MDEV-406: ANALYZE $stmt: Scans that never executed will have r_rows=NULL

mysql-test/r/analyze_stmt.result

@@ -74,4 +74,9 @@ analyze select a, a in (select t0.b from t0 where t0.b+1=t1.b+1) from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	3	3	100.00	100.00	
 2	DEPENDENT SUBQUERY	t0	ALL	NULL	NULL	NULL	NULL	10	3	100.00	33.33	Using where
+# Try a subquery that is never executed
+analyze select a, a in (select t0.b from t0 where t0.b+1=t1.b+1) from t1 where t1.a > 5;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	3	3	100.00	0.00	Using where
+2	DEPENDENT SUBQUERY	t0	ALL	NULL	NULL	NULL	NULL	10	NULL	100.00	NULL	Using where
 drop table t0,t1;

mysql-test/t/analyze_stmt.test

@@ -58,5 +58,8 @@ insert into t1 values (1,1),(2,2),(3,3);
 --echo # See .test file for the right values of r_rows and r_filtered.
 analyze select a, a in (select t0.b from t0 where t0.b+1=t1.b+1) from t1;
 
+--echo # Try a subquery that is never executed
+analyze select a, a in (select t0.b from t0 where t0.b+1=t1.b+1) from t1 where t1.a > 5;
+
 drop table t0,t1;
 

sql/sql_explain.cc

@@ -552,11 +552,18 @@ int Explain_table_access::print_explain(select_result_sink *output, uint8 explai
 
   /* `r_rows` */
   if (is_analyze)
+  {
+    if (!tracker.has_scans())
+    {
+      item_list.push_back(item_null);
+    }
+    else
     {
       ha_rows avg_rows= tracker.get_avg_rows();
       item_list.push_back(new Item_int((longlong) (ulonglong) avg_rows,
                                         MY_INT64_NUM_DECIMAL_DIGITS));
     }
+  }
 
   /* `filtered` */
   if (explain_flags & DESCRIBE_EXTENDED || is_analyze)
@@ -571,6 +578,12 @@ int Explain_table_access::print_explain(select_result_sink *output, uint8 explai
 
   /* `r_filtered` */
   if (is_analyze)
+  {
+    if (!tracker.has_scans())
+    {
+      item_list.push_back(item_null);
+    }
+    else
     {
       double r_filtered;
       if (tracker.r_rows > 0)
@@ -579,6 +592,7 @@ int Explain_table_access::print_explain(select_result_sink *output, uint8 explai
         r_filtered= 100.0;
       item_list.push_back(new Item_float(r_filtered, 2));
     }
+  }
 
   /* `Extra` */
   StringBuffer<256> extra_buf;

sql/sql_explain.h

@@ -28,6 +28,7 @@ public:
   ha_rows r_rows_after_table_cond; /* Rows after applying the table condition */
   ha_rows r_rows_after_where; /* Rows after applying attached part of WHERE */
 
+  bool has_scans() { return (r_scans != 0); }
   ha_rows get_avg_rows()
   {
     return r_scans ? (ha_rows)rint((double) r_rows / r_scans): 0;