From 398cf255ad50db86ca665b75582317d4e795242a Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Fri, 24 Jan 2014 10:42:38 +0200
Subject: [PATCH] In GIN recompression code, use mmemove rather than memcpy,
for vacuum.
When vacuuming a data leaf page, any compressed posting lists that are not
modified, are copied back to the buffer from a later location in the same
buffer rather than from a palloc'd copy. IOW, they are just moved
downwards in the same buffer. Because the source and destination addresses
can overlap, we must use memmove rather than memcpy.
Report and fix by Alexander Korotkov.
---
src/backend/access/gin/gindatapage.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/src/backend/access/gin/gindatapage.c b/src/backend/access/gin/gindatapage.c
index 8504f4c7afc..ebdacd40c55 100644
--- a/src/backend/access/gin/gindatapage.c
+++ b/src/backend/access/gin/gindatapage.c
@@ -753,6 +753,13 @@ ginVacuumPostingTreeLeaf(Relation indexrel, Buffer buffer, GinVacuumState *gvs)
* *prdata is filled with WAL information about this operation. The caller
* is responsible for inserting to the WAL, along with any other information
* about the operation that triggered this recompression.
+ *
+ * NOTE: The segment pointers can point directly to the same buffer, with
+ * the limitation that any earlier segment must not overlap with an original,
+ * later segment. In other words, some segments may point the original buffer
+ * as long as you don't make any segments larger. Currently, leafRepackItems
+ * satisies this rule because it rewrites all segments after the first
+ * modified one, and vacuum can only make segments shorter.
*/
static void
dataPlaceToPageLeafRecompress(Buffer buf, disassembledLeaf *leaf,
@@ -798,7 +805,13 @@ dataPlaceToPageLeafRecompress(Buffer buf, disassembledLeaf *leaf,
if (!modified)
unmodifiedsize += segsize;
else
- memcpy(ptr, seginfo->seg, segsize);
+ {
+ /*
+ * Use memmove rather than memcpy, in case the segment points
+ * to the same buffer
+ */
+ memmove(ptr, seginfo->seg, segsize);
+ }
ptr += segsize;
newsize += segsize;
}
--
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