An iterator template that wraps a RowIterator, such that all calls to Init() and Read() are timed (all others are passed through unchanged, and possibly even inlined, since all RowIterator implementations are final). More...

#include <timing_iterator.h>

Inheritance diagram for TimingIterator< RealIterator >:

[legend]

Public Member Functions
template<class... Args>
	TimingIterator (THD *thd, Args &&...args)

bool	Init () override
	Initialize or reinitialize the iterator. More...

int	Read () override
	Read a single row. 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	UnlockRow () override

void	StartPSIBatchMode () override
	Start performance schema batch mode, if supported (otherwise ignored). More...

void	EndPSIBatchModeIfStarted () override
	Ends performance schema batch mode, if started. More...

void	SetOverrideProfiler (const IteratorProfiler *profiler) override

const IteratorProfiler *	GetProfiler () const override
	Get profiling data for this iterator (for 'EXPLAIN ANALYZE'). More...

RealIterator *	real_iterator () override
	If this iterator is wrapping a different iterator (e.g. More...

const RealIterator *	real_iterator () const override

Public Member Functions inherited from RowIterator
	RowIterator (THD *thd)

virtual	~RowIterator ()=default

	RowIterator (const RowIterator &)=delete

	RowIterator (RowIterator &&)=default

Private Attributes
IteratorProfilerImpl	m_profiler
	This maintains the profiling measurements. More...

const IteratorProfiler *	m_override_profiler {nullptr}
	For iterators over materialized tables we must make profiling measurements in a different way. More...

RealIterator	m_iterator

Additional Inherited Members
Protected Member Functions inherited from RowIterator
THD *	thd () const

Detailed Description

template<class RealIterator>
class TimingIterator< RealIterator >

An iterator template that wraps a RowIterator, such that all calls to Init() and Read() are timed (all others are passed through unchanged, and possibly even inlined, since all RowIterator implementations are final).

This is used for EXPLAIN ANALYZE.

Note that MaterializeIterator does not use this class. Doing so would give misleading measurements. MaterializeIterator has an internal member iterator (m_table_iterator) that iterates over the materialized result. Calls to Init()/Read() on that iterator goes via Init()/Read() on the MaterializeIterator. And the internal iterator is listed above MaterializeIterator in 'EXPLAIN ANALYZE' output. Its elapsed time values should thus include both the cost of materialization and iterating over the result, while the entry for MaterializeIterator should only show the time spent on materialization. But if we used TimingIterator, the entry for MaterializeIterator would give the sum of time spent on both materialization and iteration, and the entry for the internal iterator would only show the time spent on iterating over the materialized result. (See also Bug #33834146 "'EXPLAIN ANALYZE' cost estimates and elapsed time values are not cumulative"). This also applies to TemptableAggregateIterator. These classes therefore have other mechanisms for obtaining profiling data.

Constructor & Destructor Documentation

◆ TimingIterator()

template<class RealIterator >

template<class... Args>

TimingIterator< RealIterator >::TimingIterator	(	THD *	thd,
		Args &&...	args
	)

inline

Member Function Documentation

◆ EndPSIBatchModeIfStarted()

template<class RealIterator >

void TimingIterator< RealIterator >::EndPSIBatchModeIfStarted ( )

inlineoverridevirtual

Ends performance schema batch mode, if started.

It's always safe to call this.

Iterators that have children (composite iterators) must forward the EndPSIBatchModeIfStarted() call to every iterator they could conceivably have called StartPSIBatchMode() on. This ensures that after such a call to on the root iterator, all handlers are out of batch mode.

Reimplemented from RowIterator.

◆ GetProfiler()

template<class RealIterator >

const IteratorProfiler * TimingIterator< RealIterator >::GetProfiler ( ) const

inlineoverridevirtual

Get profiling data for this iterator (for 'EXPLAIN ANALYZE').

Valid for TimingIterator, MaterializeIterator and TemptableAggregateIterator only.

