CostOfItem Class Reference

This class represents the cost of evaluating an Item. More...

#include <item.h>

Public Member Functions
void	Compute (const Item &item)
	Set '*this' to represent the cost of 'item'. More...
void	MarkExpensive ()
void	AddStrFieldCost ()
	Add the cost of accessing a Field_str. More...
void	AddFieldCost ()
	Add the cost of accessing any other Field. More...
bool	IsExpensive () const
double	FieldCost () const
	Get the cost of field access when evaluating the Item associated with this object. More...

Private Member Functions
void	ComputeInternal (const Item &root)
	Compute the cost of 'root' and its descendants. More...

Private Attributes
bool	m_computed {false}
	True if 'ComputeInternal()' has been called. More...
bool	m_is_expensive {false}
	True if the associated Item calls user defined functions or stored procedures. More...
uint8	m_str_fields {0}
	The number of Field_str objects accessed by the associated Item. More...
uint8	m_other_fields {0}
	The number of other Field objects accessed by the associated Item. More...

Static Private Attributes
static constexpr double	kStrFieldCost = 1.8
	The cost of accessing a Field_str, relative to other Field types. More...
static constexpr double	kOtherFieldCost = 1.0
	The cost of accessing a Field other than Field_str. 1.0 by definition. More...

Detailed Description

This class represents the cost of evaluating an Item.

See also

SortPredicates to see how this is used.

Member Function Documentation

AddFieldCost()

void CostOfItem::AddFieldCost ( )

inline

Add the cost of accessing any other Field.

AddStrFieldCost()

void CostOfItem::AddStrFieldCost ( )

inline

Add the cost of accessing a Field_str.

Compute()

void CostOfItem::Compute ( const Item &  item )

inline

Set '*this' to represent the cost of 'item'.

ComputeInternal()

void CostOfItem::ComputeInternal ( const Item &  root )

private

Compute the cost of 'root' and its descendants.

FieldCost()

double CostOfItem::FieldCost ( ) const

inline

Get the cost of field access when evaluating the Item associated with this object.

The cost unit is arbitrary, but the relative cost of different items reflect the fact that operating on Field_str is more expensive than other Field subclasses.

IsExpensive()

bool CostOfItem::IsExpensive ( ) const

inline

MarkExpensive()

void CostOfItem::MarkExpensive ( )

inline

Member Data Documentation

kOtherFieldCost

constexpr double CostOfItem::kOtherFieldCost = 1.0

staticconstexprprivate

The cost of accessing a Field other than Field_str. 1.0 by definition.

kStrFieldCost

constexpr double CostOfItem::kStrFieldCost = 1.8

staticconstexprprivate

The cost of accessing a Field_str, relative to other Field types.

(The value was determined using benchmarks.)

m_computed

bool CostOfItem::m_computed {false}

private

True if 'ComputeInternal()' has been called.

m_is_expensive

bool CostOfItem::m_is_expensive {false}

private

True if the associated Item calls user defined functions or stored procedures.

m_other_fields

uint8 CostOfItem::m_other_fields {0}

private

The number of other Field objects accessed by the associated Item.

m_str_fields

uint8 CostOfItem::m_str_fields {0}

private

The number of Field_str objects accessed by the associated Item.

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The documentation for this class was generated from the following files:

• sql/item.h
• sql/item.cc