></dd><dt><span class="term"><em class="replaceable"><code>new_name</code></em></span></dt><dd><p>
New name for the table.
</p></dd><dt><span class="term"><em class="replaceable"><code>data_type</code></em></span></dt><dd><p>
Data type of the new column, or new data type for an existing
column.
</p></dd><dt><span class="term"><em class="replaceable"><code>table_constraint</code></em></span></dt><dd><p>
New table constraint for the table.
</p></dd><dt><span class="term"><em class="replaceable"><code>constraint_name</code></em></span></dt><dd><p>
Name of a new or existing constraint.
</p></dd><dt><span class="term"><code class="literal">CASCADE</code></span></dt><dd><p>
Automatically drop objects that depend on the dropped column
or constraint (for example, views referencing the column),
and in turn all objects that depend on those objects
(see <a class="xref" href="ddl-depend.html" title="5.13. Dependency Tracking">Section 5.13</a>).
</p></dd><dt><span class="term"><code class="literal">RESTRICT</code></span></dt><dd><p>
Refuse to drop the column or constraint if there are any dependent
objects. This is the default behavior.
</p></dd><dt><span class="term"><em class="replaceable"><code>trigger_name</code></em></span></dt><dd><p>
Name of a single trigger to disable or enable.
</p></dd><dt><span class="term"><code class="literal">ALL</code></span></dt><dd><p>
Disable or enable all triggers belonging to the table.
(This requires superuser privilege if any of the triggers are
internally generated constraint triggers such as those that are used
to implement foreign key constraints or deferrable uniqueness and
exclusion constraints.)
</p></dd><dt><span class="term"><code class="literal">USER</code></span></dt><dd><p>
Disable or enable all triggers belonging to the table except for
internally generated constraint triggers such as those that are used
to implement foreign key constraints or deferrable uniqueness and
exclusion constraints.
</p></dd><dt><span class="term"><em class="replaceable"><code>index_name</code></em></span></dt><dd><p>
The name of an existing index.
</p></dd><dt><span class="term"><em class="replaceable"><code>storage_parameter</code></em></span></dt><dd><p>
The name of a table storage parameter.
</p></dd><dt><span class="term"><em class="replaceable"><code>value</code></em></span></dt><dd><p>
The new value for a table storage parameter.
This might be a number or a word depending on the parameter.
</p></dd><dt><span class="term"><em class="replaceable"><code>parent_table</code></em></span></dt><dd><p>
A parent table to associate or de-associate with this table.
</p></dd><dt><span class="term"><em class="replaceable"><code>new_owner</code></em></span></dt><dd><p>
The user name of the new owner of the table.
</p></dd><dt><span class="term"><em class="replaceable"><code>new_tablespace</code></em></span></dt><dd><p>
The name of the tablespace to which the table will be moved.
</p></dd><dt><span class="term"><em class="replaceable"><code>new_schema</code></em></span></dt><dd><p>
The name of the schema to which the table will be moved.
</p></dd><dt><span class="term"><em class="replaceable"><code>partition_name</code></em></span></dt><dd><p>
The name of the table to attach as a new partition or to detach from this table.
</p></dd><dt><span class="term"><em class="replaceable"><code>partition_bound_spec</code></em></span></dt><dd><p>
The partition bound specification for a new partition. Refer to
<a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> for more details on the syntax of the same.
</p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.35.7"><h2>Notes</h2><p>
The key word <code class="literal">COLUMN</code> is noise and can be omitted.
</p><p>
When a column is added with <code class="literal">ADD COLUMN</code> and a
non-volatile <code class="literal">DEFAULT</code> is specified, the default is
evaluated at the time of the statement and the result stored in the
table's metadata. That value will be used for the column for all existing
rows. If no <code class="literal">DEFAULT</code> is specified, NULL is used. In
neither case is a rewrite of the table required.
</p><p>
Adding a column with a volatile <code class="literal">DEFAULT</code> 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 <code class="literal">USING</code> 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 <code class="literal">oid</code> column also requires
rewriting the entire table. Table and/or index rebuilds may take a
significant amount of time for a large table; and will temporarily require
as much as double the disk space.
</p><p>
Adding a <code class="literal">CHECK</code> or <code class="literal">NOT NULL</code> constraint requires
scanning the table to verify that existing rows meet the constraint,
but does not require a table rewrite.
</p><p>
Similarly, when attaching a new partition it may be scanned to verify that
existing rows meet the partition constraint.
</p><p>
The main reason for providing the option to specify multiple changes
in a single <code class="command">ALTER TABLE</code> is that multiple table scans or
rewrites can thereby be combined into a single pass over the table.
</p><p>
The <code class="literal">DROP COLUMN</code> 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. (These statements do
not apply when dropping the system <code class="literal">oid</code> column; that is done
with an immediate rewrite.)
