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

3 files changed, 21 insertions, 0 deletions

diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 76d91e547b7..6e8a2b59e83 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -938,6 +938,17 @@ typedef struct PartitionRangeDatum
 } PartitionRangeDatum;
 
 /*
+ * PartitionDesc - info about single partition for ALTER TABLE SPLIT PARTITION command
+ */
+typedef struct SinglePartitionSpec
+{
+	NodeTag		type;
+
+	RangeVar   *name;			/* name of partition */
+	PartitionBoundSpec *bound;	/* FOR VALUES, if attaching */
+} SinglePartitionSpec;
+
+/*
  * PartitionCmd - info for ALTER TABLE/INDEX ATTACH/DETACH PARTITION commands
  */
 typedef struct PartitionCmd
@@ -945,6 +956,8 @@ typedef struct PartitionCmd
 	NodeTag		type;
 	RangeVar   *name;			/* name of partition to attach/detach */
 	PartitionBoundSpec *bound;	/* FOR VALUES, if attaching */
+	List	   *partlist;		/* list of partitions, for SPLIT PARTITION
+								 * command */
 	bool		concurrent;
 } PartitionCmd;
 
@@ -2398,6 +2411,7 @@ typedef enum AlterTableType
 	AT_AttachPartition,			/* ATTACH PARTITION */
 	AT_DetachPartition,			/* DETACH PARTITION */
 	AT_DetachPartitionFinalize, /* DETACH PARTITION FINALIZE */
+	AT_MergePartitions,			/* MERGE PARTITIONS */
 	AT_AddIdentity,				/* ADD IDENTITY */
 	AT_SetIdentity,				/* SET identity column options */
 	AT_DropIdentity,			/* DROP IDENTITY */
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 2d4a0c6a079..a4834eb9b7d 100644
--- a/src/include/parser/kwlist.h
+++ b/src/include/parser/kwlist.h
@@ -334,6 +334,7 @@ PG_KEYWORD("parameter", PARAMETER, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("parser", PARSER, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("partial", PARTIAL, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("partition", PARTITION, UNRESERVED_KEYWORD, BARE_LABEL)
+PG_KEYWORD("partitions", PARTITIONS, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("passing", PASSING, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("password", PASSWORD, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("path", PATH, UNRESERVED_KEYWORD, BARE_LABEL)
diff --git a/src/include/partitioning/partbounds.h b/src/include/partitioning/partbounds.h
index 3d9cc1031f7..0329d7bd848 100644
--- a/src/include/partitioning/partbounds.h
+++ b/src/include/partitioning/partbounds.h
@@ -143,4 +143,10 @@ extern int	partition_range_datum_bsearch(FmgrInfo *partsupfunc,
 extern int	partition_hash_bsearch(PartitionBoundInfo boundinfo,
 								   int modulus, int remainder);
 
+extern void calculate_partition_bound_for_merge(Relation parent,
+												List *partNames,
+												List *partOids,
+												PartitionBoundSpec *spec,
+												ParseState *pstate);
+
 #endif							/* PARTBOUNDS_H */