MySQL  8.0.27
Source Code Documentation
TimingIterator< RealIterator > Class Template Referencefinal

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 >:
RowIterator

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...
 
std::string TimingString () const override
 
RowIteratorreal_iterator () override
 If this iterator is wrapping a different iterator (e.g. More...
 
const RowIteratorreal_iterator () const override
 
- Public Member Functions inherited from RowIterator
 RowIterator (THD *thd)
 
virtual ~RowIterator ()=default
 

Private Types

using steady_clock = std::chrono::steady_clock
 
template<class T >
using duration = std::chrono::duration< T >
 

Private Member Functions

steady_clock::time_point now () const
 

Private Attributes

uint64_t m_num_rows = 0
 
uint64_t m_num_init_calls = 0
 
steady_clock::time_point::duration m_time_spent_in_first_row {0}
 
steady_clock::time_point::duration m_time_spent_in_other_rows {0}
 
bool m_first_row
 
RealIterator m_iterator
 

Additional Inherited Members

- Protected Member Functions inherited from RowIterator
THDthd () 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.

If RealIterator has a type called keeps_own_timing (no matter what it is), it must contain num_rows() and num_init_calls() accessors that override the number of Init() calls that is counted. This is useful for MaterializeIterator, where most Init() calls don't actually cause a rematerialization and are effectively free.

See also NewIterator, below.

Member Typedef Documentation

◆ duration

template<class RealIterator >
template<class T >
using TimingIterator< RealIterator >::duration = std::chrono::duration<T>
private

◆ steady_clock

template<class RealIterator >
using TimingIterator< RealIterator >::steady_clock = std::chrono::steady_clock
private

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.

◆ Init()

template<class RealIterator >
bool TimingIterator< RealIterator >::Init ( )
overridevirtual

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 TABLE_REF) and allow you to read the records anew.

Implements RowIterator.

◆ now()

template<class RealIterator >
steady_clock::time_point TimingIterator< RealIterator >::now ( ) const
inlineprivate

◆ Read()

template<class RealIterator >
int TimingIterator< RealIterator >::Read ( )
overridevirtual

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
0OK
-1End of records
1Error

Implements RowIterator.

◆ real_iterator() [1/2]

template<class RealIterator >
const RowIterator* TimingIterator< RealIterator >::real_iterator ( ) const
inlineoverridevirtual

Reimplemented from RowIterator.

◆ real_iterator() [2/2]

template<class RealIterator >
RowIterator* 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.

◆ 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:

  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.

◆ TimingString()

template<class RealIterator >
std::string TimingIterator< RealIterator >::TimingString
overridevirtual

Reimplemented from RowIterator.

◆ UnlockRow()

template<class RealIterator >
void TimingIterator< RealIterator >::UnlockRow ( )
inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_first_row

template<class RealIterator >
bool TimingIterator< RealIterator >::m_first_row
private

◆ m_iterator

template<class RealIterator >
RealIterator TimingIterator< RealIterator >::m_iterator
private

◆ m_num_init_calls

template<class RealIterator >
uint64_t TimingIterator< RealIterator >::m_num_init_calls = 0
private

◆ m_num_rows

template<class RealIterator >
uint64_t TimingIterator< RealIterator >::m_num_rows = 0
private

◆ m_time_spent_in_first_row

template<class RealIterator >
steady_clock::time_point::duration TimingIterator< RealIterator >::m_time_spent_in_first_row {0}
private

◆ m_time_spent_in_other_rows

template<class RealIterator >
steady_clock::time_point::duration TimingIterator< RealIterator >::m_time_spent_in_other_rows {0}
private

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