collect: small optimization on garbage collect

This changes a query like:

    SELECT date FROM user WHERE reported = 1 AND date != '2020-04-13' AND date != '2020-04-12'  AND date != '2020-04-11'  AND date != '2020-04-10'  AND date != '2020-04-09'  AND date != '2020-04-08'  AND date != '2020-04-07'  AND date != '2020-04-06'  AND date != '2020-04-05' AND date != '2020-04-04'  AND date != '2020-04-03'  AND date != '2020-04-02'  AND date != '2020-04-01'  AND date != '2020-03-31'  AND date != '2020-03-30'  LIMIT 1

    EXPLAIN QUERY PLAN ...
    0|0|0|SCAN TABLE user USING COVERING INDEX user_date_reported_index

which took ~3 seconds on a 1.5Go collector.db into:

    SELECT date FROM user WHERE reported = 1 AND (date < '2020-03-30'  OR  date > '2020-04-13')  LIMIT 1

    EXPLAIN QUERY PLAN ...
    0|0|0|SEARCH TABLE user USING COVERING INDEX user_date_reported_index (date<?)
    0|0|0|SEARCH TABLE user USING COVERING INDEX user_date_reported_index (date>?)

which is instant.

See merge request !201

slapos/collect/db.py

@@ -285,11 +285,12 @@ class Database:
         reported.
     """
     date_list = self._getGarbageCollectionDateList(days_to_preserve)
-    where_clause = "reported = 1" 
-    for _date in date_list:
-      where_clause += " AND date != '%s' " % _date
+    where_clause = "reported = 1"
+    if date_list:
+      where_clause += " AND (date < '%s' " % min(date_list)
+      where_clause += " OR date > '%s') " % max(date_list)
 
-    vacuum = 0 
+    vacuum = False
     delete_sql = "DELETE FROM %s WHERE %s"
     select_sql = "SELECT date FROM %s WHERE %s LIMIT 1"
 
@@ -298,7 +299,7 @@ class Database:
       if table not in self.preserve_table_list:
         if self._execute(select_sql % (table, where_clause)).fetchone() is not None:
           self._execute(delete_sql % (table, where_clause))
-          vacuum = 1
+          vacuum = True
 
     self.commit()
     if vacuum: