Don't trust deferred-unique indexes for join removal.

The uniqueness condition might fail to hold intra-transaction, and assuming
it does can give incorrect query results.  Per report from Marti Raudsepp,
though this is not his proposed patch.

Back-patch to 9.0, where both these features were introduced.  In the
released branches, add the new IndexOptInfo field to the end of the struct,
to try to minimize ABI breakage for third-party code that may be examining
that struct.

5 files changed, 17 insertions, 4 deletions

diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 681f5f85bc7..96b09fbce71 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -1764,6 +1764,7 @@ _outIndexOptInfo(StringInfo str, IndexOptInfo *node)
 	WRITE_NODE_FIELD(indpred);
 	WRITE_BOOL_FIELD(predOK);
 	WRITE_BOOL_FIELD(unique);
+	WRITE_BOOL_FIELD(immediate);
 	WRITE_BOOL_FIELD(hypothetical);
 }
 
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index fc96f4f1dac..58b8797b738 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -2179,10 +2179,11 @@ relation_has_unique_index_for(PlannerInfo *root, RelOptInfo *rel,
 		int			c;
 
 		/*
-		 * If the index is not unique or if it's a partial index that doesn't
-		 * match the query, it's useless here.
+		 * If the index is not unique, or not immediately enforced, or if it's
+		 * a partial index that doesn't match the query, it's useless here.
 		 */
-		if (!ind->unique || (ind->indpred != NIL && !ind->predOK))
+		if (!ind->unique || !ind->immediate ||
+			(ind->indpred != NIL && !ind->predOK))
 			continue;
 
 		/*
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 513b6e4d740..a92d2485289 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -316,6 +316,7 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 				ChangeVarNodes((Node *) info->indpred, 1, varno, 0);
 			info->predOK = false;		/* set later in indxpath.c */
 			info->unique = index->indisunique;
+			info->immediate = index->indimmediate;
 			info->hypothetical = false;
 
 			/*
@@ -973,6 +974,11 @@ join_selectivity(PlannerInfo *root,
  * Detect whether there is a unique index on the specified attribute
  * of the specified relation, thus allowing us to conclude that all
  * the (non-null) values of the attribute are distinct.
+ *
+ * This function does not check the index's indimmediate property, which
+ * means that uniqueness may transiently fail to hold intra-transaction.
+ * That's appropriate when we are making statistical estimates, but beware
+ * of using this for any correctness proofs.
  */
 bool
 has_unique_index(RelOptInfo *rel, AttrNumber attno)
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 8e304097db8..7a42ff03466 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -4121,7 +4121,9 @@ get_join_variables(PlannerInfo *root, List *args, SpecialJoinInfo *sjinfo,
  *		commonly the same as the exposed type of the variable argument,
  *		but can be different in binary-compatible-type cases.
  *	isunique: TRUE if we were able to match the var to a unique index,
- *		implying its values are unique for this query.
+ *		implying its values are unique for this query.  (Caution: this
+ *		should be trusted for statistical purposes only, since we do not
+ *		check indimmediate.)
  *
  * Caller is responsible for doing ReleaseVariableStats() before exiting.
  */
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index f6592697e44..0aec1637c93 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -491,6 +491,9 @@ typedef struct IndexOptInfo
 	bool		amsearchnulls;	/* can AM search for NULL/NOT NULL entries? */
 	bool		amhasgettuple;	/* does AM have amgettuple interface? */
 	bool		amhasgetbitmap; /* does AM have amgetbitmap interface? */
+
+	/* Added at end of struct to avoid ABI breakage in released branches */
+	bool		immediate;		/* is uniqueness enforced immediately? */
 } IndexOptInfo;