Directly test QuerySets to avoid extra DB calls

.exists() will perform the simplest and fastest possible query to check
for existance of results.  However if they do exist they're not cached
and another query must be made to retrieve the results.

treeherder/log_parser/crossreference.py

@@ -41,20 +41,17 @@ def crossreference_job(job):
 @transaction.atomic
 def _crossreference(job):
     failure_lines = FailureLine.objects.filter(job_guid=job.guid)
+    text_log_errors = TextLogError.objects.filter(step__job=job).order_by('line_number')
 
-    text_log_errors = TextLogError.objects.filter(
-        step__job=job).order_by('line_number')
-
-    # If we don't have both failure lines and text log errors this will never succeed
-    if not (failure_lines.exists() and text_log_errors.exists()):
+    if not failure_lines and text_log_errors:
         return False
 
-    match_iter = structured_iterator(list(failure_lines.all()))
+    match_iter = structured_iterator(failure_lines)
     failure_line, repr_str = next(match_iter)
 
     # For each error in the text log, try to match the next unmatched
     # structured log line
-    for error in list(text_log_errors.all()):
+    for error in text_log_errors:
         if repr_str and error.line.strip().endswith(repr_str):
             logger.debug("Matched '%s'", error.line)
             TextLogErrorMetadata.objects.get_or_create(text_log_error=error,