</p><p>
To force immediate reclamation of space occupied by a dropped column,
you can execute one of the forms of <code class="command">ALTER TABLE</code> that
performs a rewrite of the whole table. This results in reconstructing
each row with the dropped column replaced by a null value.
</p><p>
The rewriting forms of <code class="command">ALTER TABLE</code> are not MVCC-safe.
After a table rewrite, the table will appear empty to concurrent
transactions, if they are using a snapshot taken before the rewrite
occurred. See <a class="xref" href="mvcc-caveats.html" title="13.5. Caveats">Section 13.5</a> for more details.
</p><p>
The <code class="literal">USING</code> option of <code class="literal">SET DATA TYPE</code> can actually
specify any expression involving the old values of the row; that is, it
can refer to other columns as well as the one being converted. This allows
very general conversions to be done with the <code class="literal">SET DATA TYPE</code>
syntax. Because of this flexibility, the <code class="literal">USING</code>
expression is not applied to the column's default value (if any); the
result might not be a constant expression as required for a default.
This means that when there is no implicit or assignment cast from old to
new type, <code class="literal">SET DATA TYPE</code> might fail to convert the default even
though a <code class="literal">USING</code> clause is supplied. In such cases,
drop the default with <code class="literal">DROP DEFAULT</code>, perform the <code class="literal">ALTER
TYPE</code>, and then use <code class="literal">SET DEFAULT</code> to add a suitable new
default. Similar considerations apply to indexes and constraints involving
the column.
</p><p>
If a table has any descendant tables, it is not permitted to add,
rename, or change the type of a column in the parent table without doing
the same to the descendants. This ensures that the descendants always
have columns matching the parent. Similarly, a <code class="literal">CHECK</code>
constraint cannot be renamed in the parent without also renaming it in
all descendants, so that <code class="literal">CHECK</code> constraints also match
between the parent and its descendants. (That restriction does not apply
to index-based constraints, however.)
Also, because selecting from the parent also selects from its descendants,
a constraint on the parent cannot be marked valid unless it is also marked
valid for those descendants. In all of these cases, <code class="command">ALTER TABLE
ONLY</code> will be rejected.
</p><p>
A recursive <code class="literal">DROP COLUMN</code> 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 <code class="literal">DROP
COLUMN</code> (i.e., <code class="command">ALTER TABLE ONLY ... DROP
COLUMN</code>) never removes any descendant columns, but
instead marks them as independently defined rather than inherited.
A nonrecursive <code class="literal">DROP COLUMN</code> command will fail for a
partitioned table, because all partitions of a table must have the same
columns as the partitioning root.
</p><p>
The actions for identity columns (<code class="literal">ADD
GENERATED</code>, <code class="literal">SET</code> etc., <code class="literal">DROP
IDENTITY</code>), as well as the actions
<code class="literal">TRIGGER</code>, <code class="literal">CLUSTER</code>, <code class="literal">OWNER</code>,
and <code class="literal">TABLESPACE</code> never recurse to descendant tables;
that is, they always act as though <code class="literal">ONLY</code> were specified.
Adding a constraint recurses only for <code class="literal">CHECK</code> constraints
that are not marked <code class="literal">NO INHERIT</code>.
</p><p>
Changing any part of a system catalog table is not permitted.
</p><p>
Refer to <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> for a further description of valid
parameters. <a class="xref" href="ddl.html" title="Chapter 5. Data Definition">Chapter 5</a> has further information on
inheritance.
</p></div><div class="refsect1" id="id-1.9.3.35.8"><h2>Examples</h2><p>
To add a column of type <code class="type">varchar</code> to a table:
</p><pre class="programlisting">
ALTER TABLE distributors ADD COLUMN address varchar(30);
</pre><p>
</p><p>
To drop a column from a table:
</p><pre class="programlisting">
ALTER TABLE distributors DROP COLUMN address RESTRICT;
</pre><p>
</p><p>
To change the types of two existing columns in one operation:
</p><pre class="programlisting">
ALTER TABLE distributors
ALTER COLUMN address TYPE varchar(80),
ALTER COLUMN name TYPE varchar(100);
</pre><p>
</p><p>
To change an integer column containing Unix timestamps to <code class="type">timestamp
with time zone</code> via a <code class="literal">USING</code> clause:
</p><pre class="programlisting">
ALTER TABLE foo
ALTER COLUMN foo_timestamp SET DATA TYPE timestamp with time zone
USING
timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';
</pre><p>
</p><p>
The same, when the column has a default expression that won't automatically
cast to the new data type:
</p><pre class="programlisting">
ALTER TABLE foo
ALTER COLUMN foo_timestamp DROP DEFAULT,
ALTER COLUMN foo_timestamp TYPE timestamp with time zone
USING
timestamp with time zone 'epoch' + foo_timestamp * interval '1 second',
ALTER COLUMN foo_timestamp SET DEFAULT now();
</pre><p>
</p><p>
To rename an existing column:
</p><pre class="programlisting">
ALTER TABLE distributors RENAME COLUMN address TO city;
</pre><p>
</p><p>
To rename an existing table:
</p><pre class="programlisting">
ALTER TABLE distributors RENAME TO suppliers;
</pre><p>
</p><p>
To rename an existing constraint:
</p><pre class="programlisting">
ALTER TABLE distributors RENAME CONSTRAINT zipchk TO zip_check;
</pre><p>
</p><p>
To add a not-null constraint to a column:
</p><pre class="programlisting">
ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;
</pre><p>
To remove a not-null constraint from a column:
</p><pre class="programlisting">
ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;
</pre><p>
</p><p>
To add a check constraint to a table and all its children:
</p><pre class="programlisting">
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
</pre><p>
</p><p>
To add a check constraint only to a table and not to its children:
</p><pre class="programlisting">
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5) NO INHERIT;
</pre><p>
(The check constraint will not be inherited by future children, either.)
