Implement ALTER TABLE ... MERGE PARTITIONS ... command

This new DDL command merges several partitions into the one partition of the
target table.  The target partition is created using new
createPartitionTable() function with parent partition as the template.

This commit comprises quite naive implementation which works in single process
and holds the ACCESS EXCLUSIVE LOCK on the parent table during all the
operations including the tuple routing.  This is why this new DDL command
can't be recommended for large partitioned tables under a high load.  However,
this implementation come in handy in certain cases even as is.
Also, it could be used as a foundation for future implementations with lesser
locking and possibly parallel.

Discussion: https://postgr.es/m/c73a1746-0cd0-6bdd-6b23-3ae0b7c0c582%40postgrespro.ru
Author: Dmitry Koval
Reviewed-by: Matthias van de Meent, Laurenz Albe, Zhihong Yu, Justin Pryzby
Reviewed-by: Alvaro Herrera, Robert Haas, Stephane Tachoires, Jian He

Fixes (summary information).
Authors: Alexander Korotkov, Tender Wang, Richard Guo, Dagfinn Ilmari Mannsaker
Authors: Fujii Masao, Jian He
Reviewed-by: Alexander Korotkov, Robert Haas, Justin Pryzby, Pavel Borisov
Reviewed-by: Masahiko Sawada
Reported-by: Alexander Lakhin, Justin Pryzby, Kyotaro Horiguchi
Reported-by: Daniel Gustafsson, Tom Lane, Noah Misch

Author: akorotkov

doc/src/sgml/ddl.sgml

@@ -4450,6 +4450,25 @@ ALTER TABLE measurement_y2006m02 ADD UNIQUE (city_id, logdate);
 ALTER INDEX measurement_city_id_logdate_key
     ATTACH PARTITION measurement_y2006m02_city_id_logdate_key;
 ...
+</programlisting>
+    </para>
+
+    <para>
+     There is also an option for merging multiple table partitions into
+     a single partition using the
+     <link linkend="sql-altertable-merge-partitions"><command>ALTER TABLE ... MERGE PARTITIONS</command></link>.
+     This feature simplifies the management of partitioned tables by allowing
+     users to combine partitions that are no longer needed as
+     separate entities.  It's important to note that this operation is not
+     supported for hash-partitioned tables and acquires an
+     <literal>ACCESS EXCLUSIVE</literal> lock, which could impact high-load
+     systems due to the lock's restrictive nature.  For example, we can
+     merge three monthly partitions into one quarter partition:
+<programlisting>
+ALTER TABLE measurement
+    MERGE PARTITIONS (measurement_y2006m01,
+                      measurement_y2006m02,
+                      measurement_y2006m03) INTO measurement_y2006q1;
 </programlisting>
     </para>
    </sect3>

doc/src/sgml/ref/alter_table.sgml

@@ -37,6 +37,8 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
     ATTACH PARTITION <replaceable class="parameter">partition_name</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }
 ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
     DETACH PARTITION <replaceable class="parameter">partition_name</replaceable> [ CONCURRENTLY | FINALIZE ]
+ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
+    MERGE PARTITIONS (<replaceable class="parameter">partition_name1</replaceable>, <replaceable class="parameter">partition_name2</replaceable> [, ...]) INTO <replaceable class="parameter">partition_name</replaceable>
 
 <phrase>where <replaceable class="parameter">action</replaceable> is one of:</phrase>
 
@@ -1147,14 +1149,101 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     </listitem>
    </varlistentry>
 
