MDEV-19474: Histogram statistics are used even with optimizer_use_condition_selectivity=3

Make sure to use the histogram statistics only when optimizer_use_condition_selectivity > 3
even if they were read (this may happen that the histogram statistics were read when
optimizer_use_condition_selectivity was > 3 and then optimizer_use_condition_selectivity was
set to < 4.

5 files changed, 98 insertions, 6 deletions

diff --git a/mysql-test/r/selectivity.result b/mysql-test/r/selectivity.result
index 9bb197ad056..8f2226d8a6c 100644
--- a/mysql-test/r/selectivity.result
+++ b/mysql-test/r/selectivity.result
@@ -1881,4 +1881,36 @@ a	b
 5	5
 set optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
 drop table t1;
+#
+# MDEV-19474: Histogram statistics are used even with optimizer_use_condition_selectivity=3
+#
+CREATE TABLE t1(a int);
+INSERT INTO t1 values (1),(2),(2),(3),(4);
+SET use_stat_tables='preferably';
+SET optimizer_use_condition_selectivity=4;
+SET histogram_size= 255;
+ANALYZE TABLE t1 PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	39.84	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 2)
+SET optimizer_use_condition_selectivity=3;
+# filtered should show 25 %
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	25.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 2)
+FLUSH TABLES;
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	25.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 2)
+set optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
+DROP TABLE t1;
 # End of 10.1 tests
diff --git a/mysql-test/r/selectivity_innodb.result b/mysql-test/r/selectivity_innodb.result
index d0e31374784..205d248bfa4 100644
--- a/mysql-test/r/selectivity_innodb.result
+++ b/mysql-test/r/selectivity_innodb.result
@@ -1891,6 +1891,38 @@ a	b
 5	5
 set optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
 drop table t1;
+#
+# MDEV-19474: Histogram statistics are used even with optimizer_use_condition_selectivity=3
+#
+CREATE TABLE t1(a int);
+INSERT INTO t1 values (1),(2),(2),(3),(4);
+SET use_stat_tables='preferably';
+SET optimizer_use_condition_selectivity=4;
+SET histogram_size= 255;
+ANALYZE TABLE t1 PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	39.84	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 2)
+SET optimizer_use_condition_selectivity=3;
+# filtered should show 25 %
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	25.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 2)
+FLUSH TABLES;
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	25.00	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 2)
+set optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
+DROP TABLE t1;
 # End of 10.1 tests
 set optimizer_switch=@save_optimizer_switch_for_selectivity_test;
 set @tmp_ust= @@use_stat_tables;
diff --git a/mysql-test/t/selectivity.test b/mysql-test/t/selectivity.test
index 1d0256b1b7e..bd36767db1f 100644
--- a/mysql-test/t/selectivity.test
+++ b/mysql-test/t/selectivity.test
@@ -1278,5 +1278,30 @@ SELECT * from t1 WHERE a = 5 and b = 5;
 set optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
 drop table t1;
 
+--echo #
+--echo # MDEV-19474: Histogram statistics are used even with optimizer_use_condition_selectivity=3
+--echo #
+
+CREATE TABLE t1(a int);
+INSERT INTO t1 values (1),(2),(2),(3),(4);
+SET use_stat_tables='preferably';
+SET optimizer_use_condition_selectivity=4;
+SET histogram_size= 255;
+
+ANALYZE TABLE t1 PERSISTENT FOR ALL;
+
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+SET optimizer_use_condition_selectivity=3;
+
+--echo # filtered should show 25 %
+
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+FLUSH TABLES;
+
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=2;
+set optimizer_use_condition_selectivity= @save_optimizer_use_condition_selectivity;
+
+DROP TABLE t1;
+
 --echo # End of 10.1 tests
 
diff --git a/sql/sql_statistics.cc b/sql/sql_statistics.cc
index 11e2db9ae44..cafe11dbea6 100644
--- a/sql/sql_statistics.cc
+++ b/sql/sql_statistics.cc
@@ -3091,7 +3091,8 @@ bool statistics_for_tables_is_needed(THD *thd, TABLE_LIST *tables)
         return TRUE;
       if (table_share->stats_cb.stats_is_read)
         tl->table->stats_is_read= TRUE;
-      if (table_share->stats_cb.histograms_are_read)
+      if (table_share->stats_cb.histograms_are_read &&
+          thd->variables.optimizer_use_condition_selectivity > 3)
         tl->table->histograms_are_read= TRUE;
     } 
   }
@@ -3282,7 +3283,8 @@ int read_statistics_for_tables(THD *thd, TABLE_LIST *tables)
         (void) read_histograms_for_table(thd, tl->table, stat_tables);
         table_share->stats_cb.histograms_are_read= TRUE;
       }
-      if (table_share->stats_cb.histograms_are_read)
+      if (table_share->stats_cb.histograms_are_read &&
+          thd->variables.optimizer_use_condition_selectivity > 3)
         tl->table->histograms_are_read= TRUE;
     }
   }  
@@ -3857,7 +3859,7 @@ double get_column_range_cardinality(Field *field,
           col_stats->min_max_values_are_provided())
       {
         Histogram *hist= &col_stats->histogram;
-        if (hist->is_available())
+        if (table->histograms_are_read && hist->is_available())
         {
           store_key_image_to_rec(field, (uchar *) min_endp->key,
                                  field->key_length());
@@ -3901,10 +3903,10 @@ double get_column_range_cardinality(Field *field,
         max_mp_pos= 1.0;
 
       Histogram *hist= &col_stats->histogram;
-      if (!hist->is_available())
-        sel= (max_mp_pos - min_mp_pos);
-      else
+      if (table->histograms_are_read && hist->is_available())
         sel= hist->range_selectivity(min_mp_pos, max_mp_pos);
+      else
+        sel= (max_mp_pos - min_mp_pos);
       res= col_non_nulls * sel;
       set_if_bigger(res, col_stats->get_avg_frequency());
     }
diff --git a/sql/table.cc b/sql/table.cc
index 192faa0d00e..48817de4215 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -4229,6 +4229,7 @@ void TABLE::init(THD *thd, TABLE_LIST *tl)
   cond_selectivity_sampling_explain= NULL;
   quick_condition_rows=0;
   initialize_quick_structures();
+  histograms_are_read= FALSE;
 #ifdef HAVE_REPLICATION
   /* used in RBR Triggers */
   master_had_triggers= 0;