MDEV-18156 Assertion `0' failed or `btr_validate_index(index, 0, false)' in row_upd_sec_index_entry or error code 126: Index is corrupted upon DELETE with PAD_CHAR_TO_FULL_LENGTH

This change takes into account a column's GENERATED ALWAYS AS
expression dependcy on sql_mode's PAD_CHAR_TO_FULL_LENGTH and
NO_UNSIGNED_SUBTRACTION flags.

Indexed virtual columns as well as persistent generated columns are
now not allowed to have such dependencies to avoid inconsistent data
or index files on sql_mode changes.
So an error is now returned in cases like this:

  CREATE OR REPLACE TABLE t1
  (
    a CHAR(5),
    v VARCHAR(5) AS (a) PERSISTENT -- CHAR->VARCHAR or CHAR->TEXT = ERROR
  );

Functions RPAD() and RTRIM() can now remove dependency on
PAD_CHAR_TO_FULL_LENGTH. So this can be used instead:

  CREATE OR REPLACE TABLE t1
  (
    a CHAR(5),
    v VARCHAR(5) AS (RTRIM(a)) PERSISTENT
  );

Note, unlike CHAR->VARCHAR and CHAR->TEXT this still works,
not RPAD(a) is needed:

  CREATE OR REPLACE TABLE t1
  (
    a CHAR(5),
    v CHAR(5) AS (a) PERSISTENT -- CHAR->CHAR is OK
  );

More sql_mode flags may affect values of generated columns.
They will be addressed separately.

See comments in sql_mode.h for implementation details.

17 files changed, 472 insertions, 4 deletions

diff --git a/sql/CMakeLists.txt b/sql/CMakeLists.txt
index 769a1b53331..963243c9767 100644
--- a/sql/CMakeLists.txt
+++ b/sql/CMakeLists.txt
@@ -135,7 +135,7 @@ SET (SQL_SOURCE
                my_apc.cc mf_iocache_encr.cc item_jsonfunc.cc
                my_json_writer.cc
                rpl_gtid.cc rpl_parallel.cc
-               sql_type.cc
+               sql_type.cc sql_mode.cc
                item_windowfunc.cc sql_window.cc
 	       sql_cte.cc
 	       ${WSREP_SOURCES}
diff --git a/sql/field.cc b/sql/field.cc
index d7ac03328a0..e5359a7520e 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -1428,6 +1428,43 @@ void Field::load_data_set_value(const char *pos, uint length,
 }
 
 
+void Field::error_generated_column_function_is_not_allowed(THD *thd) const
+{
+  StringBuffer<64> tmp;
+  vcol_info->expr->print(&tmp, (enum_query_type)
+                               (QT_TO_SYSTEM_CHARSET |
+                                QT_ITEM_IDENT_SKIP_DB_NAMES |
+                                QT_ITEM_IDENT_SKIP_TABLE_NAMES));
+  my_error(ER_GENERATED_COLUMN_FUNCTION_IS_NOT_ALLOWED, MYF(0),
+           tmp.c_ptr(), vcol_info->get_vcol_type_name(),
+           const_cast<const char*>(field_name));
+}
+
+
+/*
+  Check if an indexed or a persistent virtual column depends on sql_mode flags
+  that it cannot handle.
+  See sql_mode.h for details.
+*/
+bool Field::check_vcol_sql_mode_dependency(THD *thd) const
+{
+  DBUG_ASSERT(vcol_info);
+  if ((flags & PART_KEY_FLAG) != 0 || stored_in_db())
+  {
+    Sql_mode_dependency dep=
+        vcol_info->expr->value_depends_on_sql_mode() &
+        Sql_mode_dependency(~0, ~can_handle_sql_mode_dependency_on_store());
+    if (dep)
+    {
+      error_generated_column_function_is_not_allowed(thd);
+      dep.push_dependency_warnings(thd);
+      return true;
+    }
+  }
+  return false;
+}
+
+
 /**
   Numeric fields base class constructor.
 */
@@ -1464,6 +1501,12 @@ void Field_num::prepend_zeros(String *value) const
 }
 
