doc/src/sgml/ref/create_foreign_table.sgml

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doc/src/sgml/ref/create_foreign_table.sgml
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<refentry id="sql-createforeigntable">
 <indexterm zone="sql-createforeigntable">
  <primary>CREATE FOREIGN TABLE</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE FOREIGN TABLE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE FOREIGN TABLE</refname>
  <refpurpose>define a new foreign table</refpurpose>
 </refnamediv>

 <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>table_constraint</replaceable> }
    [, ... ]
] )
[ INHERITS ( <replaceable>parent_table</replaceable> [, ... ] ) ]
  SERVER <replaceable class="parameter">server_name</replaceable>
[ OPTIONS ( [ mpp_execute 'any | master | all segments' ], <replaceable class="parameter">option</replaceable> '<replaceable class="parameter">value</replaceable>' [, ... ] ) ]

CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="parameter">table_name</replaceable>
  PARTITION OF <replaceable class="parameter">parent_table</replaceable> [ (
  { <replaceable class="parameter">column_name</replaceable> [ WITH OPTIONS ] [ <replaceable class="parameter">column_constraint</replaceable> [ ... ] ]
    | <replaceable>table_constraint</replaceable> }
    [, ... ]
) ]
{ FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }
  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 |
  CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
  DEFAULT <replaceable>default_expr</replaceable> |
  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED }

<phrase>and <replaceable class="parameter">table_constraint</replaceable> is:</phrase>

[ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]

<phrase>and <replaceable class="parameter">partition_bound_spec</replaceable> is:</phrase>

IN ( <replaceable class="parameter">partition_bound_expr</replaceable> [, ...] ) |
FROM ( { <replaceable class="parameter">partition_bound_expr</replaceable> | MINVALUE | MAXVALUE } [, ...] )
  TO ( { <replaceable class="parameter">partition_bound_expr</replaceable> | MINVALUE | MAXVALUE } [, ...] ) |
WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REMAINDER <replaceable class="parameter">numeric_literal</replaceable> )
</synopsis>
 </refsynopsisdiv>

 <refsect1 id="sql-createforeigntable-description">
  <title>Description</title>

  <para>
   <command>CREATE FOREIGN TABLE</command> creates a new foreign table
   in the current database. The table will be owned by the user issuing the
   command.
  </para>

  <para>
   If a schema name is given (for example, <literal>CREATE FOREIGN TABLE
   myschema.mytable ...</literal>) then the table is created in the specified
   schema.  Otherwise it is created in the current schema.
   The name of the foreign table must be
   distinct from the name of any other foreign table, table, sequence, index,
   view, or materialized view in the same schema.
  </para>

  <para>
   <command>CREATE FOREIGN TABLE</command> also automatically creates a data
   type that represents the composite type corresponding to one row of
   the foreign table.  Therefore, foreign tables cannot have the same
   name as any existing data type in the same schema.
  </para>

  <para>
   If <literal>PARTITION OF</literal> clause is specified then the table is
   created as a partition of <literal>parent_table</literal> with specified
   bounds.
  </para>

  <para>
   To be able to create a foreign table, you must have <literal>USAGE</literal>
   privilege on the foreign server, as well as <literal>USAGE</literal>
   privilege on all column types used in the table.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>

