Implement ALTER TABLE ... SPLIT PARTITION ... command

This new DDL command splits a single partition into several parititions.
Just like ALTER TABLE ... MERGE PARTITIONS ... command, new patitions are
created using 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

@@ -4400,6 +4400,25 @@ ALTER TABLE measurement
                       measurement_y2006m03) INTO measurement_y2006q1;
 </programlisting>
     </para>
+
+    <para>
+     Similarly to merging multiple table partitions, there is an option for
+     splitting a single partition into multiple using the
+     <link linkend="sql-altertable-split-partition"><command>ALTER TABLE ... SPLIT PARTITION</command></link>.
+     This feature could come in handy when one partition grows too big
+     and needs to be split into multiple.  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 split
+     the quarter partition back to monthly partitions: 
+<programlisting>
+ALTER TABLE measurement SPLIT PARTITION measurement_y2006q1 INTO
+   (PARTITION measurement_y2006m01 FOR VALUES FROM ('2006-01-01') TO ('2006-02-01'),
+    PARTITION measurement_y2006m02 FOR VALUES FROM ('2006-02-01') TO ('2006-03-01'),
+    PARTITION measurement_y2006m03 FOR VALUES FROM ('2006-03-01') TO ('2006-04-01'));
+</programlisting>
+    </para>
+
    </sect3>
 
    <sect3 id="ddl-partitioning-declarative-limitations">

doc/src/sgml/ref/alter_table.sgml

@@ -37,6 +37,10 @@ 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>
+    SPLIT PARTITION <replaceable class="parameter">partition_name</replaceable> INTO
+        (PARTITION <replaceable class="parameter">partition_name1</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT },
+         PARTITION <replaceable class="parameter">partition_name2</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT } [, ...])
 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>
@@ -1121,6 +1125,44 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     </listitem>
    </varlistentry>
 
+   <varlistentry id="sql-altertable-split-partition">
+    <term><literal>SPLIT PARTITION <replaceable class="parameter">partition_name</replaceable> INTO (PARTITION <replaceable class="parameter">partition_name1</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }, PARTITION <replaceable class="parameter">partition_name2</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT } [, ...])</literal></term>
+
+    <listitem>
+     <para>
+      This form splits a single partition of the target table. Hash-partitioning
+      is not supported. Bounds of new partitions should not overlap with new and
+      existing partitions (except <replaceable class="parameter">partition_name</replaceable>).
+      If the split partition is a DEFAULT partition, one of the new partitions must be DEFAULT.
+      In case one of the new partitions or one of existing partitions is DEFAULT,
+      new partitions <replaceable class="parameter">partition_name1</replaceable>,
+      <replaceable class="parameter">partition_name2</replaceable>, ... can have spaces
+      between partitions bounds.  If the partitioned table does not have a DEFAULT
+      partition, the DEFAULT partition can be defined as one of the new partitions.
+     </para>
+     <para>
+      In case new partitions do not contain a DEFAULT partition and the partitioned table
+      does not have a DEFAULT partition, the following must be true: sum bounds of
+      new partitions <replaceable class="parameter">partition_name1</replaceable>,
+      <replaceable class="parameter">partition_name2</replaceable>, ... should be
+      equal to bound of split partition <replaceable class="parameter">partition_name</replaceable>.
+      One of the new partitions <replaceable class="parameter">partition_name1</replaceable>,
+      <replaceable class="parameter">partition_name2</replaceable>, ... can have
+      the same name as split partition <replaceable class="parameter">partition_name</replaceable>
+      (this is suitable in case of splitting a DEFAULT partition: we split it, but after
+      splitting we have a partition with the same name).
+      Only simple, non-partitioned partition can be split.
+     </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>
+
    <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>
 
@@ -1188,7 +1230,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    All the forms of ALTER TABLE that act on a single table, except
    <literal>RENAME</literal>, <literal>SET SCHEMA</literal>,
    <literal>ATTACH PARTITION</literal>, <literal>DETACH PARTITION</literal>,
-   and <literal>MERGE PARTITIONS</literal> can be combined into
+   <literal>SPLIT 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
@@ -1432,7 +1475,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       <listitem>
        <para>
         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.
+        or the name of split partition, or the name of the new merged partition.
        </para>
       </listitem>
      </varlistentry>
@@ -1848,6 +1891,24 @@ ALTER TABLE measurement
     DETACH PARTITION measurement_y2015m12;
 </programlisting></para>
 
+  <para>
+   To split a single partition of the range-partitioned table:
+<programlisting>
+ALTER TABLE sales_range SPLIT PARTITION sales_feb_mar_apr2023 INTO
+   (PARTITION sales_feb2023 FOR VALUES FROM ('2023-02-01') TO ('2023-03-01'),
+    PARTITION sales_mar2023 FOR VALUES FROM ('2023-03-01') TO ('2023-04-01'),
+    PARTITION sales_apr2023 FOR VALUES FROM ('2023-04-01') TO ('2023-05-01'));
+</programlisting></para>
+
+  <para>
+   To split a single partition of the list-partitioned table:
+<programlisting>
+ALTER TABLE sales_list SPLIT PARTITION sales_all INTO
+   (PARTITION sales_west FOR VALUES IN ('Lisbon', 'New York', 'Madrid'),
+    PARTITION sales_east FOR VALUES IN ('Bejing', 'Delhi', 'Vladivostok'),
+    PARTITION sales_central FOR VALUES IN ('Warsaw', 'Berlin', 'Kyiv'));
+</programlisting></para>
+
   <para>
    To merge several partitions into one partition of the target table:
 <programlisting>