Optimize roles_is_member_of() with a Bloom filter.

When the list of roles gathered by roles_is_member_of() grows very
large, a Bloom filter is created to help avoid some linear searches
through the list.  The threshold for creating the Bloom filter is
set arbitrarily high and may require future adjustment.

Suggested-by: Tom Lane
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAGvXd3OSMbJQwOSc-Tq-Ro1CAz%3DvggErdSG7pv2s6vmmTOLJSg%40mail.gmail.com

Author: nathan-bossart

src/backend/utils/adt/acl.c

@@ -36,6 +36,7 @@
 #include "common/hashfn.h"
 #include "foreign/foreign.h"
 #include "funcapi.h"
+#include "lib/bloomfilter.h"
 #include "lib/qunique.h"
 #include "miscadmin.h"
 #include "utils/acl.h"
@@ -78,6 +79,13 @@ static Oid	cached_role[] = {InvalidOid, InvalidOid, InvalidOid};
 static List *cached_roles[] = {NIL, NIL, NIL};
 static uint32 cached_db_hash;
 
+/*
+ * If the list of roles gathered by roles_is_member_of() grows larger than the
+ * below threshold, a Bloom filter is created to speed up list membership
+ * checks.  This threshold is set arbitrarily high to avoid the overhead of
+ * creating the Bloom filter until it seems likely to provide a net benefit.
+ */
+#define ROLES_LIST_BLOOM_THRESHOLD 1024
 
 static const char *getid(const char *s, char *n, Node *escontext);
 static void putid(char *p, const char *s);
@@ -4918,6 +4926,53 @@ RoleMembershipCacheCallback(Datum arg, int cacheid, uint32 hashvalue)
 	cached_role[ROLERECURSE_SETROLE] = InvalidOid;
 }
 
+/*
+ * A helper function for roles_is_member_of() that provides an optimized
+ * implementation of list_append_unique_oid() via a Bloom filter.  The caller
+ * (i.e., roles_is_member_of()) is responsible for freeing bf once it is done
+ * using this function.
+ */
+static inline List *
+roles_list_append(List *roles_list, bloom_filter **bf, Oid role)
+{
+	unsigned char *roleptr = (unsigned char *) &role;
+
+	/*
+	 * If there is a previously-created Bloom filter, use it to try to
+	 * determine whether the role is missing from the list.  If it says yes,
+	 * that's a hard fact and we can go ahead and add the role.  If it says
+	 * no, that's only probabilistic and we'd better search the list.  Without
+	 * a filter, we must always do an ordinary linear search through the
+	 * existing list.
+	 */
+	if ((*bf && bloom_lacks_element(*bf, roleptr, sizeof(Oid))) ||
+		!list_member_oid(roles_list, role))
+	{
+		/*
+		 * If the list is large, we take on the overhead of creating and
+		 * populating a Bloom filter to speed up future calls to this
+		 * function.
+		 */
+		if (*bf == NULL &&
+			list_length(roles_list) > ROLES_LIST_BLOOM_THRESHOLD)
+		{
+			*bf = bloom_create(ROLES_LIST_BLOOM_THRESHOLD * 10, work_mem, 0);
+			foreach_oid(roleid, roles_list)
+				bloom_add_element(*bf, (unsigned char *) &roleid, sizeof(Oid));
+		}
+
+		/*
+		 * Finally, add the role to the list and the Bloom filter, if it
+		 * exists.
+		 */
+		roles_list = lappend_oid(roles_list, role);
+		if (*bf)
+			bloom_add_element(*bf, roleptr, sizeof(Oid));
+	}
+
+	return roles_list;
+}
+
 /*
  * Get a list of roles that the specified roleid is a member of
  *
@@ -4946,6 +5001,7 @@ roles_is_member_of(Oid roleid, enum RoleRecurseType type,
 	ListCell   *l;
 	List	   *new_cached_roles;
 	MemoryContext oldctx;
+	bloom_filter *bf = NULL;
 
 	Assert(OidIsValid(admin_of) == PointerIsValid(admin_role));
 	if (admin_role != NULL)
@@ -5023,16 +5079,22 @@ roles_is_member_of(Oid roleid, enum RoleRecurseType type,
 			 * graph, we must test for having already seen this role. It is
 			 * legal for instance to have both A->B and A->C->B.
 			 */
-			roles_list = list_append_unique_oid(roles_list, otherid);
+			roles_list = roles_list_append(roles_list, &bf, otherid);
 		}
 		ReleaseSysCacheList(memlist);
 
 		/* implement pg_database_owner implicit membership */
 		if (memberid == dba && OidIsValid(dba))
-			roles_list = list_append_unique_oid(roles_list,
-												ROLE_PG_DATABASE_OWNER);
+			roles_list = roles_list_append(roles_list, &bf,
+										   ROLE_PG_DATABASE_OWNER);
 	}
 
+	/*
+	 * Free the Bloom filter created by roles_list_append(), if there is one.
+	 */
+	if (bf)
+		bloom_free(bf);
+
 	/*
 	 * Copy the completed list into TopMemoryContext so it will persist.
 	 */