diff --git a/histogram.py b/histogram.py
index 80ddd95..3e78e2a 100755
--- a/histogram.py
+++ b/histogram.py
@@ -1,36 +1,43 @@
 #!/usr/bin/env python
 
 import io
 import os
 
 from collections import Counter
 
 from swh.provenance import get_provenance
 
 
 # TODO: take conninfo as command line arguments.
 conninfo = {
     "cls": "local",
     "db": {"host": "/var/run/postgresql", "port": "5436", "dbname": "provenance"},
 }
 
 
 if __name__ == "__main__":
     # Get provenance object for both databases and query its lists of content.
     provenance = get_provenance(**conninfo)
 
     tables = ["directory_in_rev", "content_in_dir"]
 
     for table in tables:
         provenance.cursor.execute(f"""SELECT depths.depth, COUNT(depths.depth)
-                                        FROM (SELECT (CHAR_LENGTH(ENCODE(location.path, 'escape')) - CHAR_LENGTH(REPLACE(ENCODE(location.path, 'escape'), '/', ''))) / CHAR_LENGTH('/') AS depth
+                                        FROM (SELECT 
+                                                CASE location.path
+                                                    WHEN '' THEN 0
+                                                    WHEN '.' THEN 0
+                                                    ELSE 1 + CHAR_LENGTH(ENCODE(location.path, 'escape')) - 
+                                                             CHAR_LENGTH(REPLACE(ENCODE(location.path, 'escape'), '/', ''))
+                                                END AS depth
                                                 FROM {table}
                                                 JOIN location
                                                   ON {table}.loc=location.id
                                              ) AS depths
                                         GROUP BY depths.depth
                                         ORDER BY depths.depth""")
-        with io.open(conninfo["db"]["dbname"] + f"_{table}.csv", "w") as outfile:
+        filename = "depths_" + conninfo["db"]["dbname"] + f"_{table}.csv"
+        with io.open(filename, "w") as outfile:
             outfile.write(f"{table} depth,{table} count\n")
             for depth, count in provenance.cursor.fetchall():
                 outfile.write(f"{depth},{count}\n")