MDEV-6519: Assertion `join->best_read < double(...)' failed after adding a key to a TokuDB table...

- calculate_cond_selectivity_for_table() should handle the case
  where index statistics is not available (zeros are returned 
  in rec_per_key)

sql/opt_range.cc

@@ -3547,7 +3547,19 @@ bool calculate_cond_selectivity_for_table(THD *thd, TABLE *table, Item *cond)
             */
             double f1= key_info->actual_rec_per_key(i-1);
             double f2= key_info->actual_rec_per_key(i);
-            table->cond_selectivity*= f1 / f2;
+            double selectivity_mult;
+            if (f1 > 0 && f2 > 0)
+              selectivity_mult= f1 / f2;
+            else
+            {
+              /* 
+                No statistics available, assume the selectivity is proportional
+                to the number of key parts.
+                (i=0 means 1 keypart, i=1 means 2 keyparts, so use i+1)
+              */
+              selectivity_mult= ((double)(i+1)) / i;
+            }
+            table->cond_selectivity*= selectivity_mult;
           }
         }
       }

storage/tokudb/mysql-test/tokudb_mariadb/r/mdev6519.result

+drop table if exists t1;
+SET @tmp_ust= @@use_stat_tables;
+SET @tmp_oucs= @@optimizer_use_condition_selectivity;
+SET use_stat_tables = PREFERABLY;
+SET optimizer_use_condition_selectivity = 2;
+CREATE TABLE t1 (
+code CHAR(2),
+name VARCHAR(32),
+population INT,
+house_seats TINYINT,
+PRIMARY KEY(code),
+KEY (house_seats)
+) ENGINE=TokuDB;
+INSERT INTO t1 VALUES ('AL','Alabama',4833722,7),('AK','Alaska',735132,1);
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+ALTER TABLE t1 ADD KEY (name(4),house_seats);
+SELECT * FROM t1 WHERE 
+population BETWEEN 1000000 AND 2000000 
+AND name LIKE 'New %'
+AND house_seats IN (1, 2, 3)
+AND code IN ('NJ', 'NM');
+code	name	population	house_seats
+drop table t1;
+SET use_stat_tables = @tmp_ust;
+SET optimizer_use_condition_selectivity = @tmp_oucs;

storage/tokudb/mysql-test/tokudb_mariadb/t/mdev6519.test

+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+
+SET @tmp_ust= @@use_stat_tables;
+SET @tmp_oucs= @@optimizer_use_condition_selectivity;
+
+SET use_stat_tables = PREFERABLY;
+SET optimizer_use_condition_selectivity = 2;
+
+CREATE TABLE t1 (
+  code CHAR(2),
+  name VARCHAR(32),
+  population INT,
+  house_seats TINYINT,
+  PRIMARY KEY(code),
+  KEY (house_seats)
+) ENGINE=TokuDB;
+
+INSERT INTO t1 VALUES ('AL','Alabama',4833722,7),('AK','Alaska',735132,1);
+ANALYZE TABLE t1; 
+ALTER TABLE t1 ADD KEY (name(4),house_seats);
+
+SELECT * FROM t1 WHERE 
+population BETWEEN 1000000 AND 2000000 
+AND name LIKE 'New %'
+AND house_seats IN (1, 2, 3)
+AND code IN ('NJ', 'NM');
+
+drop table t1;
+SET use_stat_tables = @tmp_ust;
+SET optimizer_use_condition_selectivity = @tmp_oucs;
+