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Allow CHECK constraints to be placed on foreign tables.

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As with NOT NULL constraints, we consider that such constraints are merely
reports of constraints that are being enforced by the remote server (or
other underlying storage mechanism).  Their only real use is to allow
planner optimizations, for example in constraint-exclusion checks.  Thus,
the code changes here amount to little more than removal of the error that
was formerly thrown for applying CHECK to a foreign table.

(In passing, do a bit of cleanup of the ALTER FOREIGN TABLE reference page,
which had accumulated some weird decisions about ordering etc.)

Shigeru Hanada and Etsuro Fujita, reviewed by Kyotaro Horiguchi and
Ashutosh Bapat.
This commit is contained in:
Tom Lane
2014-12-17 17:00:53 -05:00
parent ce01548d4f
commit fc2ac1fb41
11 changed files with 437 additions and 67 deletions
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119
doc/src/sgml/ref/alter_foreign_table.sgml
View File

@@ -42,6 +42,8 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET ( <replaceable class="PARAMETER">attribute_option</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] )
ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> RESET ( <replaceable class="PARAMETER">attribute_option</replaceable> [, ... ] )
ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> OPTIONS ( [ ADD | SET | DROP ] <replaceable class="PARAMETER">option</replaceable> ['<replaceable class="PARAMETER">value</replaceable>'] [, ... ])
ADD <replaceable class="PARAMETER">table_constraint</replaceable>
DROP CONSTRAINT [ IF EXISTS ] <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
DISABLE TRIGGER [ <replaceable class="PARAMETER">trigger_name</replaceable> | ALL | USER ]
ENABLE TRIGGER [ <replaceable class="PARAMETER">trigger_name</replaceable> | ALL | USER ]
ENABLE REPLICA TRIGGER <replaceable class="PARAMETER">trigger_name</replaceable>
@@ -87,16 +89,6 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
</listitem>
</varlistentry>
<varlistentry>
<term><literal>IF EXISTS</literal></term>
<listitem>
<para>
Do not throw an error if the foreign table does not exist. A notice is
issued in this case.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SET DATA TYPE</literal></term>
<listitem>
@@ -152,6 +144,38 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ADD <replaceable class="PARAMETER">table_constraint</replaceable></literal></term>
<listitem>
<para>
This form adds a new constraint to a foreign table, using the same
syntax as <xref linkend="SQL-CREATEFOREIGNTABLE">.
Currently only <literal>CHECK</> constraints are supported.
</para>
<para>
Unlike the case when adding a constraint to a regular table, nothing is
done to verify the constraint is correct; rather, this action simply
declares that some new condition holds for all rows in the foreign
table. (See the discussion in <xref linkend="SQL-CREATEFOREIGNTABLE">.)
Note that constraints on foreign tables cannot be marked
<literal>NOT VALID</> since such constraints are simply declarative.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DROP CONSTRAINT [ IF EXISTS ]</literal></term>
<listitem>
<para>
This form drops the specified constraint on a foreign table.
If <literal>IF EXISTS</literal> is specified and the constraint
does not exist, no error is thrown.
In this case a notice is issued instead.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] TRIGGER</literal></term>
<listitem>
@@ -173,6 +197,21 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
</listitem>
</varlistentry>
<varlistentry>
<term><literal>OPTIONS ( [ ADD | SET | DROP ] <replaceable class="PARAMETER">option</replaceable> ['<replaceable class="PARAMETER">value</replaceable>'] [, ... ] )</literal></term>
<listitem>
<para>
Change options for the foreign table or one of its columns.
<literal>ADD</>, <literal>SET</>, and <literal>DROP</>
specify the action to be performed. <literal>ADD</> is assumed
if no operation is explicitly specified. Duplicate option names are not
allowed (although it's OK for a table option and a column option to have
the same name). Option names and values are also validated using the
foreign data wrapper library.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>RENAME</literal></term>
<listitem>
@@ -192,21 +231,6 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
</listitem>
</varlistentry>
<varlistentry>
<term><literal>OPTIONS ( [ ADD | SET | DROP ] <replaceable class="PARAMETER">option</replaceable> ['<replaceable class="PARAMETER">value</replaceable>'] [, ... ] )</literal></term>
<listitem>
<para>
Change options for the foreign table or one of its columns.
<literal>ADD</>, <literal>SET</>, and <literal>DROP</>
specify the action to be performed. <literal>ADD</> is assumed
if no operation is explicitly specified. Duplicate option names are not
allowed (although it's OK for a table option and a column option to have
the same name). Option names and values are also validated using the
foreign data wrapper library.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
@@ -218,6 +242,12 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
columns in a single command.
</para>
<para>
If the command is written as <literal>ALTER FOREIGN TABLE IF EXISTS ...</>
and the foreign table does not exist, no error is thrown. A notice is
issued in this case.
</para>
<para>
You must own the table to use <command>ALTER FOREIGN TABLE</>.
To change the schema of a foreign table, you must also have
@@ -284,12 +314,30 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">table_constraint</replaceable></term>
<listitem>
<para>
New table constraint for the foreign table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">constraint_name</replaceable></term>
<listitem>
<para>
Name of an existing constraint to drop.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CASCADE</literal></term>
<listitem>
<para>
Automatically drop objects that depend on the dropped column
(for example, views referencing the column).
or constraint (for example, views referencing the column).
</para>
</listitem>
</varlistentry>
@@ -298,7 +346,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
<term><literal>RESTRICT</literal></term>
<listitem>
<para>
Refuse to drop the column if there are any dependent
Refuse to drop the column or constraint if there are any dependent
objects. This is the default behavior.
</para>
</listitem>
@@ -365,10 +413,10 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
<para>
Consistency with the foreign server is not checked when a column is added
or removed with <literal>ADD COLUMN</literal> or
<literal>DROP COLUMN</literal>, a <literal>NOT NULL</> constraint is
added, or a column type is changed with <literal>SET DATA TYPE</>. It is
the user's responsibility to ensure that the table definition matches the
remote side.
<literal>DROP COLUMN</literal>, a <literal>NOT NULL</>
or <literal>CHECK</> constraint is added, or a column type is changed
with <literal>SET DATA TYPE</>. It is the user's responsibility to ensure
that the table definition matches the remote side.
</para>
<para>
@@ -413,4 +461,13 @@ ALTER FOREIGN TABLE myschema.distributors OPTIONS (ADD opt1 'value', SET opt2, '
extension of SQL, which disallows zero-column foreign tables.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-createforeigntable"></member>
<member><xref linkend="sql-dropforeigntable"></member>
</simplelist>
</refsect1>
</refentry>

