MySQL 9.1.0
Source Code Documentation
WindowIterator Class Referencefinal

#include <window_iterators.h>

Inheritance diagram for WindowIterator:
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Public Member Functions

 WindowIterator (THD *thd, unique_ptr_destroy_only< RowIterator > source, Temp_table_param *temp_table_param, JOIN *join, int output_slice)
 
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 StartPSIBatchMode () override
 Start performance schema batch mode, if supported (otherwise ignored). More...
 
void EndPSIBatchModeIfStarted () override
 Ends performance schema batch mode, if started. More...
 
void UnlockRow () override
 
- Public Member Functions inherited from RowIterator
 RowIterator (THD *thd)
 
virtual ~RowIterator ()=default
 
 RowIterator (const RowIterator &)=delete
 
 RowIterator (RowIterator &&)=default
 
virtual const IteratorProfilerGetProfiler () const
 Get profiling data for this iterator (for 'EXPLAIN ANALYZE'). More...
 
virtual void SetOverrideProfiler (const IteratorProfiler *profiler)
 
virtual RowIteratorreal_iterator ()
 If this iterator is wrapping a different iterator (e.g. More...
 
virtual const RowIteratorreal_iterator () const
 

Private Attributes

unique_ptr_destroy_only< RowIterator > const m_source
 The iterator we are reading from. More...
 
Temp_table_paramm_temp_table_param
 Parameters for the temporary table we are outputting to. More...
 
Windowm_window
 The window function itself. More...
 
JOINm_join
 The join we are a part of. More...
 
int m_input_slice
 The slice we will be using when reading rows. More...
 
int m_output_slice
 The slice we will be using when outputting rows. More...
 

Additional Inherited Members

- Protected Member Functions inherited from RowIterator
THDthd () const
 

Constructor & Destructor Documentation

◆ WindowIterator()

WindowIterator::WindowIterator ( THD thd,
unique_ptr_destroy_only< RowIterator source,
Temp_table_param temp_table_param,
JOIN join,
int  output_slice 
)

Member Function Documentation

◆ EndPSIBatchModeIfStarted()

void WindowIterator::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()

bool WindowIterator::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 Index_lookup) and allow you to read the records anew.

Implements RowIterator.

◆ Read()

int WindowIterator::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.

◆ SetNullRowFlag()

void WindowIterator::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()

void WindowIterator::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 WindowIterator::UnlockRow ( )
inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_input_slice

int WindowIterator::m_input_slice
private

The slice we will be using when reading rows.

◆ m_join

JOIN* WindowIterator::m_join
private

The join we are a part of.

◆ m_output_slice

int WindowIterator::m_output_slice
private

The slice we will be using when outputting rows.

◆ m_source

unique_ptr_destroy_only<RowIterator> const WindowIterator::m_source
private

The iterator we are reading from.

◆ m_temp_table_param

Temp_table_param* WindowIterator::m_temp_table_param
private

Parameters for the temporary table we are outputting to.

◆ m_window

Window* WindowIterator::m_window
private

The window function itself.


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