MDEV-12293 Assertion `table->no_keyread ||...

MDEV-12293 Assertion `table->no_keyread || !table->covering_keys.is_set(tab->index) || table->file->keyread == tab->index' failed

sometimes the optimizer starts a keyread on some index
and later (but before any actual index accesses) changes
it to a keyread on a different index

mysql-test/r/keyread.result

+create table t1 (f1 int not null, f2 int, f3 int, primary key (f1,f2), key(f2,f3)) engine=innodb;
+create view v1 as select * from t1 where f2 = 1;
+select distinct f1 from v1;
+f1
+drop view v1;
+drop table t1;

mysql-test/t/keyread.test

+--source include/have_innodb.inc
+
+#
+# MDEV-12293 Assertion `table->no_keyread || !table->covering_keys.is_set(tab->index) || table->file->keyread == tab->index' failed
+#
+create table t1 (f1 int not null, f2 int, f3 int, primary key (f1,f2), key(f2,f3)) engine=innodb;
+create view v1 as select * from t1 where f2 = 1;
+select distinct f1 from v1;
+drop view v1;
+drop table t1;

sql/handler.h

@@ -2858,10 +2858,9 @@ class handler :public Sql_alloc
   bool keyread_enabled() { return keyread < MAX_KEY; }
   int ha_start_keyread(uint idx)
   {
-    if (keyread_enabled())
-      return 0;
+    int res= keyread_enabled() ? 0 : extra(HA_EXTRA_KEYREAD);
     keyread= idx;
-    return extra(HA_EXTRA_KEYREAD);
+    return res;
   }
   int ha_end_keyread()
   {