65
doc/src/sgml/ref/create_foreign_table.sgml
View File

@@ -19,7 +19,8 @@
<refsynopsisdiv>
<synopsis>
CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="PARAMETER">table_name</replaceable> ( [
<replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ OPTIONS ( <replaceable class="PARAMETER">option</replaceable> '<replaceable class="PARAMETER">value</replaceable>' [, ... ] ) ] [ COLLATE <replaceable>collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
{ <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ OPTIONS ( <replaceable class="PARAMETER">option</replaceable> '<replaceable class="PARAMETER">value</replaceable>' [, ... ] ) ] [ COLLATE <replaceable>collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
| <replaceable>table_constraint</replaceable> }
[, ... ]
] )
SERVER <replaceable class="parameter">server_name</replaceable>
@@ -30,7 +31,13 @@ CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="PARAMETER">table_name
[ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ]
{ NOT NULL |
NULL |
CHECK ( <replaceable class="PARAMETER">expression</replaceable> ) |
DEFAULT <replaceable>default_expr</replaceable> }
<phrase>and <replaceable class="PARAMETER">table_constraint</replaceable> is:</phrase>
[ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ]
CHECK ( <replaceable class="PARAMETER">expression</replaceable> )
</synopsis>
</refsynopsisdiv>
@@ -137,6 +144,28 @@ CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="PARAMETER">table_name
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CHECK ( <replaceable class="PARAMETER">expression</replaceable> )</literal></term>
<listitem>
<para>
The <literal>CHECK</> clause specifies an expression producing a
Boolean result which each row in the foreign table is expected
to satisfy; that is, the expression should produce TRUE or UNKNOWN,
never FALSE, for all rows in the foreign table.
A check constraint specified as a column constraint should
reference that column's value only, while an expression
appearing in a table constraint can reference multiple columns.
</para>
<para>
Currently, <literal>CHECK</literal> expressions cannot contain
subqueries nor refer to variables other than columns of the
current row. The system column <literal>tableoid</literal>
may be referenced, but not any other system column.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DEFAULT
<replaceable>default_expr</replaceable></literal></term>
@@ -187,6 +216,40 @@ CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="PARAMETER">table_name
</refsect1>
<refsect1>
<title>Notes</title>
<para>
Constraints on foreign tables (such as <literal>CHECK</>
or <literal>NOT NULL</> clauses) are not enforced by the
core <productname>PostgreSQL</> system, and most foreign data wrappers
do not attempt to enforce them either; that is, the constraint is
simply assumed to hold true. There would be little point in such
enforcement since it would only apply to rows inserted or updated via
the foreign table, and not to rows modified by other means, such as
directly on the remote server. Instead, a constraint attached to a
foreign table should represent a constraint that is being enforced by
the remote server.
</para>
<para>
Some special-purpose foreign data wrappers might be the only access
mechanism for the data they access, and in that case it might be
appropriate for the foreign data wrapper itself to perform constraint
enforcement. But you should not assume that a wrapper does that
unless its documentation says so.
</para>
<para>
Although <productname>PostgreSQL</> does not attempt to enforce
constraints on foreign tables, it does assume that they are correct
for purposes of query optimization. If there are rows visible in the
foreign table that do not satisfy a declared constraint, queries on
the table might produce incorrect answers. It is the user's
responsibility to ensure that the constraint definition matches
reality.
</para>
</refsect1>
<refsect1 id="SQL-CREATEFOREIGNTABLE-examples">
<title>Examples</title>