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)

1 files changed, 6 insertions, 2 deletions

diff --git a/src/include/storage/smgr.h b/src/include/storage/smgr.h
index a8e7877f704..3430d8665e4 100644
--- a/src/include/storage/smgr.h
+++ b/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;