</p><p>
To remove a check constraint from a table and all its children:
</p><pre class="programlisting">
ALTER TABLE distributors DROP CONSTRAINT zipchk;
</pre><p>
</p><p>
To remove a check constraint from one table only:
</p><pre class="programlisting">
ALTER TABLE ONLY distributors DROP CONSTRAINT zipchk;
</pre><p>
(The check constraint remains in place for any child tables.)
</p><p>
To add a foreign key constraint to a table:
</p><pre class="programlisting">
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address);
</pre><p>
</p><p>
To add a foreign key constraint to a table with the least impact on other work:
</p><pre class="programlisting">
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) NOT VALID;
ALTER TABLE distributors VALIDATE CONSTRAINT distfk;
</pre><p>
</p><p>
To add a (multicolumn) unique constraint to a table:
</p><pre class="programlisting">
ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);
</pre><p>
</p><p>
To add an automatically named primary key constraint to a table, noting
that a table can only ever have one primary key:
</p><pre class="programlisting">
ALTER TABLE distributors ADD PRIMARY KEY (dist_id);
</pre><p>
</p><p>
To move a table to a different tablespace:
</p><pre class="programlisting">
ALTER TABLE distributors SET TABLESPACE fasttablespace;
</pre><p>
</p><p>
To move a table to a different schema:
</p><pre class="programlisting">
ALTER TABLE myschema.distributors SET SCHEMA yourschema;
</pre><p>
</p><p>
To recreate a primary key constraint, without blocking updates while the
index is rebuilt:
</p><pre class="programlisting">
CREATE UNIQUE INDEX CONCURRENTLY dist_id_temp_idx ON distributors (dist_id);
ALTER TABLE distributors DROP CONSTRAINT distributors_pkey,
ADD CONSTRAINT distributors_pkey PRIMARY KEY USING INDEX dist_id_temp_idx;
</pre><p>
To attach a partition to a range-partitioned table:
</p><pre class="programlisting">
ALTER TABLE measurement
ATTACH PARTITION measurement_y2016m07 FOR VALUES FROM ('2016-07-01') TO ('2016-08-01');
</pre><p>
To attach a partition to a list-partitioned table:
</p><pre class="programlisting">
ALTER TABLE cities
ATTACH PARTITION cities_ab FOR VALUES IN ('a', 'b');
</pre><p>
To attach a partition to a hash-partitioned table:
</p><pre class="programlisting">
ALTER TABLE orders
ATTACH PARTITION orders_p4 FOR VALUES WITH (MODULUS 4, REMAINDER 3);
</pre><p>
To attach a default partition to a partitioned table:
</p><pre class="programlisting">
ALTER TABLE cities
ATTACH PARTITION cities_partdef DEFAULT;
</pre><p>
To detach a partition from a partitioned table:
</p><pre class="programlisting">
ALTER TABLE measurement
DETACH PARTITION measurement_y2015m12;
</pre></div><div class="refsect1" id="id-1.9.3.35.9"><h2>Compatibility</h2><p>
The forms <code class="literal">ADD</code> (without <code class="literal">USING INDEX</code>),
<code class="literal">DROP [COLUMN]</code>, <code class="literal">DROP IDENTITY</code>, <code class="literal">RESTART</code>,
<code class="literal">SET DEFAULT</code>, <code class="literal">SET DATA TYPE</code> (without <code class="literal">USING</code>),
<code class="literal">SET GENERATED</code>, and <code class="literal">SET <em class="replaceable"><code>sequence_option</code></em></code>
conform with the SQL standard. The other forms are
<span class="productname">PostgreSQL</span> extensions of the SQL standard.
Also, the ability to specify more than one manipulation in a single
<code class="command">ALTER TABLE</code> command is an extension.
</p><p>
<code class="command">ALTER TABLE DROP COLUMN</code> 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.
</p></div><div class="refsect1" id="id-1.9.3.35.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-altersystem.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-altertablespace.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">ALTER SYSTEM </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> ALTER TABLESPACE</td></tr></table></div></body></html>
