Support identity columns in partitioned tables

Previously, identity columns were disallowed on partitioned tables.
(The reason was mainly that no one had gotten around to working
through all the details to make it work.)  This makes it work now.

Some details on the behavior:

* A newly created partition inherits identity property

  The partitions of a partitioned table are integral part of the
  partitioned table.  A partition inherits identity columns from the
  partitioned table.  An identity column of a partition shares the
  identity space with the corresponding column of the partitioned
  table.  In other words, the same identity column across all
  partitions of a partitioned table share the same identity space.
  This is effected by sharing the same underlying sequence.

  When INSERTing directly into a partition, the sequence associated
  with the topmost partitioned table is used to calculate the value of
  the corresponding identity column.

  In regular inheritance, identity columns and their properties in a
  child table are independent of those in its parent tables.  A child
  table does not inherit identity columns or their properties
  automatically from the parent.  (This is unchanged.)

* Attached partition inherits identity column

  A table being attached as a partition inherits the identity property
  from the partitioned table.  This should be fine since we expect
  that the partition table's column has the same type as the
  partitioned table's corresponding column.  If the table being
  attached is a partitioned table, the identity properties are
  propagated down its partition hierarchy.

  An identity column in the partitioned table is also marked as NOT
  NULL.  The corresponding column in the partition needs to be marked
  as NOT NULL for the attach to succeed.

* Drop identity property when detaching partition

  A partition's identity column shares the identity space
  (i.e. underlying sequence) as the corresponding column of the
  partitioned table.  If a partition is detached it can longer share
  the identity space as before.  Hence the identity columns of the
  partition being detached loose their identity property.

  When identity of a column of a regular table is dropped it retains
  the NOT NULL constraint that came with the identity property.
  Similarly the columns of the partition being detached retain the NOT
  NULL constraints that came with identity property, even though the
  identity property itself is lost.

  The sequence associated with the identity property is linked to the
  partitioned table (and not the partition being detached).  That
  sequence is not dropped as part of detach operation.

* Partitions with their own identity columns are not allowed.

* The usual ALTER operations (add identity column, add identity
  property to existing column, alter properties of an indentity
  column, drop identity property) are supported for partitioned
  tables.  Changing a column only in a partitioned table or a
  partition is not allowed; the change needs to be applied to the
  whole partition hierarchy.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/CAExHW5uOykuTC+C6R1yDSp=o8Q83jr8xJdZxgPkxfZ1Ue5RRGg@mail.gmail.com

Author: petere

doc/src/sgml/ddl.sgml

@@ -339,7 +339,9 @@ INSERT INTO people (id, name, address) VALUE (<emphasis>DEFAULT</emphasis>, 'C',
   </para>
 
   <para>
-   Identity columns are currently not supported for partitioned tables.
+   Partitions inherit identity columns from the partitioned table.  They
+   cannot have their own identity columns.  The properties of a given identity
+   column are consistent across all the partitions in the partition hierarchy.
   </para>
  </sect1>