explain_access_path.h

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#ifndef SQL_JOIN_OPTIMIZER_EXPLAIN_ACCESS_PATH_H
#define SQL_JOIN_OPTIMIZER_EXPLAIN_ACCESS_PATH_H
 
#include <string>
 
struct AccessPath;
class Query_expression;
class JOIN;
class THD;
 
/// Print out an access path and all of its children (if any) in a tree.
std::string PrintQueryPlan(THD *ethd, const THD *query_thd,
                           Query_expression *unit);
/// For debugging purposes.
std::string PrintQueryPlan(int level, AccessPath *path, JOIN *join,
                           bool is_root_of_join);
 
/**
  Generate a digest based on the subplan that the given access path
  represents. This can be used by developers to force a given subplan,
  to investigate e.g. whether a given choice is actually faster in practice,
  force-apply a plan from the old join optimizer (or at least the types of
  subplans that are ever considered; e.g. aggregation through temporary
  tables are not) into the hypergraph join optimizer (to see how it's costed),
  or whether a given plan is even generated. If DEBUG contains
  force_subplan_0x<token>, subplans with the given token are unconditionally
  preferred over all others.
 
  The token returned is “0x@<digest@>”, where @<digest@> is the first 64 bits
  of the SHA-256 sum of this string:
 
    desc1,desc2,...,[child1_desc:]0xchild1,[child2_desc:]0xchild2,@<more
  children@>
 
  where desc1, desc2, etc. are the description lines given by EXPLAIN,
  and 0xchild1 is the token for children. The normal way to generate such
  tokens is to use SET DEBUG='+d,subplan_tokens' and look at the EXPLAIN
  FORMAT=tree, but in a pinch, you can also write them by hand and use
  sha256sum or a similar tool.
 
  Only the hypergraph join optimizer honors token preferences, but EXPLAIN
  FORMAT=tree shows computed tokens for both optimizers.
 */
std::string GetForceSubplanToken(AccessPath *path, JOIN *join);
 
#endif  // SQL_JOIN_OPTIMIZER_EXPLAIN_ACCESS_PATH_H