 
+sql_mode_t Field_num::can_handle_sql_mode_dependency_on_store() const
+{
+  return MODE_PAD_CHAR_TO_FULL_LENGTH;
+}
+
+
 Item *Field_num::get_equal_zerofill_const_item(THD *thd, const Context &ctx,
                                                Item *const_item)
 {
@@ -5654,6 +5697,12 @@ my_time_t Field_timestampf::get_timestamp(const uchar *pos,
 
 
 /*************************************************************/
+sql_mode_t Field_temporal::can_handle_sql_mode_dependency_on_store() const
+{
+  return MODE_PAD_CHAR_TO_FULL_LENGTH;
+}
+
+
 uint Field_temporal::is_equal(Create_field *new_field)
 {
   return new_field->sql_type == real_type() &&
@@ -7271,6 +7320,18 @@ longlong Field_string::val_int(void)
 }
 
 
+sql_mode_t Field_string::value_depends_on_sql_mode() const
+{
+  return has_charset() ? MODE_PAD_CHAR_TO_FULL_LENGTH : sql_mode_t(0);
+};
+
+
+sql_mode_t Field_string::can_handle_sql_mode_dependency_on_store() const
+{
+  return has_charset() ? MODE_PAD_CHAR_TO_FULL_LENGTH : sql_mode_t(0);
+}
+
+
 String *Field_string::val_str(String *val_buffer __attribute__((unused)),
 			      String *val_ptr)
 {
@@ -8881,6 +8942,12 @@ bool Field_geom::load_data_set_null(THD *thd)
 ** If one uses this string in a number context one gets the type number.
 ****************************************************************************/
 
+sql_mode_t Field_enum::can_handle_sql_mode_dependency_on_store() const
+{
+  return MODE_PAD_CHAR_TO_FULL_LENGTH;
+}
+
+
 enum ha_base_keytype Field_enum::key_type() const
 {
   switch (packlength) {
diff --git a/sql/field.h b/sql/field.h
index ac7794081bc..9333d1492fd 100644
--- a/sql/field.h
+++ b/sql/field.h
@@ -697,6 +697,7 @@ protected:
     val_str(&result);
     return to->store(result.ptr(), result.length(), charset());
   }
+  void error_generated_column_function_is_not_allowed(THD *thd) const;
   static void do_field_int(Copy_field *copy);
   static void do_field_real(Copy_field *copy);
   static void do_field_string(Copy_field *copy);
@@ -1174,6 +1175,16 @@ public:
   }
 
   bool stored_in_db() const { return !vcol_info || vcol_info->stored_in_db; }
+  bool check_vcol_sql_mode_dependency(THD *) const;
+
+  virtual sql_mode_t value_depends_on_sql_mode() const
+  {
+    return 0;
+  }
+  virtual sql_mode_t can_handle_sql_mode_dependency_on_store() const
+  {
+    return 0;
+  }
 
   inline THD *get_thd() const
   { return likely(table) ? table->in_use : current_thd; }
@@ -1653,6 +1664,7 @@ public:
   enum Derivation derivation(void) const { return DERIVATION_NUMERIC; }
   uint repertoire(void) const { return MY_REPERTOIRE_NUMERIC; }
   CHARSET_INFO *charset(void) const { return &my_charset_numeric; }
+  sql_mode_t can_handle_sql_mode_dependency_on_store() const;
   Item *get_equal_const_item(THD *thd, const Context &ctx, Item *const_item)
   {
     return (flags & ZEROFILL_FLAG) ?
@@ -2389,6 +2401,7 @@ public:
                field_name_arg)
     { flags|= BINARY_FLAG; }
   Item_result result_type () const { return STRING_RESULT; }   
+  sql_mode_t can_handle_sql_mode_dependency_on_store() const;
   int  store_hex_hybrid(const char *str, uint length)
   {
     return store(str, length, &my_charset_bin);
@@ -3190,6 +3203,8 @@ public:
   { return charset() == &my_charset_bin ? FALSE : TRUE; }
   Field *make_new_field(MEM_ROOT *root, TABLE *new_table, bool keep_type);
   virtual uint get_key_image(uchar *buff,uint length, imagetype type);
+  sql_mode_t value_depends_on_sql_mode() const;
+  sql_mode_t can_handle_sql_mode_dependency_on_store() const;
 private:
   int do_save_field_metadata(uchar *first_byte);
 };
@@ -3599,6 +3614,7 @@ public:
   enum_field_types type() const { return MYSQL_TYPE_STRING; }
   enum Item_result cmp_type () const { return INT_RESULT; }
   enum ha_base_keytype key_type() const;
+  sql_mode_t can_handle_sql_mode_dependency_on_store() const;
   Copy_func *get_copy_func(const Field *from) const
   {
     if (eq_def(from))
diff --git a/sql/item.h b/sql/item.h
index 2239544dc88..ff4cfd6c1b8 100644
--- a/sql/item.h
+++ b/sql/item.h
@@ -1216,6 +1216,32 @@ public:
   double val_real_from_decimal();
   double val_real_from_date();
 
