explain_access_path.h File Reference

#include <string>

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Functions
std::string	PrintQueryPlan (THD *ethd, const THD *query_thd, Query_expression *unit)
	Print out an access path and all of its children (if any) in a tree. More...
std::string	PrintQueryPlan (int level, AccessPath *path, JOIN *join, bool is_root_of_join)
	For debugging purposes. More...
std::string	GetForceSubplanToken (AccessPath *path, JOIN *join)
	Generate a digest based on the subplan that the given access path represents. More...

Function Documentation

GetForceSubplanToken()

std::string GetForceSubplanToken	(	AccessPath *	path,
		JOIN *	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.

PrintQueryPlan() [1/2]

std::string PrintQueryPlan	(	int	level,
		AccessPath *	path,
		JOIN *	join,
		bool	is_root_of_join
	)

For debugging purposes.

PrintQueryPlan() [2/2]

std::string PrintQueryPlan	(	THD *	ethd,
		const THD *	query_thd,
		Query_expression *	unit
	)

Print out an access path and all of its children (if any) in a tree.