WeedoutIterator Class Reference

Like semijoin materialization, weedout works on the basic idea that a semijoin is just like an inner join as we long as we can get rid of the duplicates somehow. More...

#include <composite_iterators.h>

Inheritance diagram for WeedoutIterator:

Public Member Functions
	WeedoutIterator (THD *thd, unique_ptr_destroy_only< RowIterator > source, SJ_TMP_TABLE *sj, table_map tables_to_get_rowid_for)
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	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
bool	Init ()
	Initialize or reinitialize the iterator. More...
int	Read ()
	Read a single row. More...
virtual const IteratorProfiler *	GetProfiler () const
	Get profiling data for this iterator (for 'EXPLAIN ANALYZE'). More...
virtual void	SetOverrideProfiler (const IteratorProfiler *profiler)
virtual void	StartPSIBatchMode ()
	Start performance schema batch mode, if supported (otherwise ignored). More...
virtual RowIterator *	real_iterator ()
	If this iterator is wrapping a different iterator (e.g. More...
virtual const RowIterator *	real_iterator () const
uint64_t	num_init_calls () const
	Returns the number of times Init() has been called on this iterator. More...
uint64_t	num_rows () const
	Returns the number of times Read() has returned a row successfully from this iterator. More...
uint64_t	num_full_reads () const
	Returns the number of times the iterator has been fully read. More...

Private Member Functions
bool	DoInit () override
int	DoRead () override

Private Attributes
unique_ptr_destroy_only< RowIterator >	m_source
SJ_TMP_TABLE *	m_sj
const table_map	m_tables_to_get_rowid_for

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

Detailed Description

Like semijoin materialization, weedout works on the basic idea that a semijoin is just like an inner join as we long as we can get rid of the duplicates somehow.

(This is advantageous, because inner joins can be reordered, whereas semijoins generally can't.) However, unlike semijoin materialization, weedout removes duplicates after the join, not before it. Consider something like

SELECT * FROM t1 WHERE a IN ( SELECT b FROM t2 );

Semijoin materialization solves this by materializing t2, with deduplication, and then joining. Weedout joins t1 to t2 and then leaves only one output row per t1 row. The disadvantage is that this potentially needs to discard more rows; the (potential) advantage is that we deduplicate on t1 instead of t2.

Weedout, unlike materialization, works in a streaming fashion; rows are output (or discarded) as they come in, with a temporary table used for recording the row IDs we've seen before. (We need to deduplicate on t1's row IDs, not its contents.) See SJ_TMP_TABLE for details about the table format.

Constructor & Destructor Documentation

WeedoutIterator()

WeedoutIterator::WeedoutIterator	(	THD *	thd,
		unique_ptr_destroy_only< RowIterator >	source,
		SJ_TMP_TABLE *	sj,
		table_map	tables_to_get_rowid_for
	)

Member Function Documentation

DoInit()

bool WeedoutIterator::DoInit ( )

overrideprivatevirtual

Implements RowIterator.

DoRead()

int WeedoutIterator::DoRead ( )

overrideprivatevirtual

Implements RowIterator.

EndPSIBatchModeIfStarted()

void WeedoutIterator::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.

SetNullRowFlag()

void WeedoutIterator::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.

UnlockRow()

void WeedoutIterator::UnlockRow ( )

inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

m_sj

SJ_TMP_TABLE* WeedoutIterator::m_sj

private

m_source

unique_ptr_destroy_only<RowIterator> WeedoutIterator::m_source

private

m_tables_to_get_rowid_for

const table_map WeedoutIterator::m_tables_to_get_rowid_for

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