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

Author: akorotkov

doc/src/sgml/ddl.sgml

@@ -4379,6 +4379,25 @@ 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
+     administrators 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 +37,9 @@ 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>
 
@@ -1118,14 +1121,74 @@ 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 into the one partition of the target table.
+      Hash-partitioning is not supported.  If DEFAULT 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 is necessary that the ranges
+         of the partitions <replaceable class="parameter">partition_name1</replaceable>,
+         <replaceable class="parameter">partition_name2</replaceable> [, ...] can
+         be merged into one range without spaces and overlaps (otherwise an error
+         will be generated).  The combined range will be the range for the partition
+         <replaceable class="parameter">partition_name</replaceable>.
+        </para>
+       </listitem>
+       <listitem>
+        <para>
+         For list-partitioned tables the values lists of all partitions
+         <replaceable class="parameter">partition_name1</replaceable>,
+         <replaceable class="parameter">partition_name2</replaceable> [, ...] are
+         combined and form a list of values of partition
+         <replaceable class="parameter">partition_name</replaceable>.
+        </para>
+       </listitem>
+      </itemizedlist>
+      If DEFAULT 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 DEFAULT partition.
+        </para>
+       </listitem>
+       <listitem>
+        <para>
+         For range- and list-partitioned tables the ranges and lists of values
+         of the merged partitions can be any.
+        </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>
+     <note>
+      <para>
+       This command acquires an <literal>ACCESS EXCLUSIVE</literal> lock.
+       This is a significant limitation, which limits the usage of this
+       command with large partitioned tables under a high load.
+      </para>
+     </note>
+    </listitem>
+   </varlistentry>
+
   </variablelist>
   </para>
 
   <para>
    All the forms of ALTER TABLE 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
@@ -1368,7 +1431,8 @@ 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>
@@ -1784,6 +1848,13 @@ 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>