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

2 files changed, 110 insertions, 1 deletions

diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 6ea68722e3d..1fcdb25142f 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -761,7 +761,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	ORDER ORDINALITY OTHERS OUT_P OUTER_P
 	OVER OVERLAPS OVERLAY OVERRIDING OWNED OWNER
 
-	PARALLEL PARAMETER PARSER PARTIAL PARTITION PASSING PASSWORD PATH
+	PARALLEL PARAMETER PARSER PARTIAL PARTITION PARTITIONS PASSING PASSWORD PATH
 	PERIOD PLACING PLAN PLANS POLICY
 
 	POSITION PRECEDING PRECISION PRESERVE PREPARE PREPARED PRIMARY
@@ -2317,6 +2317,7 @@ partition_cmd:
 					n->subtype = AT_AttachPartition;
 					cmd->name = $3;
 					cmd->bound = $4;
+					cmd->partlist = NULL;
 					cmd->concurrent = false;
 					n->def = (Node *) cmd;
 
@@ -2331,6 +2332,7 @@ partition_cmd:
 					n->subtype = AT_DetachPartition;
 					cmd->name = $3;
 					cmd->bound = NULL;
+					cmd->partlist = NULL;
 					cmd->concurrent = $4;
 					n->def = (Node *) cmd;
 
@@ -2344,6 +2346,21 @@ partition_cmd:
 					n->subtype = AT_DetachPartitionFinalize;
 					cmd->name = $3;
 					cmd->bound = NULL;
+					cmd->partlist = NULL;
+					cmd->concurrent = false;
+					n->def = (Node *) cmd;
+					$$ = (Node *) n;
+				}
+			/* ALTER TABLE <name> MERGE PARTITIONS () INTO <partition_name> */
+			| MERGE PARTITIONS '(' qualified_name_list ')' INTO qualified_name
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					PartitionCmd *cmd = makeNode(PartitionCmd);
+
+					n->subtype = AT_MergePartitions;
+					cmd->name = $7;
+					cmd->bound = NULL;
+					cmd->partlist = $4;
 					cmd->concurrent = false;
 					n->def = (Node *) cmd;
 					$$ = (Node *) n;
@@ -2360,6 +2377,7 @@ index_partition_cmd:
 					n->subtype = AT_AttachPartition;
 					cmd->name = $3;
 					cmd->bound = NULL;
+					cmd->partlist = NULL;
 					cmd->concurrent = false;
 					n->def = (Node *) cmd;
 
@@ -17668,6 +17686,7 @@ unreserved_keyword:
 			| PARSER
 			| PARTIAL
 			| PARTITION
+			| PARTITIONS
 			| PASSING
 			| PASSWORD
 			| PATH
@@ -18292,6 +18311,7 @@ bare_label_keyword:
 			| PARSER
 			| PARTIAL
 			| PARTITION
+			| PARTITIONS
 			| PASSING
 			| PASSWORD
 			| PATH
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index c7efd8d8cee..48a23c7ca42 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -58,6 +58,8 @@
 #include "parser/parse_type.h"
 #include "parser/parse_utilcmd.h"
 #include "parser/parser.h"
+#include "partitioning/partdesc.h"
+#include "partitioning/partbounds.h"
 #include "rewrite/rewriteManip.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
@@ -3414,6 +3416,80 @@ transformRuleStmt(RuleStmt *stmt, const char *queryString,
 
 
 /*
+ * transformPartitionCmdForMerge
+ *		Analyze the ALTER TABLLE ... MERGE PARTITIONS command
+ *
+ * Does simple checks for merged partitions. Calculates bound of result
+ * partition.
+ */
+static void
+transformPartitionCmdForMerge(CreateStmtContext *cxt, PartitionCmd *partcmd)
+{
+	Oid			defaultPartOid;
+	Oid			partOid;
+	Relation	parent = cxt->rel;
+	PartitionKey key;
+	char		strategy;
+	ListCell   *listptr,
+			   *listptr2;
+	bool		isDefaultPart = false;
+	List	   *partOids = NIL;
+
+	if (parent->rd_rel->relkind != RELKIND_PARTITIONED_TABLE)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+				 errmsg("\"%s\" is not a partitioned table", RelationGetRelationName(parent))));
+
+	key = RelationGetPartitionKey(parent);
+	strategy = get_partition_strategy(key);
+
+	if (strategy == PARTITION_STRATEGY_HASH)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("partition of hash-partitioned table cannot be merged")));
+
+	/* Is current partition a DEFAULT partition? */
+	defaultPartOid = get_default_oid_from_partdesc(
+												   RelationGetPartitionDesc(parent, true));
+
+	foreach(listptr, partcmd->partlist)
+	{
+		RangeVar   *name = (RangeVar *) lfirst(listptr);
+
+		/* Partitions in the list should have different names. */
+		for_each_cell(listptr2, partcmd->partlist, lnext(partcmd->partlist, listptr))
+		{
+			RangeVar   *name2 = (RangeVar *) lfirst(listptr2);
+
+			if (equal(name, name2))
+				ereport(ERROR,
+						(errcode(ERRCODE_DUPLICATE_TABLE),
+						 errmsg("partition with name \"%s\" already used", name->relname)),
+						parser_errposition(cxt->pstate, name2->location));
+		}
+
+		/* Search DEFAULT partition in the list. */
+		partOid = RangeVarGetRelid(name, NoLock, false);
+		if (partOid == defaultPartOid)
+			isDefaultPart = true;
+		partOids = lappend_oid(partOids, partOid);
+	}
+
+	/* Allocate bound of result partition. */
+	Assert(partcmd->bound == NULL);
+	partcmd->bound = makeNode(PartitionBoundSpec);
+
+	/* Fill partition bound. */
+	partcmd->bound->strategy = strategy;
+	partcmd->bound->location = -1;
+	partcmd->bound->is_default = isDefaultPart;
+	if (!isDefaultPart)
+		calculate_partition_bound_for_merge(parent, partcmd->partlist,
+											partOids, partcmd->bound,
+											cxt->pstate);
+}
+
+/*
  * transformAlterTableStmt -
  *		parse analysis for ALTER TABLE
  *
@@ -3683,6 +3759,19 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 				newcmds = lappend(newcmds, cmd);
 				break;
 
+			case AT_MergePartitions:
+				{
+					PartitionCmd *partcmd = (PartitionCmd *) cmd->def;
+
+					if (list_length(partcmd->partlist) < 2)
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+								 errmsg("list of new partitions should contains at least two items")));
+					transformPartitionCmdForMerge(&cxt, partcmd);
+					newcmds = lappend(newcmds, cmd);
+					break;
+				}
+
 			default:
 
 				/*