+  /*
+    Returns true if this item can be calculated during
+    value_depends_on_sql_mode()
+  */
+  bool value_depends_on_sql_mode_const_item()
+  {
+    /*
+      Currently we use value_depends_on_sql_mode() only for virtual
+      column expressions. They should not contain any expensive items.
+      If we ever get a crash on the assert below, it means
+      check_vcol_func_processor() is badly implemented for this item.
+    */
+    DBUG_ASSERT(!is_expensive());
+    /*
+      It should return const_item() actually.
+      But for some reasons Item_field::const_item() returns true
+      at value_depends_on_sql_mode() call time.
+      This should be checked and fixed.
+    */
+    return basic_const_item();
+  }
+  virtual Sql_mode_dependency value_depends_on_sql_mode() const
+  {
+    return Sql_mode_dependency();
+  }
+
   // Get TIME, DATE or DATETIME using proper sql_mode flags for the field type
   bool get_temporal_with_sql_mode(MYSQL_TIME *ltime);
   // Check NULL value for a TIME, DATE or DATETIME expression
@@ -2694,6 +2720,10 @@ public:
   {
     return MONOTONIC_STRICT_INCREASING;
   }
+  Sql_mode_dependency value_depends_on_sql_mode() const
+  {
+    return Sql_mode_dependency(0, field->value_depends_on_sql_mode());
+  }
   longlong val_int_endpoint(bool left_endp, bool *incl_endp);
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate);
   bool get_date_result(MYSQL_TIME *ltime,ulonglong fuzzydate);
@@ -4102,6 +4132,7 @@ public:
   inline Item **arguments() const { return args; }
   inline uint argument_count() const { return arg_count; }
   inline void remove_arguments() { arg_count=0; }
+  Sql_mode_dependency value_depends_on_sql_mode_bit_or() const;
 };
 
 
@@ -4334,6 +4365,10 @@ public:
   bool const_item() const { return const_item_cache; }
   table_map used_tables() const { return used_tables_cache; }
   Item* build_clone(THD *thd, MEM_ROOT *mem_root);
+  Sql_mode_dependency value_depends_on_sql_mode() const
+  {
+    return Item_args::value_depends_on_sql_mode_bit_or().soft_to_hard();
+  }
 };
 
 
diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index 8bf130b297f..2c3a96f3f47 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -531,6 +531,23 @@ bool Item_func::setup_args_and_comparator(THD *thd, Arg_comparator *cmp)
 }
 
 
+/*
+  Comparison operators remove arguments' dependency on PAD_CHAR_TO_FULL_LENGTH
+  in case of PAD SPACE comparison collations: trailing spaces do not affect
+  the comparison result for such collations.
+*/
+Sql_mode_dependency
+Item_bool_rowready_func2::value_depends_on_sql_mode() const
+{
+  if (compare_collation()->state & MY_CS_NOPAD)
+    return Item_func::value_depends_on_sql_mode();
+  return ((args[0]->value_depends_on_sql_mode() |
+           args[1]->value_depends_on_sql_mode()) &
+          Sql_mode_dependency(~0, ~MODE_PAD_CHAR_TO_FULL_LENGTH)).
+         soft_to_hard();
+}
+
+
 bool Item_bool_rowready_func2::fix_length_and_dec()
 {
   max_length= 1;				     // Function returns 0 or 1
@@ -5284,6 +5301,29 @@ bool Item_func_like::with_sargable_pattern() const
 }
 
