st_position Struct Reference

#include <sql_select.h>

Inheritance diagram for st_position:

List of all members.

Public Member Functions
void	no_semijoin ()
void	set_prefix_costs (double read_time_arg, double row_count_arg)
Public Attributes
double	records_read
double	read_time
JOIN_TAB *	table
Key_use *	key
table_map	ref_depend_map
bool	use_join_buffer
Cost_estimate	prefix_cost
double	prefix_record_count
uint	sj_strategy
uint	n_sj_tables
table_map	dups_producing_tables
uint	first_loosescan_table
table_map	loosescan_need_tables
uint	loosescan_key
uint	loosescan_parts
uint	first_firstmatch_table
table_map	first_firstmatch_rtbl
table_map	firstmatch_need_tables
uint	first_dupsweedout_table
table_map	dupsweedout_tables
uint	sjm_scan_last_inner
table_map	sjm_scan_need_tables

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Detailed Description

A position of table within a join order. This structure is primarily used as a part of join->positions and join->best_positions arrays.

One POSITION element contains information about:

• Which table is accessed
• Which access method was chosen = Its cost and #of output records
• Semi-join strategy choice. Note that there are two different representation formats: 1. The one used during join optimization 2. The one used at plan refinement/code generation stage. We call fix_semijoin_strategies_for_picked_join_order() to switch between #1 and #2. See that function's comment for more details.

• Semi-join optimization state. When we're running join optimization, we main a state for every semi-join strategy which are various variables that tell us if/at which point we could consider applying the strategy. The variables are really a function of join prefix but they are too expensive to re-caclulate for every join prefix we consider, so we maintain current state in join->positions[#tables_in_prefix]. See advance_sj_state() for details.

This class has to stay a POD, because it is memcpy'd in many places.

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Member Function Documentation

void st_position::no_semijoin

(

)

[inline]

Even if the query has no semijoin, two sj-related members are read and must thus have been set, by this function.

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Member Data Documentation

table_map st_position::dups_producing_tables

Bitmap of semi-join inner tables that are in the join prefix and for which there's no provision yet for how to eliminate semi-join duplicates which they produce.

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The documentation for this struct was generated from the following file:

• sql_select.h