Hi hackers!
Experimenting with the new pluggable storage API, I found that amcanbackward
flag is not checked in build_index_paths() before
build_index_pathkeys(... BackwardScanDirection) call when we are building
paths for ORDER BY. And this flag is even not copied into IndexOptInfo struct.
Obviously, this can lead to misuse of Backward Index [Only] Scan plans.
Attached patch with the corresponding fix.
There are no test cases because now only btree supports ordered scans but it
supports backward scans too.
--
Nikita Glukhov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
>From 2bb9a17a8efb166339bcd5618157728a4b596430 Mon Sep 17 00:00:00 2001
From: Nikita Glukhov <n.glu...@postgrespro.ru>
Date: Sun, 13 May 2018 16:37:00 +0300
Subject: [PATCH] Disable backward scans on indices that do not support it
---
src/backend/optimizer/path/indxpath.c | 2 +-
src/backend/optimizer/util/plancat.c | 1 +
src/include/nodes/relation.h | 1 +
3 files changed, 3 insertions(+), 1 deletion(-)
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index f295558..caa688c 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -1084,7 +1084,7 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
/*
* 5. If the index is ordered, a backwards scan might be interesting.
*/
- if (index_is_ordered && pathkeys_possibly_useful)
+ if (index_is_ordered && pathkeys_possibly_useful && index->amcanbackward)
{
index_pathkeys = build_index_pathkeys(root, index,
BackwardScanDirection);
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 8369e3a..470d38e 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -271,6 +271,7 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
info->amsearcharray = amroutine->amsearcharray;
info->amsearchnulls = amroutine->amsearchnulls;
info->amcanparallel = amroutine->amcanparallel;
+ info->amcanbackward = amroutine->amcanbackward;
info->amhasgettuple = (amroutine->amgettuple != NULL);
info->amhasgetbitmap = (amroutine->amgetbitmap != NULL);
info->amcostestimate = amroutine->amcostestimate;
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 3b28d19..593ee48 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -798,6 +798,7 @@ typedef struct IndexOptInfo
bool amhasgettuple; /* does AM have amgettuple interface? */
bool amhasgetbitmap; /* does AM have amgetbitmap interface? */
bool amcanparallel; /* does AM support parallel scan? */
+ bool amcanbackward; /* does AM support backward scan? */
/* Rather than include amapi.h here, we declare amcostestimate like this */
void (*amcostestimate) (); /* AM's cost estimator */
} IndexOptInfo;
--
2.7.4
