EQRefIterator Class Reference

Like RefIterator, but used in situations where we're guaranteed to have exactly zero or one rows for each reference (due to e.g. More...

#include <ref_row_iterators.h>

Inheritance diagram for EQRefIterator:

Public Member Functions
	EQRefIterator (THD *thd, TABLE *table, Index_lookup *ref, ha_rows *examined_rows)
bool	Init () override
	Read row using unique key: eq_ref access method implementation. More...
int	Read () override
	Read row using unique key: eq_ref access method implementation. More...
void	UnlockRow () override
	Since EQRefIterator may buffer a record, do not unlock it if it was not used in this invocation of EQRefIterator::Read(). More...
void	StartPSIBatchMode () override
	Start performance schema batch mode, if supported (otherwise ignored). More...
Public Member Functions inherited from TableRowIterator
	TableRowIterator (THD *thd, TABLE *table)
void	UnlockRow () override
	The default implementation of unlock-row method of RowIterator, used in all access methods except EQRefIterator. More...
void	SetNullRowFlag (bool is_null_row) override
	Mark the current row buffer as containing a NULL row or not, so that if you read from it and the flag is true, you'll get only NULLs no matter what is actually in the buffer (typically some old leftover row). More...
void	StartPSIBatchMode () override
	Start performance schema batch mode, if supported (otherwise ignored). More...
void	EndPSIBatchModeIfStarted () override
	Ends performance schema batch mode, if started. More...
Public Member Functions inherited from RowIterator
	RowIterator (THD *thd)
virtual	~RowIterator ()=default
	RowIterator (const RowIterator &)=delete
	RowIterator (RowIterator &&)=default
virtual const IteratorProfiler *	GetProfiler () const
	Get profiling data for this iterator (for 'EXPLAIN ANALYZE'). More...
virtual void	SetOverrideProfiler (const IteratorProfiler *profiler)
virtual RowIterator *	real_iterator ()
	If this iterator is wrapping a different iterator (e.g. More...
virtual const RowIterator *	real_iterator () const

Private Attributes
Index_lookup *const	m_ref
bool	m_first_record_since_init
ha_rows *const	m_examined_rows

Additional Inherited Members
Protected Member Functions inherited from TableRowIterator
int	HandleError (int error)
void	PrintError (int error)
TABLE *	table () const
Protected Member Functions inherited from RowIterator
THD *	thd () const

Detailed Description

Like RefIterator, but used in situations where we're guaranteed to have exactly zero or one rows for each reference (due to e.g.

unique constraints). It adds extra buffering to reduce the number of calls to the storage engine in the case where many consecutive rows on the left side contain the same value.

Constructor & Destructor Documentation

EQRefIterator()

EQRefIterator::EQRefIterator	(	THD *	thd,
		TABLE *	table,
		Index_lookup *	ref,
		ha_rows *	examined_rows
	)

Member Function Documentation

Init()

bool EQRefIterator::Init ( void  )

overridevirtual

Read row using unique key: eq_ref access method implementation.

This is the "read_first" function for the eq_ref access method. The difference from ref access function is that it has a one-element lookup cache, maintained in record[0]. Since the eq_ref access method will always return the same row, it is not necessary to read the row more than once, regardless of how many times it is needed in execution. This cache element is used when a row is needed after it has been read once, unless a key conversion error has occurred, or the cache has been disabled.

Return values

0	- Ok
-1	- Row not found
1	- Error

Implements RowIterator.

Read()

int EQRefIterator::Read ( )

overridevirtual

Read row using unique key: eq_ref access method implementation.

The difference from RefIterator is that it has a one-element lookup cache, maintained in record[0]. Since the eq_ref access method will always return the same row, it is not necessary to read the row more than once, regardless of how many times it is needed in execution. This cache element is used when a row is needed after it has been read once, unless a key conversion error has occurred, or the cache has been disabled.

Return values

0	- Ok
-1	- Row not found
1	- Error

Implements RowIterator.

StartPSIBatchMode()

void EQRefIterator::StartPSIBatchMode ( )

inlineoverridevirtual

Start performance schema batch mode, if supported (otherwise ignored).

PFS batch mode is a mitigation to reduce the overhead of performance schema, typically applied at the innermost table of the entire join. If you start it before scanning the table and then end it afterwards, the entire set of handler calls will be timed only once, as a group, and the costs will be distributed evenly out. This reduces timer overhead.

If you start PFS batch mode, you must also take care to end it at the end of the scan, one way or the other. Do note that this is true even if the query ends abruptly (LIMIT is reached, or an error happens). The easiest workaround for this is to simply call EndPSIBatchModeIfStarted() on the root iterator at the end of the scan. See the PFSBatchMode class for a useful helper.

The rules for starting batch and ending mode are:

1. If you are an iterator with exactly one child (FilterIterator etc.), forward any StartPSIBatchMode() calls to it.
2. If you drive an iterator (read rows from it using a for loop or similar), use PFSBatchMode as described above.
3. If you have multiple children, ignore the call and do your own handling of batch mode as appropriate. For materialization, #2 would typically apply. For joins, it depends on the join type (e.g., NestedLoopIterator applies batch mode only when scanning the innermost table).

The upshot of this is that when scanning a single table, batch mode will typically be activated for that table (since we call StartPSIBatchMode() on the root iterator, and it will trickle all the way down to the table iterator), but for a join, the call will be ignored and the join iterator will activate batch mode by itself as needed.

Reimplemented from RowIterator.

UnlockRow()

void EQRefIterator::UnlockRow ( )

overridevirtual

Since EQRefIterator may buffer a record, do not unlock it if it was not used in this invocation of EQRefIterator::Read().

Only count locks, thus remembering if the record was left unused, and unlock already when pruning the current value of Index_lookup buffer.

See also

EQRefIterator::Read()

Implements RowIterator.

Member Data Documentation

m_examined_rows

ha_rows* const EQRefIterator::m_examined_rows

private

m_first_record_since_init

bool EQRefIterator::m_first_record_since_init

private

m_ref

Index_lookup* const EQRefIterator::m_ref

private

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

• sql/iterators/ref_row_iterators.h
• sql/iterators/ref_row_iterators.cc