Buffering GiST index build algorithm.

When building a GiST index that doesn't fit in cache, buffers are attached
to some internal nodes in the index. This speeds up the build by avoiding
random I/O that would otherwise be needed to traverse all the way down the
tree to the find right leaf page for tuple.

Alexander Korotkov

src/backend/access/common/reloptions.c

@@ -219,6 +219,17 @@ static relopt_real realRelOpts[] =
 
 static relopt_string stringRelOpts[] =
 {
+	{
+		{
+			"buffering",
+			"Enables buffering build for this GiST index",
+			RELOPT_KIND_GIST
+		},
+		4,
+		false,
+		gistValidateBufferingOption,
+		"auto"
+	},
 	/* list terminator */
 	{{NULL}}
 };