ha_innodb.cc:

  Fix bug #12779 : never give a row count estimate of 0 to the MySQL query optimizer, as then left join optimizer may beleive it KNOWS that the table is empty; note that this fix may change query optimization of many other queries where one table is empty; note that the proper fix would be to make the query optimizer to know that the row count estimates it receives really are just estimates, it cannot assume they are certain


sql/ha_innodb.cc:
  Fix bug #12779 : never give a row count estimate of 0 to the MySQL query optimizer, as then left join optimizer may beleive it KNOWS that the table is empty; note that this fix may change query optimization of many other queries where one table is empty; note that the proper fix would be to make the query optimizer to know that the row count estimates it receives really are just estimates, it cannot assume they are certain

1 files changed, 25 insertions, 1 deletions

diff --git a/sql/ha_innodb.cc b/sql/ha_innodb.cc
index bcd07198f5c..3ae56f13478 100644
--- a/sql/ha_innodb.cc
+++ b/sql/ha_innodb.cc
@@ -4109,6 +4109,7 @@ ha_innobase::info(
 	dict_table_t*	ib_table;
 	dict_index_t*	index;
 	ha_rows		rec_per_key;
+	ib_longlong	n_rows;
 	ulong		j;
 	ulong		i;
 
@@ -4151,7 +4152,30 @@ ha_innobase::info(
  	}
 
 	if (flag & HA_STATUS_VARIABLE) {
-    		records = (ha_rows)ib_table->stat_n_rows;
+		n_rows = ib_table->stat_n_rows;
+
+		/* Because we do not protect stat_n_rows by any mutex in a
+		delete, it is theoretically possible that the value can be
+		smaller than zero! TODO: fix this race.
+
+		The MySQL optimizer seems to assume in a left join that n_rows
+		is an accurate estimate if it is zero. Of course, it is not,
+		since we do not have any locks on the rows yet at this phase.
+		Since SHOW TABLE STATUS seems to call this function with the
+		HA_STATUS_TIME flag set, while the left join optizer does not
+		set that flag, we add one to a zero value if the flag is not
+		set. That way SHOW TABLE STATUS will show the best estimate,
+		while the optimizer never sees the table empty. */
+
+		if (n_rows < 0) {
+			n_rows = 0;
+		}
+
+		if (n_rows == 0 && !(flag & HA_STATUS_TIME)) {
+			n_rows++;
+		}
+
+    		records = (ha_rows)n_rows;
     		deleted = 0;
     		data_file_length = ((ulonglong)
 				ib_table->stat_clustered_index_size)