   <varlistentry>
    <term><literal>IF NOT EXISTS</literal></term>
    <listitem>
     <para>
      Do not throw an error if a relation with the same name already exists.
      A notice is issued in this case.  Note that there is no guarantee that
      the existing relation is anything like the one that would have been
      created.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the table to be created.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">column_name</replaceable></term>
    <listitem>
     <para>
      The name of a column to be created in the new table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">data_type</replaceable></term>
    <listitem>
     <para>
      The data type of the column. This can include array
      specifiers. For more information on the data types supported by
      <productname>PostgreSQL</productname>, refer to <xref
      linkend="datatype"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>COLLATE <replaceable>collation</replaceable></literal></term>
    <listitem>
     <para>
      The <literal>COLLATE</literal> clause assigns a collation to
      the column (which must be of a collatable data type).
      If not specified, the column data type's default collation is used.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>INHERITS ( <replaceable>parent_table</replaceable> [, ... ] )</literal></term>
    <listitem>
     <para>
      The optional <literal>INHERITS</literal> clause specifies a list of
      tables from which the new foreign table automatically inherits
      all columns.  Parent tables can be plain tables or foreign tables.
      See the similar form of
      <link linkend="sql-createtable"><command>CREATE TABLE</command></link> for more details.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>PARTITION OF <replaceable>parent_table</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }</literal></term>
    <listitem>
     <para>
      This form can be used to create the foreign table as partition of
      the given parent table with specified partition bound values.
      See the similar form of
      <link linkend="sql-createtable"><command>CREATE TABLE</command></link> for more details.
      Note that it is currently not allowed to create the foreign table as a
      partition of the parent table if there are <literal>UNIQUE</literal>
      indexes on the parent table.  (See also
      <link linkend="sql-altertable"><command>ALTER TABLE ATTACH PARTITION</command></link>.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CONSTRAINT <replaceable class="parameter">constraint_name</replaceable></literal></term>
    <listitem>
     <para>
      An optional name for a column or table constraint.  If the
      constraint is violated, the constraint name is present in error messages,
      so constraint names like <literal>col must be positive</literal> can be used
      to communicate helpful constraint information to client applications.
      (Double-quotes are needed to specify constraint names that contain spaces.)
      If a constraint name is not specified, the system generates a name.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>NOT NULL</literal></term>
    <listitem>
     <para>
      The column is not allowed to contain null values.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>NULL</literal></term>
    <listitem>
     <para>
      The column is allowed to contain null values. This is the default.
     </para>

     <para>
      This clause is only provided for compatibility with
      non-standard SQL databases.  Its use is discouraged in new
      applications.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] </literal></term>
    <listitem>
     <para>
      The <literal>CHECK</literal> 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>

     <para>
      A constraint marked with <literal>NO INHERIT</literal> will not propagate to
      child tables.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEFAULT
    <replaceable>default_expr</replaceable></literal></term>
    <listitem>
     <para>
      The <literal>DEFAULT</literal> 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><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED</literal><indexterm><primary>generated column</primary></indexterm></term>
    <listitem>
     <para>
      This clause creates the column as a <firstterm>generated
      column</firstterm>.  The column cannot be written to, and when read the
      result of the specified expression will be returned.
     </para>

     <para>
      The keyword <literal>STORED</literal> is required to signify that the
      column will be computed on write.  (The computed value will be presented
      to the foreign-data wrapper for storage and must be returned on
      reading.)
     </para>

     <para>
      The generation expression can refer to other columns in the table, but
      not other generated columns.  Any functions and operators used must be
      immutable.  References to other tables are not allowed.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">server_name</replaceable></term>
    <listitem>
     <para>
      The name of an existing foreign server to use for the foreign table.
      For details on defining a server, see <xref
      linkend="sql-createserver"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>OPTIONS ( [ mpp_execute 'any | master | all segments' ], <replaceable class="parameter">option</replaceable> '<replaceable class="parameter">value</replaceable>' [, ...] )</literal></term>
    <listitem>
     <para>
      Options to be associated with the new foreign table or one of its
      columns.
      The allowed option names and values are specific to each foreign
      data wrapper and are validated using the foreign-data wrapper's
      validator function.  Duplicate option names are not allowed (although
      it's OK for a table option and a column option to have the same name).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">mpp_execute</replaceable></term>
    <listitem>
     <para>
     MASTER, FDW requests for data from master.

     ANY, FDW requests for data from master or one any segment, depends on
     which path costs less.

     ALL SEGMENTS, FDW requests for data from all segments, wrappers need to
     have a policy matching the segments to data.

     If this option is not set, a mpp_execute option setting at the foreign
     server or FDW level is used, in that precedence order.

     For instance, file_fdw probes the mpp_execute value, then load different
     files based on the segment number. But something like gpfdist on the
     foreign side doesn't need this, which hands out a different slice of the
     data to each request, all segments could request the same location.

