--- a/cubicweb/server/querier.py	Thu Aug 01 02:51:52 2019 +0200
+++ b/cubicweb/server/querier.py	Thu Aug 01 05:42:45 2019 +0200
@@ -18,6 +18,7 @@
 """Helper classes to execute RQL queries on a set of sources, performing
 security checking and data aggregation.
 """
+import uuid
 from itertools import repeat
 
 from rql import RQLSyntaxError, CoercionError
@@ -164,6 +165,8 @@
         self.querier = querier
         self.schema = querier.schema
         self.rqlhelper = cnx.vreg.rqlhelper
+        # tracing token for debugging
+        self.rql_query_tracing_token = None
 
     def annotate_rqlst(self):
         if not self.rqlst.annotated:
@@ -179,6 +182,7 @@
     def execute(self):
         """execute a plan and return resulting rows"""
         for step in self.steps:
+            step.rql_query_tracing_token = self.rql_query_tracing_token
             result = step.execute()
         # the latest executed step contains the full query result
         return result
@@ -552,6 +556,7 @@
         # make an execution plan
         plan = self.plan_factory(rqlst, args, cnx)
         plan.cache_key = cachekey
+        plan.rql_query_tracing_token = str(uuid.uuid4())
         self._planner.build_plan(plan)
         # execute the plan
         try:

--- a/cubicweb/server/session.py	Thu Aug 01 02:51:52 2019 +0200
+++ b/cubicweb/server/session.py	Thu Aug 01 05:42:45 2019 +0200
@@ -790,11 +790,12 @@
         return service.call(**kwargs)
 
     @_open_only
-    def system_sql(self, sql, args=None, rollback_on_failure=True):
+    def system_sql(self, sql, args=None, rollback_on_failure=True, rql_query_tracing_token=None):
         """return a sql cursor on the system database"""
         source = self.repo.system_source
         try:
-            return source.doexec(self, sql, args, rollback=rollback_on_failure)
+            return source.doexec(self, sql, args, rollback=rollback_on_failure,
+                                 rql_query_tracing_token=rql_query_tracing_token)
         except (source.OperationalError, source.InterfaceError):
             if not rollback_on_failure:
                 raise

--- a/cubicweb/server/sources/native.py	Thu Aug 01 02:51:52 2019 +0200
+++ b/cubicweb/server/sources/native.py	Thu Aug 01 05:42:45 2019 +0200
@@ -499,7 +499,7 @@
                 continue
         raise AuthenticationError()
 
-    def syntax_tree_search(self, cnx, union, args=None, cachekey=None):
+    def syntax_tree_search(self, cnx, union, args=None, cachekey=None, rql_query_tracing_token=None):
         """return result from this source for a rql query (actually from
         a rql syntax tree and a solution dictionary mapping each used
         variable to a possible type). If cachekey is given, the query
@@ -523,7 +523,7 @@
                 self._cache[cachekey] = sql, qargs, cbs
         args = self.merge_args(args, qargs)
         assert isinstance(sql, str), repr(sql)
-        cursor = cnx.system_sql(sql, args)
+        cursor = cnx.system_sql(sql, args, rql_query_tracing_token=rql_query_tracing_token)
         results = self.process_result(cursor, cnx, cbs)
         assert dbg_results(results)
         return results
@@ -680,7 +680,7 @@
         self.doexec(cnx, sql, attrs)
 
     @statsd_timeit
-    def doexec(self, cnx, query, args=None, rollback=True):
+    def doexec(self, cnx, query, args=None, rollback=True, rql_query_tracing_token=None):
         """Execute a query.
         it's a function just so that it shows up in profiling
         """

--- a/cubicweb/server/ssplanner.py	Thu Aug 01 02:51:52 2019 +0200
+++ b/cubicweb/server/ssplanner.py	Thu Aug 01 05:42:45 2019 +0200
@@ -344,6 +344,7 @@
     def __init__(self, plan, union):
         Step.__init__(self, plan)
         self.union = union
+        self.rql_query_tracing_token = None
 
     def execute(self):
         """call .syntax_tree_search with the given syntax tree on each
@@ -365,7 +366,8 @@
             cachekey = union.as_string()
         # get results for query
         source = cnx.repo.system_source
-        result = source.syntax_tree_search(cnx, union, args, cachekey)
+        result = source.syntax_tree_search(cnx, union, args, cachekey,
+                                           rql_query_tracing_token=self.rql_query_tracing_token)
         return result
 
     def mytest_repr(self):