MDEV-20519: Query plan regression with optimizer_use_condition_selectivity > 1

The issue here is the wrong estimate of the cardinality of a partial join,
the cardinality is too high because the function table_cond_selectivity()
returns an absurd number 100 while selectivity cannot be greater than 1.

When accessing table t by outer reference t1.a via index we do not perform any
range analysis for t. Yet we see TABLE::quick_key_parts[key] and
TABLE->quick_rows[key] contain a non-zero value though these should have been
remained untouched and equal to 0.

Thus real cause of the problem is that TABLE::init does not clean the arrays
TABLE::quick_key_parts[] and TABLE::>quick_rows[].
It should have done it because the TABLE structure created for any
instance of a table can be reused for many queries.

1 files changed, 1 insertions, 0 deletions

diff --git a/sql/table.h b/sql/table.h
index 98ec9f005ea..44803b5aacd 100644
--- a/sql/table.h
+++ b/sql/table.h
@@ -1450,6 +1450,7 @@ public:
   }
 
   bool update_const_key_parts(COND *conds);
+  void initialize_quick_structures();
 
   my_ptrdiff_t default_values_offset() const
   { return (my_ptrdiff_t) (s->default_values - record[0]); }