 
+/*
+  subject LIKE pattern
+  removes subject's dependency on PAD_CHAR_TO_FULL_LENGTH
+  if pattern ends with the '%' wildcard.
+*/
+Sql_mode_dependency Item_func_like::value_depends_on_sql_mode() const
+{
+  if (!args[1]->value_depends_on_sql_mode_const_item())
+    return Item_func::value_depends_on_sql_mode();
+  StringBuffer<64> patternbuf;
+  String *pattern= args[1]->val_str_ascii(&patternbuf);
+  if (!pattern || !pattern->length())
+    return Sql_mode_dependency();                  // Will return NULL or 0
+  DBUG_ASSERT(pattern->charset()->mbminlen == 1);
+  if (pattern->ptr()[pattern->length() - 1] != '%')
+    return Item_func::value_depends_on_sql_mode();
+  return ((args[0]->value_depends_on_sql_mode() |
+           args[1]->value_depends_on_sql_mode()) &
+          Sql_mode_dependency(~0, ~MODE_PAD_CHAR_TO_FULL_LENGTH)).
+         soft_to_hard();
+}
+
+
 SEL_TREE *Item_func_like::get_mm_tree(RANGE_OPT_PARAM *param, Item **cond_ptr)
 {
   MEM_ROOT *tmp_root= param->mem_root;
diff --git a/sql/item_cmpfunc.h b/sql/item_cmpfunc.h
index 5045e39fc5a..0a5abfe6273 100644
--- a/sql/item_cmpfunc.h
+++ b/sql/item_cmpfunc.h
@@ -489,6 +489,7 @@ public:
   {
     allowed_arg_cols= 0;  // Fetch this value from first argument
   }
+  Sql_mode_dependency value_depends_on_sql_mode() const;
   void print(String *str, enum_query_type query_type)
   {
     Item_func::print_op(str, query_type);
@@ -1920,6 +1921,7 @@ public:
     Item_bool_func2(thd, a, b), canDoTurboBM(FALSE), pattern(0), pattern_len(0),
     bmGs(0), bmBc(0), escape_item(escape_arg),
     escape_used_in_parsing(escape_used), use_sampling(0), negated(0) {}
+  Sql_mode_dependency value_depends_on_sql_mode() const;
   longlong val_int();
   enum Functype functype() const { return LIKE_FUNC; }
   void print(String *str, enum_query_type query_type);
diff --git a/sql/item_func.cc b/sql/item_func.cc
index 5ff1b26d734..d11b56f2aad 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -422,6 +422,15 @@ void Item_args::propagate_equal_fields(THD *thd,
 }
 
 
+Sql_mode_dependency Item_args::value_depends_on_sql_mode_bit_or() const
+{
+  Sql_mode_dependency res;
+  for (uint i= 0; i < arg_count; i++)
+    res|= args[i]->value_depends_on_sql_mode();
+  return res;
+}
+
+
 /**
   See comments in Item_cond::split_sum_func()
 */
@@ -1414,9 +1423,13 @@ bool Item_func_minus::fix_length_and_dec()
 {
   if (Item_num_op::fix_length_and_dec())
     return TRUE;
-  if (unsigned_flag &&
-      (current_thd->variables.sql_mode & MODE_NO_UNSIGNED_SUBTRACTION))
-    unsigned_flag=0;
+  m_sql_mode_dependency= Item_func::value_depends_on_sql_mode();
+  if (unsigned_flag)
+  {
+    m_sql_mode_dependency|= Sql_mode_dependency(0,MODE_NO_UNSIGNED_SUBTRACTION);
+    if (current_thd->variables.sql_mode & MODE_NO_UNSIGNED_SUBTRACTION)
+      unsigned_flag= false;
+  }
   return FALSE;
 }
 
diff --git a/sql/item_func.h b/sql/item_func.h
index 81a7e948085..5be62427852 100644
--- a/sql/item_func.h
+++ b/sql/item_func.h
@@ -773,11 +773,16 @@ public:
 
 class Item_func_minus :public Item_func_additive_op
 {
+  Sql_mode_dependency m_sql_mode_dependency;
 public:
   Item_func_minus(THD *thd, Item *a, Item *b):
     Item_func_additive_op(thd, a, b) {}
   const char *func_name() const { return "-"; }
   enum precedence precedence() const { return ADD_PRECEDENCE; }
+  Sql_mode_dependency value_depends_on_sql_mode() const
+  {
+    return m_sql_mode_dependency;
+  }
   longlong int_op();
   double real_op();
   my_decimal *decimal_op(my_decimal *);
diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index 753b6134419..dcfbd272809 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -2103,6 +2103,41 @@ void Item_func_trim::print(String *str, enum_query_type query_type)
 }
 
