Improve scalability of md.c for large relations.

So far md.c used a linked list of segments. That proved to be a problem
when processing large relations, because every smgr.c/md.c level access
to a page incurred walking through a linked list of all preceding
segments. Thus making accessing pages O(#segments).

Replace the linked list of segments hanging off SMgrRelationData with an
array of opened segments. That allows O(1) access to individual
segments, if they've previously been opened.

Discussion: <20140331101001.GE13135@alap3.anarazel.de>
Reviewed-By: Peter Geoghegan, Tom Lane (in an older version)

src/backend/storage/smgr/smgr.c

@@ -174,7 +174,7 @@ smgropen(RelFileNode rnode, BackendId backend)
 
 		/* mark it not open */
 		for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
-			reln->md_fd[forknum] = NULL;
+			reln->md_num_open_segs[forknum] = 0;
 
 		/* it has no owner yet */
 		add_to_unowned_list(reln);
@@ -379,7 +379,7 @@ smgrcreate(SMgrRelation reln, ForkNumber forknum, bool isRedo)
 	 * Exit quickly in WAL replay mode if we've already opened the file. If
 	 * it's open, it surely must exist.
 	 */
-	if (isRedo && reln->md_fd[forknum] != NULL)
+	if (isRedo && reln->md_num_open_segs[forknum] > 0)
 		return;
 
 	/*

src/include/storage/smgr.h

@@ -64,8 +64,12 @@ typedef struct SMgrRelationData
 	 */
 	int			smgr_which;		/* storage manager selector */
 
-	/* for md.c; NULL for forks that are not open */
-	struct _MdfdVec *md_fd[MAX_FORKNUM + 1];
+	/*
+	 * for md.c; per-fork arrays of the number of open segments
+	 * (md_num_open_segs) and the segments themselves (md_seg_fds).
+	 */
+	int			md_num_open_segs[MAX_FORKNUM + 1];
+	struct _MdfdVec *md_seg_fds[MAX_FORKNUM + 1];
 
 	/* if unowned, list link in list of all unowned SMgrRelations */
 	struct SMgrRelationData *next_unowned_reln;