Allow default expressions to be attached to columns of foreign tables.

There's still some discussion about exactly how postgres_fdw ought to
handle this case, but there seems no debate that we want to allow defaults
to be used for inserts into foreign tables.  So remove the core-code
restrictions that prevented it.

While at it, get rid of the special grammar productions for CREATE FOREIGN
TABLE, and instead add explicit FEATURE_NOT_SUPPORTED error checks for the
disallowed cases.  This makes the grammar a shade smaller, and more
importantly results in much more intelligible error messages for
unsupported cases.  It's also one less thing to fix if we ever start
supporting constraints on foreign tables.

2 files changed, 48 insertions, 2 deletions

diff --git a/doc/src/sgml/ref/alter_foreign_table.sgml b/doc/src/sgml/ref/alter_foreign_table.sgml
index f0b3129ab09..723aa075c57 100644
--- a/doc/src/sgml/ref/alter_foreign_table.sgml
+++ b/doc/src/sgml/ref/alter_foreign_table.sgml
@@ -32,9 +32,11 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ NULL | NOT NULL ]
+    ADD [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
     DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column_name</replaceable> [ RESTRICT | CASCADE ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable>
+    ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
+    ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> DROP DEFAULT
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> { SET | DROP } NOT NULL
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET STATISTICS <replaceable class="PARAMETER">integer</replaceable>
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET ( <replaceable class="PARAMETER">attribute_option</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] )
@@ -59,6 +61,9 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
      <para>
       This form adds a new column to the foreign table, using the same syntax as
       <xref linkend="SQL-CREATEFOREIGNTABLE">.
+      Unlike the case when adding a column to a regular table, nothing happens
+      to the underlying storage: this action simply declares that
+      some new column is now accessible through the foreign table.
      </para>
     </listitem>
    </varlistentry>
@@ -98,6 +103,18 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceab
    </varlistentry>
 
    <varlistentry>
+    <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
+    <listitem>
+     <para>
+      These forms set or remove the default value for a column.
+      Default values only apply in subsequent <command>INSERT</command>
+      or <command>UPDATE</> commands; they do not cause rows already in the
+      table to change.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><literal>SET</literal>/<literal>DROP NOT NULL</literal></term>
     <listitem>
      <para>
diff --git a/doc/src/sgml/ref/create_foreign_table.sgml b/doc/src/sgml/ref/create_foreign_table.sgml
index 0a6ac29d4d9..1ef4b5e9d74 100644
--- a/doc/src/sgml/ref/create_foreign_table.sgml
+++ b/doc/src/sgml/ref/create_foreign_table.sgml
@@ -19,12 +19,18 @@
  <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>' [, ... ] ) ] [ NULL | NOT NULL ] }
+    <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> [ ... ] ]
     [, ... ]
 ] )
   SERVER <replaceable class="parameter">server_name</replaceable>
 [ OPTIONS ( <replaceable class="PARAMETER">option</replaceable> '<replaceable class="PARAMETER">value</replaceable>' [, ... ] ) ]
 
+<phrase>where <replaceable class="PARAMETER">column_constraint</replaceable> is:</phrase>
+
+[ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ]
+{ NOT NULL |
+  NULL |
+  DEFAULT <replaceable>default_expr</replaceable> }
 </synopsis>
  </refsynopsisdiv>
 
@@ -132,6 +138,27 @@ CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="PARAMETER">table_name
    </varlistentry>
 
    <varlistentry>
+    <term><literal>DEFAULT
+    <replaceable>default_expr</replaceable></literal></term>
+    <listitem>
+     <para>
+      The <literal>DEFAULT</> clause assigns a default data value for
+      the column whose column definition it appears within.  The value
+      is any variable-free expression (subqueries and cross-references
+      to other columns in the current table are not allowed).  The
+      data type of the default expression must match the data type of the
+      column.
+     </para>
+
+     <para>
+      The default expression will be used in any insert operation that
+      does not specify a value for the column.  If there is no default
+      for a column, then the default is null.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><replaceable class="PARAMETER">server_name</replaceable></term>
     <listitem>
      <para>
@@ -190,6 +217,8 @@ SERVER film_server;
    <acronym>SQL</acronym> standard; however, much as with
    <link linkend="sql-createtable"><command>CREATE TABLE</></link>,
    <literal>NULL</> constraints and zero-column foreign tables are permitted.
+   The ability to specify a default value is also a <productname>PostgreSQL</>
+   extension.
   </para>
 
  </refsect1>