+   <varlistentry id="sql-altertable-merge-partitions">
+    <term><literal>MERGE PARTITIONS (<replaceable class="parameter">partition_name1</replaceable>, <replaceable class="parameter">partition_name2</replaceable> [, ...]) INTO <replaceable class="parameter">partition_name</replaceable></literal></term>
+
+    <listitem>
+     <para>
+      This form merges several partitions of the target table into a new partition.
+      Hash-partitioned target table is not supported.
+      If <literal>DEFAULT</literal> partition is not in the
+      list of partitions <replaceable class="parameter">partition_name1</replaceable>,
+      <replaceable class="parameter">partition_name2</replaceable> [, ...]:
+      <itemizedlist>
+       <listitem>
+        <para>
+         For range-partitioned tables, the ranges of the partitions
+         <replaceable class="parameter">partition_name1</replaceable>,
+         <replaceable class="parameter">partition_name2</replaceable>, [...]
+         must be adjacent in order to be merged. Otherwise, an error will be
+         raised. The resulting combined range will be the new partition bound
+         for the partition <replaceable class="parameter">partition_name</replaceable>.
+        </para>
+       </listitem>
+       <listitem>
+        <para>
+         For list-partitioned tables, the partition bounds of
+         <replaceable class="parameter">partition_name1</replaceable>,
+         <replaceable class="parameter">partition_name2</replaceable>, [...]
+         are combined to form the new partition bound for
+         <replaceable class="parameter">partition_name</replaceable>.
+        </para>
+       </listitem>
+      </itemizedlist>
+      If <literal>DEFAULT</literal> partition is in the list of partitions <replaceable class="parameter">partition_name1</replaceable>,
+      <replaceable class="parameter">partition_name2</replaceable> [, ...]:
+      <itemizedlist>
+       <listitem>
+        <para>
+         The partition <replaceable class="parameter">partition_name</replaceable>
+         will be the new <literal>DEFAULT</literal> partition of the target table.
+        </para>
+       </listitem>
+       <listitem>
+        <para>
+         The partition bound specifications for all partitions-
+         <replaceable class="parameter">partition_name1</replaceable>,
+         <replaceable class="parameter">partition_name2</replaceable>, [...]
+         can be arbitrary.
+        </para>
+       </listitem>
+      </itemizedlist>
+      The new partition <replaceable class="parameter">partition_name</replaceable>
+      can have the same name as one of the merged partitions. Only simple,
+      non-partitioned partitions can be merged.
+     </para>
+     <para>
+      If merged partitions have different owners, an error will be generated.
+      The owner of the merged partitions will be the owner of the new partition.
+      It is the user's responsibility to setup <acronym>ACL</acronym> on the
+      new partition.
+     </para>
+     <para>
+      The indexes and identity are created later, after moving the data
+      into the new partition.
+      Extended statistics aren't copied from the parent table, for consistency with
+      <command>CREATE TABLE PARTITION OF</command>.
+      The new partition will inherit the same table access method, persistence
+      type, and tablespace as the parent table.
+     </para>
+     <para>
+      When partitions are merged, any individual objects belonging to those
+      partitions, such as constraints or statistics will be dropped. This occurs
+      because <command>ALTER TABLE MERGE PARTITIONS</command> uses the partitioned table itself as the
+      template to define these objects.
+     </para>
+     <para>
+      If merged partitions have some objects dependent on them, the command can
+      not be done (<literal>CASCADE</literal> is not used, an error will be returned).
+     </para>
+     <note>
+      <para>
+       Merging partitions acquires a <literal>ACCESS EXCLUSIVE</literal> lock on
+       the parent table, in addition to the <literal>ACCESS EXCLUSIVE</literal>
+       locks on the tables being merged and on the default partition (if any).
+      </para>
+     </note>
+    </listitem>
+   </varlistentry>
+
   </variablelist>
   </para>
 
   <para>
    All the forms of <command>ALTER TABLE</command> that act on a single table,
    except <literal>RENAME</literal>, <literal>SET SCHEMA</literal>,
-   <literal>ATTACH PARTITION</literal>, and
-   <literal>DETACH PARTITION</literal> can be combined into
+   <literal>ATTACH PARTITION</literal>, <literal>DETACH PARTITION</literal>,
+   and <literal>MERGE PARTITIONS</literal> can be combined into
    a list of multiple alterations to be applied together.  For example, it
    is possible to add several columns and/or alter the type of several
    columns in a single command.  This is particularly useful with large
@@ -1397,7 +1486,18 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       <term><replaceable class="parameter">partition_name</replaceable></term>
       <listitem>
        <para>
-        The name of the table to attach as a new partition or to detach from this table.
+        The name of the table to attach as a new partition or to detach from this table,
+        or the name of the new merged partition.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry id="sql-altertable-parms-partition-name1">
+      <term><replaceable class="parameter">partition_name1</replaceable></term>
+      <term><replaceable class="parameter">partition_name2</replaceable></term>
+      <listitem>
+       <para>
+        The names of the tables being merged into the new partition.
        </para>
       </listitem>
      </varlistentry>
@@ -1830,6 +1930,13 @@ ALTER TABLE measurement
     DETACH PARTITION measurement_y2015m12;
 </programlisting></para>
 
