Add some additional core functions to support join pushdown for FDWs.

GetExistingLocalJoinPath() is useful for handling EvalPlanQual rechecks
properly, and GetUserMappingById() is needed to make sure you're using
the right credentials.

Shigeru Hanada, Etsuro Fujita, Ashutosh Bapat, Robert Haas

1 files changed, 162 insertions, 2 deletions

diff --git a/src/backend/foreign/foreign.c b/src/backend/foreign/foreign.c
index 47c00af74f..213217966c 100644
--- a/src/backend/foreign/foreign.c
+++ b/src/backend/foreign/foreign.c
@@ -160,6 +160,54 @@ GetForeignServerByName(const char *srvname, bool missing_ok)
 	return GetForeignServer(serverid);
 }
 
+/*
+ * GetUserMappingById - look up the user mapping by its OID.
+ */
+UserMapping *
+GetUserMappingById(Oid umid)
+{
+	Datum		datum;
+	HeapTuple	tp;
+	bool		isnull;
+	UserMapping *um;
+
+	tp = SearchSysCache1(USERMAPPINGOID, ObjectIdGetDatum(umid));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for user mapping %u", umid);
+
+	um = (UserMapping *) palloc(sizeof(UserMapping));
+	um->umid = umid;
+
+	/* Extract the umuser */
+	datum = SysCacheGetAttr(USERMAPPINGOID,
+							tp,
+							Anum_pg_user_mapping_umuser,
+							&isnull);
+	Assert(!isnull);
+	um->userid = DatumGetObjectId(datum);
+
+	/* Extract the umserver */
+	datum = SysCacheGetAttr(USERMAPPINGOID,
+							tp,
+							Anum_pg_user_mapping_umserver,
+							&isnull);
+	Assert(!isnull);
+	um->serverid = DatumGetObjectId(datum);
+
+	/* Extract the umoptions */
+	datum = SysCacheGetAttr(USERMAPPINGOID,
+							tp,
+							Anum_pg_user_mapping_umoptions,
+							&isnull);
+	if (isnull)
+		um->options = NIL;
+	else
+		um->options = untransformRelOptions(datum);
+
+	ReleaseSysCache(tp);
+
+	return um;
+}
 
 /*
  * GetUserMapping - look up the user mapping.
@@ -240,8 +288,8 @@ find_user_mapping(Oid userid, Oid serverid)
 
 	/* Not found for the specific user -- try PUBLIC */
 	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
-							 ObjectIdGetDatum(InvalidOid),
-							 ObjectIdGetDatum(serverid));
+						 ObjectIdGetDatum(InvalidOid),
+						 ObjectIdGetDatum(serverid));
 
 	if (!HeapTupleIsValid(tp))
 		ereport(ERROR,
@@ -732,3 +780,115 @@ get_foreign_server_oid(const char *servername, bool missing_ok)
 				 errmsg("server \"%s\" does not exist", servername)));
 	return oid;
 }
+
+/*
+ * Get a copy of an existing local path for a given join relation.
+ *
+ * This function is usually helpful to obtain an alternate local path for EPQ
+ * checks.
+ *
+ * Right now, this function only supports unparameterized foreign joins, so we
+ * only search for unparameterized path in the given list of paths. Since we
+ * are searching for a path which can be used to construct an alternative local
+ * plan for a foreign join, we look for only MergeJoin, HashJoin or NestLoop
+ * paths.
+ *
+ * If the inner or outer subpath of the chosen path is a ForeignScan, we
+ * replace it with its outer subpath.  For this reason, and also because the
+ * planner might free the original path later, the path returned by this
+ * function is a shallow copy of the original.  There's no need to copy
+ * the substructure, so we don't.
+ *
+ * Since the plan created using this path will presumably only be used to
+ * execute EPQ checks, efficiency of the path is not a concern. But since the
+ * list passed is expected to be from RelOptInfo, it's anyway sorted by total
+ * cost and hence we are likely to choose the most efficient path, which is
+ * all for the best.
+ */
+extern Path *
+GetExistingLocalJoinPath(RelOptInfo *joinrel)
+{
+	ListCell   *lc;
+
+	Assert(joinrel->reloptkind == RELOPT_JOINREL);
+
+	foreach(lc, joinrel->pathlist)
+	{
+		Path	   *path = (Path *) lfirst(lc);
+		JoinPath   *joinpath = NULL;
+
+		/* Skip parameterised or non-parallel-safe paths. */
+		if (path->param_info != NULL || !path->parallel_safe)
+			continue;
+
+		switch (path->pathtype)
+		{
+			case T_HashJoin:
+				{
+					HashPath   *hash_path = makeNode(HashPath);
+
+					memcpy(hash_path, path, sizeof(HashPath));
+					joinpath = (JoinPath *) hash_path;
+				}
+				break;
+
+			case T_NestLoop:
+				{
+					NestPath   *nest_path = makeNode(NestPath);
+
+					memcpy(nest_path, path, sizeof(NestPath));
+					joinpath = (JoinPath *) nest_path;
+				}
+				break;
+
+			case T_MergeJoin:
+				{
+					MergePath  *merge_path = makeNode(MergePath);
+
+					memcpy(merge_path, path, sizeof(MergePath));
+					joinpath = (JoinPath *) merge_path;
+				}
+				break;
+
+			default:
+
+				/*
+				 * Just skip anything else. We don't know if corresponding
+				 * plan would build the output row from whole-row references
+				 * of base relations and execute the EPQ checks.
+				 */
+				break;
+		}
+
+		/* This path isn't good for us, check next. */
+		if (!joinpath)
+			continue;
+
+		/*
+		 * If either inner or outer path is a ForeignPath corresponding to a
+		 * pushed down join, replace it with the fdw_outerpath, so that we
+		 * maintain path for EPQ checks built entirely of local join
+		 * strategies.
+		 */
+		if (IsA(joinpath->outerjoinpath, ForeignPath))
+		{
+			ForeignPath *foreign_path;
+
+			foreign_path = (ForeignPath *) joinpath->outerjoinpath;
+			if (foreign_path->path.parent->reloptkind == RELOPT_JOINREL)
+				joinpath->outerjoinpath = foreign_path->fdw_outerpath;
+		}
+
+		if (IsA(joinpath->innerjoinpath, ForeignPath))
+		{
+			ForeignPath *foreign_path;
+
+			foreign_path = (ForeignPath *) joinpath->innerjoinpath;
+			if (foreign_path->path.parent->reloptkind == RELOPT_JOINREL)
+				joinpath->innerjoinpath = foreign_path->fdw_outerpath;
+		}
+
+		return (Path *) joinpath;
+	}
+	return NULL;
+}