 
+/*
+  RTRIM(expr)
+  TRIM(TRAILING ' ' FROM expr)
+  remove argument's soft dependency on PAD_CHAR_TO_FULL_LENGTH:
+*/
+Sql_mode_dependency Item_func_trim::value_depends_on_sql_mode() const
+{
+  DBUG_ASSERT(fixed);
+  if (arg_count == 1) // RTRIM(expr)
+    return (args[0]->value_depends_on_sql_mode() &
+            Sql_mode_dependency(~0, ~MODE_PAD_CHAR_TO_FULL_LENGTH)).
+           soft_to_hard();
+  // TRIM(... FROM expr)
+  DBUG_ASSERT(arg_count == 2);
+  if (!args[1]->value_depends_on_sql_mode_const_item())
+    return Item_func::value_depends_on_sql_mode();
+  StringBuffer<64> trimstrbuf;
+  String *trimstr= args[1]->val_str(&trimstrbuf);
+  if (!trimstr)
+    return Sql_mode_dependency();                  // will return NULL
+  if (trimstr->length() == 0)
+    return Item_func::value_depends_on_sql_mode(); // will trim nothing
+  if (trimstr->lengthsp() != 0)
+    return Item_func::value_depends_on_sql_mode(); // will trim not only spaces
+  if (trimstr->length() > trimstr->charset()->mbminlen ||
+      trimstr->numchars() > 1)
+    return Item_func::value_depends_on_sql_mode(); // more than one space
+  // TRIM(TRAILING ' ' FROM expr)
+  return ((args[0]->value_depends_on_sql_mode() |
+           args[1]->value_depends_on_sql_mode()) &
+          Sql_mode_dependency(~0, ~MODE_PAD_CHAR_TO_FULL_LENGTH)).
+         soft_to_hard();
+}
+
+
 /* Item_func_password */
 
 bool Item_func_password::fix_fields(THD *thd, Item **ref)
@@ -3116,6 +3151,38 @@ bool Item_func_pad::fix_length_and_dec()
 }
 
 
+/*
+  PAD(expr,length,' ')
+  removes argument's soft dependency on PAD_CHAR_TO_FULL_LENGTH if the result
+  is longer than the argument's maximim possible length.
+*/
+Sql_mode_dependency Item_func_rpad::value_depends_on_sql_mode() const
+{
+  DBUG_ASSERT(fixed);
+  DBUG_ASSERT(arg_count == 3);
+  if (!args[1]->value_depends_on_sql_mode_const_item() ||
+      !args[2]->value_depends_on_sql_mode_const_item())
+    return Item_func::value_depends_on_sql_mode();
+  Longlong_hybrid len= args[1]->to_longlong_hybrid();
+  if (args[1]->null_value || len.neg())
+    return Sql_mode_dependency();                  // will return NULL
+  if (len.abs() > 0 && len.abs() < args[0]->max_char_length())
+    return Item_func::value_depends_on_sql_mode();
+  StringBuffer<64> padstrbuf;
+  String *padstr= args[2]->val_str(&padstrbuf);
+  if (!padstr || !padstr->length())
+    return Sql_mode_dependency();                  // will return NULL
+  if (padstr->lengthsp() != 0)
+    return Item_func::value_depends_on_sql_mode(); // will pad not only spaces
+  // RPAD(expr, length, ' ')  -- with a long enough length
+  return ((args[0]->value_depends_on_sql_mode() |
+           args[1]->value_depends_on_sql_mode()) &
+          Sql_mode_dependency(~0, ~MODE_PAD_CHAR_TO_FULL_LENGTH)).
+         soft_to_hard();
+}
+
+
+
 String *Item_func_rpad::val_str(String *str)
 {
   DBUG_ASSERT(fixed == 1);
diff --git a/sql/item_strfunc.h b/sql/item_strfunc.h
index a398b4972cb..df8761534a1 100644
--- a/sql/item_strfunc.h
+++ b/sql/item_strfunc.h
@@ -489,6 +489,7 @@ protected:
 public:
   Item_func_trim(THD *thd, Item *a, Item *b): Item_str_func(thd, a, b) {}
   Item_func_trim(THD *thd, Item *a): Item_str_func(thd, a) {}
+  Sql_mode_dependency value_depends_on_sql_mode() const;
   String *val_str(String *);
   bool fix_length_and_dec();
   const char *func_name() const { return "trim"; }
@@ -504,6 +505,10 @@ class Item_func_ltrim :public Item_func_trim
 public:
   Item_func_ltrim(THD *thd, Item *a, Item *b): Item_func_trim(thd, a, b) {}
   Item_func_ltrim(THD *thd, Item *a): Item_func_trim(thd, a) {}
+  Sql_mode_dependency value_depends_on_sql_mode() const
+  {
+    return Item_func::value_depends_on_sql_mode();
+  }
   String *val_str(String *);
   const char *func_name() const { return "ltrim"; }
   const char *mode_name() const { return "leading"; }
@@ -950,6 +955,7 @@ public:
     Item_func_pad(thd, arg1, arg2, arg3) {}
   String *val_str(String *);
   const char *func_name() const { return "rpad"; }
+  Sql_mode_dependency value_depends_on_sql_mode() const;
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_func_rpad>(thd, mem_root, this); }
 };
diff --git a/sql/mysqld.h b/sql/mysqld.h
index 26229c8cc5d..2947734901c 100644
--- a/sql/mysqld.h
+++ b/sql/mysqld.h
@@ -19,6 +19,7 @@
 
