<!--
doc/src/sgml/ref/update.sgml
PostgreSQL documentation
-->

<refentry id="sql-update">
 <indexterm zone="sql-update">
  <primary>UPDATE</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>UPDATE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>UPDATE</refname>
  <refpurpose>update rows of a table</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
[ WITH [ RECURSIVE ] <replaceable class="parameter">with_query</replaceable> [, ...] ]
UPDATE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">alias</replaceable> ]
    SET { <replaceable class="parameter">column_name</replaceable> = { <replaceable class="parameter">expression</replaceable> | DEFAULT } |
          ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = [ ROW ] ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) |
          ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = ( <replaceable class="parameter">sub-SELECT</replaceable> )
        } [, ...]
    [ FROM <replaceable class="parameter">from_item</replaceable> [, ...] ]
    [ WHERE <replaceable class="parameter">condition</replaceable> | WHERE CURRENT OF <replaceable class="parameter">cursor_name</replaceable> ]
    [ RETURNING [ WITH ( { OLD | NEW } AS <replaceable class="parameter">output_alias</replaceable> [, ...] ) ]
                { * | <replaceable class="parameter">output_expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] } [, ...] ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>UPDATE</command> changes the values of the specified
   columns in all rows that satisfy the condition. Only the columns to
   be modified need be mentioned in the <literal>SET</literal> clause;
   columns not explicitly modified retain their previous values.
  </para>

  <para>
   There are two ways to modify a table using information contained in
   other tables in the database: using sub-selects, or specifying
   additional tables in the <literal>FROM</literal> clause. Which
   technique is more appropriate depends on the specific
   circumstances.
  </para>

  <para>
   The optional <literal>RETURNING</literal> clause causes <command>UPDATE</command>
   to compute and return value(s) based on each row actually updated.
   Any expression using the table's columns, and/or columns of other
   tables mentioned in <literal>FROM</literal>, can be computed.
   By default, the new (post-update) values of the table's columns are used,
   but it is also possible to request the old (pre-update) values.
   The syntax of the <literal>RETURNING</literal> list is identical to that of the
   output list of <command>SELECT</command>.
  </para>

  <para>
   You must have the <literal>UPDATE</literal> privilege on the table,
   or at least on the column(s) that are listed to be updated.
   You must also have the <literal>SELECT</literal>
   privilege on any column whose values are read in the
   <replaceable class="parameter">expressions</replaceable> or
   <replaceable class="parameter">condition</replaceable>.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">with_query</replaceable></term>
    <listitem>
     <para>
      The <literal>WITH</literal> clause allows you to specify one or more
      subqueries that can be referenced by name in the <command>UPDATE</command>
      query. See <xref linkend="queries-with"/> and <xref linkend="sql-select"/>
      for details.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the table to update.
      If <literal>ONLY</literal> is specified before the table name, matching rows
      are updated in the named table only.  If <literal>ONLY</literal> is not
      specified, matching rows are also updated in any tables inheriting from
      the named table.  Optionally, <literal>*</literal> can be specified after the
      table name to explicitly indicate that descendant tables are included.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">alias</replaceable></term>
    <listitem>
     <para>
      A substitute name for the target table. When an alias is
      provided, it completely hides the actual name of the table.  For
      example, given <literal>UPDATE foo AS f</literal>, the remainder of the
      <command>UPDATE</command> statement must refer to this table as
      <literal>f</literal> not <literal>foo</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">column_name</replaceable></term>
    <listitem>
     <para>
      The name of a column in the table named by <replaceable
      class="parameter">table_name</replaceable>.
      The column name can be qualified with a subfield name or array
      subscript, if needed.  Do not include the table's name in the
      specification of a target column &mdash; for example,
      <literal>UPDATE table_name SET table_name.col = 1</literal> is invalid.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">expression</replaceable></term>
    <listitem>
     <para>
      An expression to assign to the column.  The expression can use the
      old values of this and other columns in the table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEFAULT</literal></term>
    <listitem>
     <para>
      Set the column to its default value (which will be NULL if no specific
      default expression has been assigned to it).  An identity column will be
      set to a new value generated by the associated sequence.  For a
      generated column, specifying this is permitted but merely specifies the
      normal behavior of computing the column from its generation expression.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">sub-SELECT</replaceable></term>
    <listitem>
     <para>
      A <literal>SELECT</literal> sub-query that produces as many output columns
      as are listed in the parenthesized column list preceding it.  The
      sub-query must yield no more than one row when executed.  If it
      yields one row, its column values are assigned to the target columns;
      if it yields no rows, NULL values are assigned to the target columns.
      The sub-query can refer to old values of the current row of the table
      being updated.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">from_item</replaceable></term>
    <listitem>
     <para>
      A table expression allowing columns from other tables to appear in
      the <literal>WHERE</literal> condition and update expressions. This
      uses the same syntax as the <link
      linkend="sql-from"><literal>FROM</literal></link> clause of
      a <command>SELECT</command> statement;
      for example, an alias for the table name can be specified.  Do not
      repeat the target table as a <replaceable>from_item</replaceable>
      unless you intend a self-join (in which case it must appear with
      an alias in the <replaceable>from_item</replaceable>).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">condition</replaceable></term>
    <listitem>
     <para>
      An expression that returns a value of type <type>boolean</type>.
      Only rows for which this expression returns <literal>true</literal>
      will be updated.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">cursor_name</replaceable></term>
    <listitem>
     <para>
      The name of the cursor to use in a <literal>WHERE CURRENT OF</literal>
      condition.  The row to be updated is the one most recently fetched
      from this cursor.  The cursor must be a non-grouping
      query on the <command>UPDATE</command>'s target table.
      Note that <literal>WHERE CURRENT OF</literal> cannot be
      specified together with a Boolean condition.  See
      <xref linkend="sql-declare"/>
      for more information about using cursors with
      <literal>WHERE CURRENT OF</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">output_alias</replaceable></term>
    <listitem>
     <para>
      An optional substitute name for <literal>OLD</literal> or
      <literal>NEW</literal> rows in the <literal>RETURNING</literal> list.
     </para>

