MDEV-9641 MDEV-9644 NULLIF assertions

* only copy args[0] to args[2] after fix_fields (when all item
  substitutions have already happened)

* change QT_ITEM_FUNC_NULLIF_TO_CASE (that allows to print NULLIF
  as CASE) to QT_ITEM_ORIGINAL_FUNC_NULLIF (that prohibits it).
  So that NULLIF-to-CASE is allowed by default and only disabled
  explicitly for SHOW VIEW|FUNCTION|PROCEDURE and mysql_make_view.
  By default it is allowed (in particular in error messages and
  debug output, that can happen anytime before or after optimizer).

1 files changed, 8 insertions, 7 deletions

diff --git a/sql/mysqld.h b/sql/mysqld.h
index 1c1478f8a63..30120d66113 100644
--- a/sql/mysqld.h
+++ b/sql/mysqld.h
@@ -665,15 +665,16 @@ enum enum_query_type
   /// If NULLIF(a,b) should print itself as
   /// CASE WHEN a_for_comparison=b THEN NULL ELSE a_for_return_value END
   /// when "a" was replaced to two different items
-  /// (e.g. by equal fields propagation in optimize_cond()).
-  /// The default behaviour is to print as NULLIF(a_for_return, b)
-  /// which should be Ok for SHOW CREATE {VIEW|PROCEDURE|FUNCTION}
-  /// as they are not affected by WHERE optimization.
-  QT_ITEM_FUNC_NULLIF_TO_CASE= (1 <<6),
+  /// (e.g. by equal fields propagation in optimize_cond())
+  /// or always as NULLIF(a, b).
+  /// The default behaviour is to use CASE syntax when
+  /// a_for_return_value is not the same as a_for_comparison.
+  /// SHOW CREATE {VIEW|PROCEDURE|FUNCTION} and other cases where the
+  /// original representation is required, should set this flag.
+  QT_ITEM_ORIGINAL_FUNC_NULLIF= (1 <<6),
 
   /// This value means focus on readability, not on ability to parse back, etc.
   QT_EXPLAIN=           QT_TO_SYSTEM_CHARSET |
-                        QT_ITEM_FUNC_NULLIF_TO_CASE |
                         QT_ITEM_IDENT_SKIP_CURRENT_DATABASE |
                         QT_ITEM_CACHE_WRAPPER_SKIP_DETAILS |
                         QT_ITEM_SUBSELECT_ID_ONLY,
@@ -682,7 +683,7 @@ enum enum_query_type
   /// Be more detailed than QT_EXPLAIN.
   /// Perhaps we should eventually include QT_ITEM_IDENT_SKIP_CURRENT_DATABASE
   /// here, as it would give better readable results
-  QT_EXPLAIN_EXTENDED=  QT_TO_SYSTEM_CHARSET | QT_ITEM_FUNC_NULLIF_TO_CASE
+  QT_EXPLAIN_EXTENDED=  QT_TO_SYSTEM_CHARSET
 };