 #include <my_global.h> /* MYSQL_PLUGIN_IMPORT, FN_REFLEN, FN_EXTLEN */
 #include "sql_basic_types.h"			/* query_id_t */
+#include "sql_mode.h"                           /* Sql_mode_dependency */
 #include "sql_bitmap.h"                         /* Bitmap */
 #include "my_decimal.h"                         /* my_decimal */
 #include "mysql_com.h"                     /* SERVER_VERSION_LENGTH */
diff --git a/sql/set_var.h b/sql/set_var.h
index 8a563f3c089..a5439a27137 100644
--- a/sql/set_var.h
+++ b/sql/set_var.h
@@ -427,6 +427,7 @@ inline bool IS_SYSVAR_AUTOSIZE(void *ptr)
 bool fix_delay_key_write(sys_var *self, THD *thd, enum_var_type type);
 
 sql_mode_t expand_sql_mode(sql_mode_t sql_mode);
+const char *sql_mode_string_representation(uint bit_number);
 bool sql_mode_string_representation(THD *thd, sql_mode_t sql_mode, LEX_STRING *ls);
 int default_regex_flags_pcre(const THD *thd);
 
diff --git a/sql/sql_mode.cc b/sql/sql_mode.cc
new file mode 100644
index 00000000000..9eea6ccdb52
--- /dev/null
+++ b/sql/sql_mode.cc
@@ -0,0 +1,34 @@
+/*
+   Copyright (c) 2019, MariaDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */
+
+#include "mysqld.h"
+#include "set_var.h"
+
+void Sql_mode_dependency::push_dependency_warnings(THD *thd)
+{
+  sql_mode_t all= m_hard | m_soft;
+  for (uint i= 0; all ; i++, all >>= 1)
+  {
+    if (all & 1)
+    {
+      // TODO-10.5: add a new error code
+      push_warning_printf(thd,
+                          Sql_condition::WARN_LEVEL_WARN, ER_UNKNOWN_ERROR,
+                          "Expression depends on the @@%s value %s",
+                          "sql_mode", sql_mode_string_representation(i));
+    }
+  }
+}
diff --git a/sql/sql_mode.h b/sql/sql_mode.h
new file mode 100644
index 00000000000..e92848fb6d1
--- /dev/null
+++ b/sql/sql_mode.h
@@ -0,0 +1,162 @@
+#ifndef SQL_MODE_H_INCLUDED
+#define SQL_MODE_H_INCLUDED
+/*
+   Copyright (c) 2019, MariaDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */
+
+#ifdef USE_PRAGMA_INTERFACE
+#pragma interface			/* gcc class implementation */
+#endif
+
+#include "sql_basic_types.h"
+
+/*
+  class Sql_mode_dependency
+
+  A combination of hard and soft dependency on sql_mode.
+  Used to watch if a GENERATED ALWAYS AS expression guarantees consitent
+  data written to its virtual column.
+
+  A virtual column can appear in an index if:
+  - the generation expression does not depend on any sql_mode flags, or
+  - the generation expression has a soft dependency on an sql_mode flag,
+    and the column knows how to handle this dependeny.
+
+  A virtual column cannot appear in an index if:
+  - its generation expression has a hard dependency
+  - its generation expression has a soft dependency, but the column
+    cannot handle it on store.
+  An error is reported in such cases.
+
+  How dependencies appear:
+  - When a column return value depends on some sql_mode flag,
+    its Item_field adds a corresponding bit to m_soft. For example,
+    Item_field for a CHAR(N) column adds the PAD_CHAR_TO_FULL_LENGTH flag.
+  - When an SQL function/operator return value depends on some sql_mode flag,
+    it adds a corresponding bit to m_soft. For example, Item_func_minus
+    adds the MODE_NO_UNSIGNED_SUBTRACTION in case of unsigned arguments.
+
+  How dependency are processed (see examples below):
+  - All SQL functions/operators bit-OR all hard dependencies from all arguments.
+  - Some soft dependencies can be handled by the underlying Field on store,
+    e.g. CHAR(N) can handle PAD_CHAR_TO_FULL_LENGTH.
+  - Some soft dependencies can be handled by SQL functions and operators,
+    e.g. RTRIM(expr) removes expr's soft dependency on PAD_CHAR_TO_FULL_LENGTH.
+    If a function or operator handles a soft dependency on a certain sql_mode
+    flag, it removes the corresponding bit from m_soft (see below).
+    Note, m_hard is not touched in such cases.
+  - When an expression with a soft dependency on a certain sql_mode flag
+    goes as an argument to an SQL function/operator which cannot handle
+    this flag, the dependency escalates from soft to hard
+    (by moving the corresponding bit from m_soft to m_hard) and cannot be