     Code snippet to probe the mpp_execute value:
<programlisting>
ForeignTable *table = GetForeignTable(foreigntableid);

if (table->exec_location == FTEXECLOCATION_ALL_SEGMENTS)
{
    ...
}
else if (table->exec_location == FTEXECLOCATION_ANY)
{
    ...
}
else if (table->exec_location == FTEXECLOCATION_COORDINATOR)
{
    ...
}
</programlisting>
     
     Code snippet to get the segment number:
<programlisting>
int segmengNumber = GpIdentity.segindex;
int totalNumberOfSegments = getgpsegmentCount();
</programlisting>
     </para>
    </listitem>
   </varlistentry>

  </variablelist>

 </refsect1>

 <refsect1>
  <title>Notes</title>

   <para>
    Constraints on foreign tables (such as <literal>CHECK</literal>
    or <literal>NOT NULL</literal> clauses) are not enforced by the
    core <productname>PostgreSQL</productname> 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</productname> 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 errors or incorrect answers.  It is the user's
    responsibility to ensure that the constraint definition matches
    reality.
   </para>

   <caution>
    <para>
     When a foreign table is used as a partition of a partitioned table,
     there is an implicit constraint that its contents must satisfy the
     partitioning rule.  Again, it is the user's responsibility to ensure
     that that is true, which is best done by installing a matching
     constraint on the remote server.
    </para>
   </caution>

   <para>
    Within a partitioned table containing foreign-table partitions,
    an <command>UPDATE</command> that changes the partition key value can
    cause a row to be moved from a local partition to a foreign-table
    partition, provided the foreign data wrapper supports tuple routing.
    However it is not currently possible to move a row from a
    foreign-table partition to another partition.
    An <command>UPDATE</command> that would require doing that will fail
    due to the partitioning constraint, assuming that that is properly
    enforced by the remote server.
   </para>

   <para>
    Similar considerations apply to generated columns.  Stored generated
    columns are computed on insert or update on the local
    <productname>PostgreSQL</productname> server and handed to the
    foreign-data wrapper for writing out to the foreign data store, but it is
    not enforced that a query of the foreign table returns values for stored
    generated columns that are consistent with the generation expression.
    Again, this might result in incorrect query results.
   </para>
 </refsect1>

 <refsect1 id="sql-createforeigntable-examples">
  <title>Examples</title>

  <para>
   Create foreign table <structname>films</structname>, which will be accessed through
   the server <structname>film_server</structname>:

<programlisting>
CREATE FOREIGN TABLE films (
    code        char(5) NOT NULL,
    title       varchar(40) NOT NULL,
    did         integer NOT NULL,
    date_prod   date,
    kind        varchar(10),
    len         interval hour to minute
)
SERVER film_server;
</programlisting></para>

  <para>
   Create foreign table <structname>measurement_y2016m07</structname>, which will be
   accessed through the server <structname>server_07</structname>, as a partition
   of the range partitioned table <structname>measurement</structname>:

<programlisting>
CREATE FOREIGN TABLE measurement_y2016m07
    PARTITION OF measurement FOR VALUES FROM ('2016-07-01') TO ('2016-08-01')
    SERVER server_07;
</programlisting></para>

 </refsect1>

 <refsect1 id="sql-createforeigntable-compatibility">
  <title>Compatibility</title>

  <para>
   The <command>CREATE FOREIGN TABLE</command> command largely conforms to the
   <acronym>SQL</acronym> standard; however, much as with
   <link linkend="sql-createtable"><command>CREATE TABLE</command></link>,
   <literal>NULL</literal> constraints and zero-column foreign tables are permitted.
   The ability to specify column default values is also
   a <productname>PostgreSQL</productname> extension.  Table inheritance, in the form
   defined by <productname>PostgreSQL</productname>, is nonstandard.
  </para>

 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterforeigntable"/></member>
   <member><xref linkend="sql-dropforeigntable"/></member>
   <member><xref linkend="sql-createtable"/></member>
   <member><xref linkend="sql-createserver"/></member>
   <member><xref linkend="sql-importforeignschema"/></member>
  </simplelist>
 </refsect1>
</refentry>