[Fix] Getting file counts query

Grouping by the file path takes too long when the user wants to filter the reports by file path filter.
The solution can be using `distinct` insted of `group by` to get the first ten files considering the report filter as well. Then it is faster to count the number of reports only for these files..

web/server/codechecker_server/api/report_server.py

@@ -3425,35 +3425,35 @@ def getFileCounts(self, run_ids, report_filter, cmp_data, limit, offset):
             filter_expression, join_tables = process_report_filter(
                 session, run_ids, report_filter, cmp_data)
 
-            extended_table = session.query(
-                Report.file_id,
-                Report.bug_id,
-                Report.id)
+            distinct_file_path = session.query(File.filepath.distinct()) \
+                .join(Report, Report.file_id == File.id)
 
             if report_filter.annotations is not None:
-                extended_table = extended_table.outerjoin(
+                distinct_file_path = distinct_file_path.outerjoin(
                     ReportAnnotations,
                     ReportAnnotations.report_id == Report.id)
-                extended_table = extended_table.group_by(Report.id)
+                distinct_file_path = distinct_file_path.group_by(
+                    Report.id)
 
-            extended_table = apply_report_filter(
-                extended_table, filter_expression, join_tables)
+            distinct_file_path = apply_report_filter(
+                distinct_file_path, filter_expression, join_tables, [File])
 
-            extended_table = extended_table.subquery()
+            if limit:
+                distinct_file_path = distinct_file_path.limit(limit) \
+                    .offset(offset)
+
+            distinct_file_path = distinct_file_path.subquery()
 
-            count_col = extended_table.c.bug_id.distinct() if \
-                report_filter.isUnique else extended_table.c.bug_id
+            count_col = Report.bug_id.distinct() if \
+                report_filter.isUnique else Report.bug_id
 
             stmt = session.query(
                     File.filepath,
                     func.count(count_col).label('report_num')) \
                 .join(
-                    extended_table, File.id == extended_table.c.file_id) \
-                .group_by(File.filepath) \
-                .order_by(desc('report_num'))
-
-            if limit:
-                stmt = stmt.limit(limit).offset(offset)
+                    Report, Report.file_id == File.id) \
+                .filter(File.filepath.in_(distinct_file_path)) \
+                .group_by(File.filepath)
 
             for fp, count in stmt:
                 results[fp] = count