     <para>
      By default, old values from the target table can be returned by writing
      <literal>OLD.<replaceable class="parameter">column_name</replaceable></literal>
      or <literal>OLD.*</literal>, and new values can be returned by writing
      <literal>NEW.<replaceable class="parameter">column_name</replaceable></literal>
      or <literal>NEW.*</literal>.  When an alias is provided, these names are
      hidden and the old or new rows must be referred to using the alias.
      For example <literal>RETURNING WITH (OLD AS o, NEW AS n) o.*, n.*</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">output_expression</replaceable></term>
    <listitem>
     <para>
      An expression to be computed and returned by the <command>UPDATE</command>
      command after each row is updated.  The expression can use any
      column names of the table named by <replaceable class="parameter">table_name</replaceable>
      or table(s) listed in <literal>FROM</literal>.
      Write <literal>*</literal> to return all columns.
     </para>

     <para>
      A column name or <literal>*</literal> may be qualified using
      <literal>OLD</literal> or <literal>NEW</literal>, or the corresponding
      <replaceable class="parameter">output_alias</replaceable> for
      <literal>OLD</literal> or <literal>NEW</literal>, to cause old or new
      values to be returned.  An unqualified column name, or
      <literal>*</literal>, or a column name or <literal>*</literal> qualified
      using the target table name or alias will return new values.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">output_name</replaceable></term>
    <listitem>
     <para>
      A name to use for a returned column.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Outputs</title>

  <para>
   On successful completion, an <command>UPDATE</command> command returns a command
   tag of the form
<screen>
UPDATE <replaceable class="parameter">count</replaceable>
</screen>
   The <replaceable class="parameter">count</replaceable> is the number
   of rows updated, including matched rows whose values did not change.
   Note that the number may be less than the number of rows that matched
   the <replaceable class="parameter">condition</replaceable> when
   updates were suppressed by a <literal>BEFORE UPDATE</literal> trigger.  If
   <replaceable class="parameter">count</replaceable> is 0, no rows were
   updated by the query (this is not considered an error).
  </para>

  <para>
   If the <command>UPDATE</command> command contains a <literal>RETURNING</literal>
   clause, the result will be similar to that of a <command>SELECT</command>
   statement containing the columns and values defined in the
   <literal>RETURNING</literal> list, computed over the row(s) updated by the
   command.
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   When a <literal>FROM</literal> clause is present, what essentially happens
   is that the target table is joined to the tables mentioned in the
   <replaceable>from_item</replaceable> list, and each output row of the join
   represents an update operation for the target table.  When using
   <literal>FROM</literal> you should ensure that the join
   produces at most one output row for each row to be modified.  In
   other words, a target row shouldn't join to more than one row from
   the other table(s).  If it does, then only one of the join rows
   will be used to update the target row, but which one will be used
   is not readily predictable.
  </para>