Reimplemented from RowIterator.

◆ Init()

template<class RealIterator >

bool TimingIterator< RealIterator >::Init ( )

inlineoverridevirtual

Initialize or reinitialize the iterator.

You must always call Init() before trying a Read() (but Init() does not imply Read()).

You can call Init() multiple times; subsequent calls will rewind the iterator (or reposition it, depending on whether the iterator takes in e.g. a Index_lookup) and allow you to read the records anew.

Implements RowIterator.

◆ Read()

template<class RealIterator >

int TimingIterator< RealIterator >::Read ( )

inlineoverridevirtual

Read a single row.

The row data is not actually returned from the function; it is put in the table's (or tables', in case of a join) record buffer, ie., table->records[0].

Return values

0	OK
-1	End of records
1	Error

Implements RowIterator.

◆ real_iterator() [1/2]

template<class RealIterator >

const RealIterator * TimingIterator< RealIterator >::real_iterator ( ) const

inlineoverridevirtual

Reimplemented from RowIterator.

◆ real_iterator() [2/2]

template<class RealIterator >

RealIterator * TimingIterator< RealIterator >::real_iterator ( )

inlineoverridevirtual

If this iterator is wrapping a different iterator (e.g.

TimingIterator<T>) and you need to down_cast<> to a specific iterator type, this allows getting at the wrapped iterator.

Reimplemented from RowIterator.

◆ SetNullRowFlag()

template<class RealIterator >

void TimingIterator< RealIterator >::SetNullRowFlag ( bool is_null_row )

inlineoverridevirtual

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).

This is used for outer joins, when an iterator hasn't produced any rows and we need to produce a NULL-complemented row. Init() or Read() won't necessarily reset this flag, so if you ever set is to true, make sure to also set it to false when needed.

Note that this can be called without Init() having been called first. For example, NestedLoopIterator can hit EOF immediately on the outer iterator, which means the inner iterator doesn't get an Init() call, but will still forward SetNullRowFlag to both inner and outer iterators.

TODO: We shouldn't need this. See the comments on AggregateIterator for a bit more discussion on abstracting out a row interface.

Implements RowIterator.

◆ SetOverrideProfiler()

template<class RealIterator >

void TimingIterator< RealIterator >::SetOverrideProfiler ( const IteratorProfiler * profiler )

inlineoverridevirtual

See also: TimingIterator .

Reimplemented from RowIterator.

◆ StartPSIBatchMode()

template<class RealIterator >

void TimingIterator< RealIterator >::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:

If you are an iterator with exactly one child (FilterIterator etc.), forward any StartPSIBatchMode() calls to it.
If you drive an iterator (read rows from it using a for loop or similar), use PFSBatchMode as described above.
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()

template<class RealIterator >

void TimingIterator< RealIterator >::UnlockRow ( )

inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_iterator

template<class RealIterator >

RealIterator TimingIterator< RealIterator >::m_iterator

private

◆ m_override_profiler

template<class RealIterator >

const IteratorProfiler* TimingIterator< RealIterator >::m_override_profiler {nullptr}

private

For iterators over materialized tables we must make profiling measurements in a different way.

This field keeps those measurements.

◆ m_profiler

template<class RealIterator >

IteratorProfilerImpl TimingIterator< RealIterator >::m_profiler

private

This maintains the profiling measurements.

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

sql/iterators/timing_iterator.h

Public Member Functions

Private Attributes

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ TimingIterator()

Member Function Documentation

◆ EndPSIBatchModeIfStarted()

◆ GetProfiler()

◆ Init()

◆ Read()

◆ real_iterator() [1/2]

◆ real_iterator() [2/2]

◆ SetNullRowFlag()

◆ SetOverrideProfiler()

◆ StartPSIBatchMode()

◆ UnlockRow()

Member Data Documentation

◆ m_iterator

◆ m_override_profiler

◆ m_profiler