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)
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	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 IteratorProfiler *	GetProfiler () const
	Get profiling data for this iterator (for 'EXPLAIN ANALYZE'). More...
virtual void	SetOverrideProfiler ([[maybe_unused]] 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

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

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.

Init()

bool WeedoutIterator::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 WeedoutIterator::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.

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