  <para>
   Because of this indeterminacy, referencing other tables only within
   sub-selects is safer, though often harder to read and slower than
   using a join.
  </para>

  <para>
   In the case of a partitioned table, updating a row might cause it to no
   longer satisfy the partition constraint of the containing partition. In that
   case, if there is some other partition in the partition tree for which this
   row satisfies its partition constraint, then the row is moved to that
   partition. If there is no such partition, an error will occur.  Behind the
   scenes, the row movement is actually a <command>DELETE</command> and
   <command>INSERT</command> operation.
  </para>

  <para>
   There is a possibility that a concurrent <command>UPDATE</command> or
   <command>DELETE</command> on the row being moved will get a serialization
   failure error.  Suppose session 1 is performing an <command>UPDATE</command>
   on a partition key, and meanwhile a concurrent session 2 for which this
   row is visible performs an <command>UPDATE</command> or
   <command>DELETE</command> operation on this row.  In such case,
   session 2's <command>UPDATE</command> or <command>DELETE</command> will
   detect the row movement and raise a serialization failure error (which
   always returns with an SQLSTATE code '40001').  Applications may wish to
   retry the transaction if this occurs.  In the usual case where the table
   is not partitioned, or where there is no row movement, session 2 would
   have identified the newly updated row and carried out the
   <command>UPDATE</command>/<command>DELETE</command> on this new row
    version.
  </para>

  <para>
   Note that while rows can be moved from local partitions to a foreign-table
   partition (provided the foreign data wrapper supports tuple routing), they
   cannot be moved from a foreign-table partition to another partition.
  </para>

  <para>
   An attempt of moving a row from one partition to another will fail if a
   foreign key is found to directly reference an ancestor of the source
   partition that is not the same as the ancestor that's mentioned in the
   <command>UPDATE</command> query.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Change the word <literal>Drama</literal> to <literal>Dramatic</literal> in the
   column <structfield>kind</structfield> of the table <structname>films</structname>:

<programlisting>
UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama';
</programlisting>
  </para>

  <para>
   Adjust temperature entries and reset precipitation to its default
   value in one row of the table <structname>weather</structname>:

<programlisting>
UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
  WHERE city = 'San Francisco' AND date = '2003-07-03';
</programlisting>
  </para>

  <para>
   Perform the same operation and return the updated entries, and the old
   precipitation value:

<programlisting>
UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
  WHERE city = 'San Francisco' AND date = '2003-07-03'
  RETURNING temp_lo, temp_hi, prcp, old.prcp AS old_prcp;
</programlisting>
  </para>

  <para>
   Use the alternative column-list syntax to do the same update:
<programlisting>
UPDATE weather SET (temp_lo, temp_hi, prcp) = (temp_lo+1, temp_lo+15, DEFAULT)
  WHERE city = 'San Francisco' AND date = '2003-07-03';
</programlisting>
  </para>

  <para>
   Increment the sales count of the salesperson who manages the
   account for Acme Corporation, using the <literal>FROM</literal>
   clause syntax:
<programlisting>
UPDATE employees SET sales_count = sales_count + 1 FROM accounts
  WHERE accounts.name = 'Acme Corporation'
  AND employees.id = accounts.sales_person;
</programlisting>
  </para>

  <para>
   Perform the same operation, using a sub-select in the
   <literal>WHERE</literal> clause:
<programlisting>
UPDATE employees SET sales_count = sales_count + 1 WHERE id =
  (SELECT sales_person FROM accounts WHERE name = 'Acme Corporation');
</programlisting>
  </para>

  <para>
   Update contact names in an accounts table to match the currently assigned
   salespeople:
<programlisting>
UPDATE accounts SET (contact_first_name, contact_last_name) =
    (SELECT first_name, last_name FROM employees
     WHERE employees.id = accounts.sales_person);
</programlisting>
   A similar result could be accomplished with a join:
<programlisting>
UPDATE accounts SET contact_first_name = first_name,
                    contact_last_name = last_name
  FROM employees WHERE employees.id = accounts.sales_person;
</programlisting>
   However, the second query may give unexpected results
   if <structname>employees</structname>.<structfield>id</structfield> is not a unique key, whereas
   the first query is guaranteed to raise an error if there are multiple
   <structfield>id</structfield> matches.  Also, if there is no match for a particular
   <structname>accounts</structname>.<structfield>sales_person</structfield> entry, the first query
   will set the corresponding name fields to NULL, whereas the second query
   will not update that row at all.
  </para>