+    handled any more on the upper level, neither by a Field on store,
+    nor by another SQL function/operator.
+
+  There are four kinds of Items:
+  1. Items that generate a soft or hard dependency, e.g.
+     - Item_field for CHAR(N) - generates soft/PAD_CHAR_TO_FULL_LENGTH
+     - Item_func_minus        - generates soft/NO_UNSIGNED_SUBTRACTION
+  2. Items that convert a soft dependency to a hard dependency.
+     This happens e.g. when an Item_func instance gets a soft dependency
+     from its arguments, and it does not know how to handle this dependency.
+     Most Item_func descendants do this.
+  3. Items that remove soft dependencies, e.g.:
+     - Item_func_rtrim - removes soft/PAD_CHAR_TO_FULL_LENGTH
+                         that came from args[0] (under certain conditions)
+     - Item_func_rpad  - removes soft/PAD_CJAR_TO_FULL_LENGTH
+                         that came from args[0] (under certain conditions)
+  4. Items that repeat soft dependency from its arguments to the caller.
+     They are not implemented yet. But functions like Item_func_coalesce,
+     Item_func_case, Item_func_case_abbreviation2 could do this.
+
+  Examples:
+
+  1. CREATE OR REPLACE TABLE t1 (a CHAR(5), v CHAR(20) AS(a), KEY(v));
+
+     Here `v` has a soft dependency on `a`.
+     The value of `a` depends on PAD_CHAR_TO_FULL_LENGTH, it can return:
+     - 'a'                         - if PAD_CHAR_TO_FULL_LENGTH is disabled
+     - 'a' followed by four spaces - if PAD_CHAR_TO_FULL_LENGTH is enabled
+     But `v` will pad trailing spaces to the full length on store anyway.
+     So Field_string handles this soft dependency on store.
+     This combination of the virtial column data type and its generation
+     expression is safe and provides consistent data in `v`, which is
+     'a' followed by four spaces, no matter what PAD_CHAR_TO_FULL_LENGTH is.
+
+  2. CREATE OR REPLACE TABLE t1 (a CHAR(5), v VARCHAR(20) AS(a), KEY(v));
+
+     Here `v` has a soft dependency on `a`. But Field_varstring does
+     not pad spaces on store, so it cannot handle this dependency.
+     This combination of the virtual column data type and its generation
+     expression is not safe. An error is returned.
+
+  3. CREATE OR REPLACE TABLE t1 (a CHAR(5), v INT AS(LENGTH(a)), KEY(v));
+
+     Here `v` has a hard dependency on `a`, because the value of `a`
+     is wrapped to the function LENGTH().
+     The value of `LENGTH(a)` depends on PAD_CHAR_TO_FULL_LENGTH, it can return:
+     - 1  - if PAD_CHAR_TO_FULL_LENGTH is disabled
+     - 4  - if PAD_CHAR_TO_FULL_LENGTH is enabled
+     This combination cannot provide consistent data stored to `v`,
+     therefore it's disallowed.
+*/
+class Sql_mode_dependency
+{
+  sql_mode_t m_hard;
+  sql_mode_t m_soft;
+public:
+  Sql_mode_dependency()
+   :m_hard(0), m_soft(0)
+  { }
+  Sql_mode_dependency(sql_mode_t hard, sql_mode_t soft)
+   :m_hard(hard), m_soft(soft)
+  { }
+  sql_mode_t hard() const { return m_hard; }
+  sql_mode_t soft() const { return m_soft; }
+  operator bool () const
+  {
+    return m_hard > 0 || m_soft > 0;
+  }
+  Sql_mode_dependency operator|(const Sql_mode_dependency &other) const
+  {
+    return Sql_mode_dependency(m_hard | other.m_hard, m_soft | other.m_soft);
+  }
+  Sql_mode_dependency operator&(const Sql_mode_dependency &other) const
+  {
+    return Sql_mode_dependency(m_hard & other.m_hard, m_soft & other.m_soft);
+  }
+  Sql_mode_dependency &operator|=(const Sql_mode_dependency &other)
+  {
+    m_hard|= other.m_hard;
+    m_soft|= other.m_soft;
+    return *this;
+  }
+  Sql_mode_dependency &operator&=(const Sql_mode_dependency &other)
+  {
+    m_hard&= other.m_hard;
+    m_soft&= other.m_soft;
+    return *this;
+  }
+  Sql_mode_dependency &soft_to_hard()
+  {
+    m_hard|= m_soft;
+    m_soft= 0;
+    return *this;
+  }
+  void push_dependency_warnings(THD *thd);
+};
+
+
+#endif // SQL_MODE_H_INCLUDED
diff --git a/sql/sql_string.h b/sql/sql_string.h
index a7652376bbc..e1d64722898 100644
--- a/sql/sql_string.h
+++ b/sql/sql_string.h
@@ -235,6 +235,11 @@ public:
     return skr;
   }
 
