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Improve scalability of md.c for large relations.

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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)
This commit is contained in:
Andres Freund
2016-09-08 17:02:43 -07:00
parent 417fefaf08
commit 45e191e3aa
3 changed files with 231 additions and 178 deletions
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8
src/include/storage/smgr.h
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@@ -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;