  <para>
   Update statistics in a summary table to match the current data:
<programlisting>
UPDATE summary s SET (sum_x, sum_y, avg_x, avg_y) =
    (SELECT sum(x), sum(y), avg(x), avg(y) FROM data d
     WHERE d.group_id = s.group_id);
</programlisting>
  </para>

  <para>
   Attempt to insert a new stock item along with the quantity of stock. If
   the item already exists, instead update the stock count of the existing
   item. To do this without failing the entire transaction, use savepoints:
<programlisting>
BEGIN;
-- other operations
SAVEPOINT sp1;
INSERT INTO wines VALUES('Chateau Lafite 2003', '24');
-- Assume the above fails because of a unique key violation,
-- so now we issue these commands:
ROLLBACK TO sp1;
UPDATE wines SET stock = stock + 24 WHERE winename = 'Chateau Lafite 2003';
-- continue with other operations, and eventually
COMMIT;
</programlisting>
  </para>

  <para>
   Change the <structfield>kind</structfield> column of the table
   <structname>films</structname> in the row on which the cursor
   <literal>c_films</literal> is currently positioned:
<programlisting>
UPDATE films SET kind = 'Dramatic' WHERE CURRENT OF c_films;
</programlisting>
  </para>

  <para id="update-limit">
   Updates affecting many rows can have negative effects on system
   performance, such as table bloat, increased replica lag, and increased
   lock contention.  In such situations it can make sense to perform the
   operation in smaller batches, possibly with a <command>VACUUM</command>
   operation on the table between batches.  While there is
   no <literal>LIMIT</literal> clause for <command>UPDATE</command>, it is
   possible to get a similar effect through the use of
   a <link linkend="queries-with">Common Table Expression</link> and a
   self-join.  With the standard <productname>PostgreSQL</productname>
   table access method, a self-join on the system
   column <link linkend="ddl-system-columns-ctid">ctid</link> is very
   efficient:
<programlisting>
WITH exceeded_max_retries AS (
  SELECT w.ctid FROM work_item AS w
    WHERE w.status = 'active' AND w.num_retries &gt; 10
    ORDER BY w.retry_timestamp
    FOR UPDATE
    LIMIT 5000
)
UPDATE work_item SET status = 'failed'
  FROM exceeded_max_retries AS emr
  WHERE work_item.ctid = emr.ctid;
</programlisting>
   This command will need to be repeated until no rows remain to be updated.
   (This use of <structfield>ctid</structfield> is only safe because
   the query is repeatedly run, avoiding the problem of changed
   <structfield>ctid</structfield>s.)
   Use of an <literal>ORDER BY</literal> clause allows the command to
   prioritize which rows will be updated; it can also prevent deadlock
   with other update operations if they use the same ordering.
   If lock contention is a concern, then <literal>SKIP LOCKED</literal>
   can be added to the <acronym>CTE</acronym> to prevent multiple commands
   from updating the same row.  However, then a
   final <command>UPDATE</command> without <literal>SKIP LOCKED</literal>
   or <literal>LIMIT</literal> will be needed to ensure that no matching
   rows were overlooked.
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   This command conforms to the <acronym>SQL</acronym> standard, except
   that the <literal>FROM</literal> and <literal>RETURNING</literal> clauses
   are <productname>PostgreSQL</productname> extensions, as is the ability
   to use <literal>WITH</literal> with <command>UPDATE</command>.
  </para>

  <para>
   Some other database systems offer a <literal>FROM</literal> option in which
   the target table is supposed to be listed again within <literal>FROM</literal>.
   That is not how <productname>PostgreSQL</productname> interprets
   <literal>FROM</literal>.  Be careful when porting applications that use this
   extension.
  </para>

  <para>
   According to the standard, the source value for a parenthesized sub-list of
   target column names can be any row-valued expression yielding the correct
   number of columns.  <productname>PostgreSQL</productname> only allows the
   source value to be a <link linkend="sql-syntax-row-constructors">row
   constructor</link> or a sub-<literal>SELECT</literal>.  An individual column's
   updated value can be specified as <literal>DEFAULT</literal> in the
   row-constructor case, but not inside a sub-<literal>SELECT</literal>.
  </para>
 </refsect1>
</refentry>
