MDEV-34319: DECLARE TYPE .. TABLE OF .. INDEX BY in stored routines

This patch adds support for associative arrays in stored procedures
for sql_mode=ORACLE.
The syntax follows Oracle's PL/SQL syntax for associative arrays -
TYPE assoc_array_t IS TABLE OF VARCHAR2(100) INDEX BY INTEGER;
or
TYPE assoc_array_t IS TABLE OF record_t INDEX BY VARCHAR2(100);
where record_t is a record type.

The following functions were added for associative arrays:
- COUNT - Retrieve the number of elements within the arra
- EXISTS - Check whether given key exists in the array
- FIRST - Retrieve the first key in the array
- LAST - Retrieve the last key in the array
- PRIOR - Retrieve the key before the given key
- NEXT - Retrieve the key after the given key
- DELETE - Remove the element with the given key or remove all elements
if no key is given

The arrays/elements can be initialized with the following methods:
- Constructor
i.e. array:= assoc_array_t('key1'=>1, 'key2'=>2, 'key3'=>3)
- Assignment
i.e. array(key):= record_t(1, 2)
- SELECT INTO
i.e. SELECT x INTO array(key)

TODOs:
- Nested tables are not supported yet.
i.e. TYPE assoc_array_t IS TABLE OF other_assoc_array_t INDEX BY INTEGER;
- Associative arrays comparisons are not supported yet.

sql/item_composite.h

@@ -0,0 +1,103 @@
+/*
+   Copyright (c) 2025, Rakuten Securities
+   Copyright (c) 2025, MariaDB plc
+
+   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
+*/
+#ifndef ITEM_COMPOSITE_INCLUDED
+#define ITEM_COMPOSITE_INCLUDED
+
+#include "item.h"
+
+class Field_composite;
+
+class Item_composite_base
+{
+public:
+  virtual ~Item_composite_base() = default;
+
+  // For associative arrays
+  /// Returns the number of columns for the elements of the array
+  virtual uint rows() const { return 1; }
+  virtual bool get_key(String *key, bool is_first) { return true; }
+  virtual bool get_next_key(const String *curr_key, String *next_key)
+  {
+    return true;
+  }
+  virtual Item *element_by_key(THD *thd, String *key) { return nullptr; }
+  virtual Item **element_addr_by_key(THD *thd, Item **addr_arg, String *key)
+  {
+    return addr_arg;
+  }
+
+  /*
+    Get the composite field for the item, when applicable.
+  */
+  virtual Field_composite *get_composite_field() const { return nullptr; }
+};
+
+
+class Item_composite: public Item_fixed_hybrid,
+                      protected Item_args,
+                      public Item_composite_base
+{
+public:
+  Item_composite(THD *thd, List<Item> &list)
+    :Item_fixed_hybrid(thd), Item_args(thd, list)
+  { }
+  Item_composite(THD *thd, Item_args *other)
+    :Item_fixed_hybrid(thd), Item_args(thd, other)
+  { }
+  Item_composite(THD *thd)
+    :Item_fixed_hybrid(thd)
+  { }
+
+  enum Type type() const override { return ROW_ITEM; }
+
+  void illegal_method_call(const char *);
+
+  void make_send_field(THD *thd, Send_field *) override
+  {
+    illegal_method_call((const char*)"make_send_field");
+  };
+  double val_real() override
+  {
+    illegal_method_call((const char*)"val");
+    return 0;
+  };
+  longlong val_int() override
+  {
+    illegal_method_call((const char*)"val_int");
+    return 0;
+  };
+  String *val_str(String *) override
+  {
+    illegal_method_call((const char*)"val_str");
+    return 0;
+  };
+  my_decimal *val_decimal(my_decimal *) override
+  {
+    illegal_method_call((const char*)"val_decimal");
+    return 0;
+  };
+  bool get_date(THD *thd, MYSQL_TIME *ltime, date_mode_t fuzzydate) override
+  {
+    illegal_method_call((const char*)"get_date");
+    return true;
+  }
+};
+
+
+#endif /* ITEM_COMPOSITE_INCLUDED */