+  size_t lengthsp() const
+  {
+    return str_charset->cset->lengthsp(str_charset, Ptr, str_length);
+  }
+
   void set(String &str,uint32 offset,uint32 arg_length)
   {
     DBUG_ASSERT(&str != this);
diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
index 04ecbb29b12..8eb75e49d8e 100644
--- a/sql/sys_vars.cc
+++ b/sql/sys_vars.cc
@@ -3160,6 +3160,14 @@ static const char *sql_mode_names[]=
   0
 };
 
+
+const char *sql_mode_string_representation(uint bit_number)
+{
+  DBUG_ASSERT(bit_number < array_elements(sql_mode_names));
+  return sql_mode_names[bit_number];
+}
+
+
 export bool sql_mode_string_representation(THD *thd, sql_mode_t sql_mode,
                                            LEX_STRING *ls)
 {
diff --git a/sql/table.cc b/sql/table.cc
index d3b4cf3a3c0..62449ebcc06 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -1073,6 +1073,12 @@ bool parse_vcol_defs(THD *thd, MEM_ROOT *mem_root, TABLE *table,
       vcol= unpack_vcol_info_from_frm(thd, mem_root, table, &expr_str,
                                     &((*field_ptr)->vcol_info), error_reported);
       *(vfield_ptr++)= *field_ptr;
+      if (vcol && field_ptr[0]->check_vcol_sql_mode_dependency(thd))
+      {
+        DBUG_ASSERT(thd->is_error());
+        *error_reported= true;
+        goto end;
+      }
       break;
     case VCOL_DEFAULT:
       vcol= unpack_vcol_info_from_frm(thd, mem_root, table, &expr_str,