+  <para>
+   To merge several partitions into one partition of the target table:
+<programlisting>
+ALTER TABLE sales_list MERGE PARTITIONS (sales_west, sales_east, sales_central)
+    INTO sales_all;
+</programlisting></para>
+
  </refsect1>
 
  <refsect1>

src/backend/catalog/dependency.c

@@ -319,6 +319,56 @@ performDeletion(const ObjectAddress *object,
 	table_close(depRel, RowExclusiveLock);
 }
 
+/*
+ * performDeletionCheck: Check whether a specific object can be safely deleted.
+ * This function does not perform any deletion; instead, it raises an error
+ * if the object cannot be deleted due to existing dependencies.
+ *
+ * It can be useful when you need delete some objects later.  See comments in
+ * performDeletion too.
+ * The behavior must specified as DROP_RESTRICT.
+ */
+void
+performDeletionCheck(const ObjectAddress *object,
+					 DropBehavior behavior, int flags)
+{
+	Relation	depRel;
+	ObjectAddresses *targetObjects;
+
+	Assert(behavior == DROP_RESTRICT);
+
+	depRel = table_open(DependRelationId, RowExclusiveLock);
+
+	AcquireDeletionLock(object, 0);
+
+	/*
+	 * Construct a list of objects we want delete later (ie, the given object plus
+	 * everything directly or indirectly dependent on it).
+	 */
+	targetObjects = new_object_addresses();
+
+	findDependentObjects(object,
+						 DEPFLAG_ORIGINAL,
+						 flags,
+						 NULL,	/* empty stack */
+						 targetObjects,
+						 NULL,	/* no pendingObjects */
+						 &depRel);
+
+	/*
+	 * Check if deletion is allowed.
+	 */
+	reportDependentObjects(targetObjects,
+						   behavior,
+						   flags,
+						   object);
+
+	/* And clean up */
+	free_object_addresses(targetObjects);
+
+	table_close(depRel, RowExclusiveLock);
+}
+
 /*
  * performMultipleDeletions: Similar to performDeletion, but act on multiple
  * objects at once.

src/backend/catalog/heap.c

@@ -102,11 +102,11 @@ static ObjectAddress AddNewRelationType(const char *typeName,
 										Oid new_row_type,
 										Oid new_array_type);
 static void RelationRemoveInheritance(Oid relid);
+static void StoreConstraints(Relation rel, List *cooked_constraints,
+							 bool is_internal);
 static Oid	StoreRelCheck(Relation rel, const char *ccname, Node *expr,
 						  bool is_enforced, bool is_validated, bool is_local,
 						  int16 inhcount, bool is_no_inherit, bool is_internal);
-static void StoreConstraints(Relation rel, List *cooked_constraints,
-							 bool is_internal);
 static bool MergeWithExistingConstraint(Relation rel, const char *ccname, Node *expr,
 										bool allow_merge, bool is_local,
 										bool is_enforced,

src/backend/catalog/pg_constraint.c

@@ -875,7 +875,7 @@ RelationGetNotNullConstraints(Oid relid, bool cooked, bool include_noinh)
 															 false)));
 			constr->is_enforced = true;
 			constr->skip_validation = !conForm->convalidated;
-			constr->initially_valid = true;
+			constr->initially_valid = conForm->convalidated;
 			constr->is_no_inherit = conForm->connoinherit;
 			notnulls = lappend(notnulls, constr);
 		}