Introduce custom path and scan providers.

This allows extension modules to define their own methods for
scanning a relation, and get the core code to use them.  It's
unclear as yet how much use this capability will find, but we
won't find out if we never commit it.

KaiGai Kohei, reviewed at various times and in various levels
of detail by Shigeru Hanada, Tom Lane, Andres Freund, Álvaro
Herrera, and myself.

2 files changed, 55 insertions, 0 deletions

diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 7b51d331777..e76b5b3f633 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -598,6 +598,29 @@ _copyForeignScan(const ForeignScan *from)
 }
 
 /*
+ * _copyCustomScan
+ */
+static CustomScan *
+_copyCustomScan(const CustomScan *from)
+{
+	CustomScan		   *newnode;
+
+	newnode = from->methods->CopyCustomScan(from);
+	Assert(nodeTag(newnode) == nodeTag(from));
+
+	CopyScanFields((const Scan *) from, (Scan *) newnode);
+	COPY_SCALAR_FIELD(flags);
+	/*
+	 * NOTE: The method field of CustomScan is required to be a pointer
+	 * to a static table of callback functions. So, we don't copy the
+	 * table itself, just reference the original one.
+	 */
+	COPY_SCALAR_FIELD(methods);
+
+	return newnode;
+}
+
+/*
  * CopyJoinFields
  *
  *		This function copies the fields of the Join node.  It is used by
@@ -4043,6 +4066,9 @@ copyObject(const void *from)
 		case T_ForeignScan:
 			retval = _copyForeignScan(from);
 			break;
+		case T_CustomScan:
+			retval = _copyCustomScan(from);
+			break;
 		case T_Join:
 			retval = _copyJoin(from);
 			break;
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 4bbfa629d69..cdf1e7ece1f 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -564,6 +564,18 @@ _outForeignScan(StringInfo str, const ForeignScan *node)
 }
 
 static void
+_outCustomScan(StringInfo str, const CustomScan *node)
+{
+	WRITE_NODE_TYPE("CUSTOMSCAN");
+
+	_outScanInfo(str, (const Scan *) node);
+	WRITE_UINT_FIELD(flags);
+	appendStringInfo(str, " :methods");
+	_outToken(str, node->methods->CustomName);
+	node->methods->TextOutCustomScan(str, node);
+}
+
+static void
 _outJoin(StringInfo str, const Join *node)
 {
 	WRITE_NODE_TYPE("JOIN");
@@ -1585,6 +1597,17 @@ _outForeignPath(StringInfo str, const ForeignPath *node)
 }
 
 static void
+_outCustomPath(StringInfo str, const CustomPath *node)
+{
+	WRITE_NODE_TYPE("CUSTOMPATH");
+	_outPathInfo(str, (const Path *) node);
+	WRITE_UINT_FIELD(flags);
+	appendStringInfo(str, " :methods");
+	_outToken(str, node->methods->CustomName);
+	node->methods->TextOutCustomPath(str, node);
+}
+
+static void
 _outAppendPath(StringInfo str, const AppendPath *node)
 {
 	WRITE_NODE_TYPE("APPENDPATH");
@@ -2855,6 +2878,9 @@ _outNode(StringInfo str, const void *obj)
 			case T_ForeignScan:
 				_outForeignScan(str, obj);
 				break;
+			case T_CustomScan:
+				_outCustomScan(str, obj);
+				break;
 			case T_Join:
 				_outJoin(str, obj);
 				break;
@@ -3063,6 +3089,9 @@ _outNode(StringInfo str, const void *obj)
 			case T_ForeignPath:
 				_outForeignPath(str, obj);
 				break;
+			case T_CustomPath:
+				_outCustomPath(str, obj);
+				break;
 			case T_AppendPath:
 				_outAppendPath(str, obj);
 				break;