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Implement UPDATE tab SET (col1,col2,...) = (SELECT ...), ...

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This SQL-standard feature allows a sub-SELECT yielding multiple columns
(but only one row) to be used to compute the new values of several columns
to be updated.  While the same results can be had with an independent
sub-SELECT per column, such a workaround can require a great deal of
duplicated computation.

The standard actually says that the source for a multi-column assignment
could be any row-valued expression.  The implementation used here is
tightly tied to our existing sub-SELECT support and can't handle other
cases; the Bison grammar would have some issues with them too.  However,
I don't feel too bad about this since other cases can be converted into
sub-SELECTs.  For instance, "SET (a,b,c) = row_valued_function(x)" could
be written "SET (a,b,c) = (SELECT * FROM row_valued_function(x))".
This commit is contained in:
Tom Lane
2014-06-18 13:22:25 -04:00
parent 230ba02d85
commit 8f889b1083
31 changed files with 805 additions and 122 deletions
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74
doc/src/sgml/ref/update.sgml
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@@ -24,7 +24,9 @@ PostgreSQL documentation
[ 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> [, ...] ) = ( { <replaceable class="PARAMETER">expression</replaceable> | DEFAULT } [, ...] ) } [, ...]
( <replaceable class="PARAMETER">column_name</replaceable> [, ...] ) = ( { <replaceable class="PARAMETER">expression</replaceable> | DEFAULT } [, ...] ) |
( <replaceable class="PARAMETER">column_name</replaceable> [, ...] ) = ( <replaceable class="PARAMETER">sub-SELECT</replaceable> )
} [, ...]
[ FROM <replaceable class="PARAMETER">from_list</replaceable> ]
[ WHERE <replaceable class="PARAMETER">condition</replaceable> | WHERE CURRENT OF <replaceable class="PARAMETER">cursor_name</replaceable> ]
[ RETURNING * | <replaceable class="parameter">output_expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...] ]
@@ -146,6 +148,21 @@ UPDATE [ ONLY ] <replaceable class="PARAMETER">table_name</replaceable> [ * ] [
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">sub-SELECT</replaceable></term>
<listitem>
<para>
A <literal>SELECT</> 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_list</replaceable></term>
<listitem>
@@ -324,6 +341,38 @@ UPDATE employees SET sales_count = sales_count + 1 WHERE id =
</programlisting>
</para>
<para>
Update contact names in an accounts table to match the currently assigned
salesmen:
<programlisting>
UPDATE accounts SET (contact_first_name, contact_last_name) =
(SELECT first_name, last_name FROM salesmen
WHERE salesmen.id = accounts.sales_id);
</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 salesmen WHERE salesmen.id = accounts.sales_id;
</programlisting>
However, the second query may give unexpected results
if <structname>salesmen</>.<structfield>id</> is not a unique key, whereas
the first query is guaranteed to raise an error if there are multiple
<structfield>id</> matches. Also, if there is no match for a particular
<structname>accounts</>.<structfield>sales_id</> 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
@@ -361,19 +410,6 @@ UPDATE films SET kind = 'Dramatic' WHERE CURRENT OF c_films;
to use <literal>WITH</> with <command>UPDATE</>.
</para>
<para>
According to the standard, the column-list syntax should allow a list
of columns to be assigned from a single row-valued expression, such
as a sub-select:
<programlisting>
UPDATE accounts SET (contact_last_name, contact_first_name) =
(SELECT last_name, first_name FROM salesmen
WHERE salesmen.id = accounts.sales_id);
</programlisting>
This is not currently implemented &mdash; the source must be a list
of independent expressions.
</para>
<para>
Some other database systems offer a <literal>FROM</> option in which
the target table is supposed to be listed again within <literal>FROM</>.
@@ -381,5 +417,15 @@ UPDATE accounts SET (contact_last_name, contact_first_name) =
<literal>FROM</>. Be careful when porting applications that use this
extension.
</para>
<para>
According to the standard, the source value for a parenthesized sub-list of
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 parenthesized list of expressions (a row constructor) or a
sub-<literal>SELECT</>. An individual column's updated value can be
specified as <literal>DEFAULT</> in the row-constructor case, but not
inside a sub-<literal>SELECT</>.
</para>
</refsect1>
</refentry>