replace_item.h

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#ifndef SQL_JOIN_OPTIMIZER_REPLACE_ITEM_H
#define SQL_JOIN_OPTIMIZER_REPLACE_ITEM_H
 
class Func_ptr;
class Item;
class Item_field;
template <class T>
class Mem_root_array;
class THD;
using Func_ptr_array = Mem_root_array<Func_ptr>;
 
/*
  Return a new item that is to be used after materialization (as given by
  items_to_copy). There are three main cases:
 
    1. The item isn't touched by materialization (e.g., because it's constant,
       or because we're not ready to compute it yet).
    2. The item is directly in the items_to_copy list, so it has its own field
       in the resulting temporary table; the corresponding new Item_field
       is returned.
    3. A _part_ of the item is in the items_to_copy list; e.g. say that we
       have an item (t1.x + 1), and t1.x is materialized into <temporary>.x.
       (In particular, this happens when having expressions that contain
       aggregate functions _and_ non-aggregates.) In this case, we go in and
       modify the item in-place, so that the appropriate sub-expressions are
       replaced; in this case, to (<temporary>.x + 1). This assumes that we
       never use the same item before and after a materialization in the
       query plan!
 */
Item *FindReplacementOrReplaceMaterializedItems(
    THD *thd, Item *item, const Func_ptr_array &items_to_copy,
    bool need_exact_match);
 
/**
  Like FindReplacementOrReplaceMaterializedItems, but only search _below_ the
  item, ie. ignore point 2 above. This can be useful if doing self-replacement,
  ie., we are replacing source items in items_to_copy and don't want to
  replace an item with its own output.
 */
void ReplaceMaterializedItems(THD *thd, Item *item,
                              const Func_ptr_array &items_to_copy,
                              bool need_exact_match);
 
#endif  // SQL_JOIN_OPTIMIZER_REPLACE_ITEM_H