StreamingIterator Class Reference

StreamingIterator is a minimal version of MaterializeIterator that does not actually materialize; instead, every Read() just forwards the call to the subquery iterator and does the required copying from one set of fields to another. More...

#include <composite_iterators.h>

Inheritance diagram for StreamingIterator:

Public Member Functions
	StreamingIterator (THD *thd, unique_ptr_destroy_only< RowIterator > subquery_iterator, Temp_table_param *temp_table_param, TABLE *table, bool provide_rowid, JOIN *join, int ref_slice)
bool	Init () override
	Initialize or reinitialize the iterator. More...
int	Read () override
	Read a single 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 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 ([[maybe_unused]] 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
unique_ptr_destroy_only< RowIterator >	m_subquery_iterator
Temp_table_param *	m_temp_table_param
ha_rows	m_row_number
JOIN *const	m_join
const int	m_output_slice
int	m_input_slice
const bool	m_provide_rowid

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

StreamingIterator is a minimal version of MaterializeIterator that does not actually materialize; instead, every Read() just forwards the call to the subquery iterator and does the required copying from one set of fields to another.

It is used for when the optimizer would normally set up a materialization, but you don't actually need one, ie. you don't want to read the rows multiple times after writing them, and you don't want to access them by index (only a single table scan). It also takes care of setting the NULL row flag on the temporary table.

Constructor & Destructor Documentation

StreamingIterator()

StreamingIterator::StreamingIterator	(	THD *	thd,
		unique_ptr_destroy_only< RowIterator >	subquery_iterator,
		Temp_table_param *	temp_table_param,
		TABLE *	table,
		bool	provide_rowid,
		JOIN *	join,
		int	ref_slice
	)

Parameters

thd	Thread handle.
subquery_iterator	The iterator to read rows from.
temp_table_param	Parameters for the temp table.
table	The table we are streaming through. Will never actually be written to, but its fields will be used.
provide_rowid	If true, generate a row ID for each row we stream. This is used if the parent needs row IDs for deduplication, in particular weedout.
join	See MaterializeIterator.
ref_slice	See MaterializeIterator.

Member Function Documentation

EndPSIBatchModeIfStarted()

void StreamingIterator::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 StreamingIterator::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 StreamingIterator::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

0	OK
-1	End of records
1	Error

Implements RowIterator.

StartPSIBatchMode()

void StreamingIterator::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 StreamingIterator::UnlockRow ( )

inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

m_input_slice

int StreamingIterator::m_input_slice

private

m_join

JOIN* const StreamingIterator::m_join

private

m_output_slice

const int StreamingIterator::m_output_slice

private

m_provide_rowid

const bool StreamingIterator::m_provide_rowid

private

m_row_number

ha_rows StreamingIterator::m_row_number

private

m_subquery_iterator

unique_ptr_destroy_only<RowIterator> StreamingIterator::m_subquery_iterator

private

m_temp_table_param

Temp_table_param* StreamingIterator::m_temp_table_param

private

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

• sql/iterators/composite_iterators.h
• sql/iterators/composite_iterators.cc