common_subexpression_elimination.h

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#ifndef SQL_JOIN_OPTIMIZER_COMMON_SUBEXPRESSION_ELIMINATION
#define SQL_JOIN_OPTIMIZER_COMMON_SUBEXPRESSION_ELIMINATION 1
 
class Item;
 
/**
  Do simple CSE (common subexpression elimination) on “item”, and return
  the answer. The CSE done is exclusively moving common expressions out
  of conjunctions-of-disjunctions, ie. it rewrites
 
    (a AND b) OR (a AND c)
 
  into
 
    a AND (b OR c)
 
  The primary motivation is that such split-out items are more versatile;
  they can be pushed independently, be made into hash join conditions etc.
  However, an added bonus is that the expressions will simply execute faster.
 
  This function does not descend into subexpressions that are not AND/OR
  conjunctions, so e.g. an expression like
 
    1 + ((a AND b) OR (a AND c))
 
  will be left as-is.
 */
Item *CommonSubexpressionElimination(Item *cond);
 
#endif  // SQL_JOIN_OPTIMIZER_COMMON_SUBEXPRESSION_ELIMINATION