NestedLoopSemiJoinWithDuplicateRemovalIterator Class Reference

An iterator that is semantically equivalent to a semijoin NestedLoopIterator immediately followed by a RemoveDuplicatesOnIndexIterator. More...

#include <composite_iterators.h>

Inheritance diagram for NestedLoopSemiJoinWithDuplicateRemovalIterator:

Public Member Functions
	NestedLoopSemiJoinWithDuplicateRemovalIterator (THD *thd, unique_ptr_destroy_only< RowIterator > source_outer, unique_ptr_destroy_only< RowIterator > source_inner, const TABLE *table, KEY *key, size_t key_len)
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 (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 > const	m_source_outer
unique_ptr_destroy_only< RowIterator > const	m_source_inner
const TABLE *	m_table_outer
KEY *	m_key
uchar *	m_key_buf
const size_t	m_key_len
bool	m_deduplicate_against_previous_row

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

Detailed Description

An iterator that is semantically equivalent to a semijoin NestedLoopIterator immediately followed by a RemoveDuplicatesOnIndexIterator.

It is used to implement the “loose scan” strategy in queries with multiple tables on the inside of a semijoin, like

... FROM t1 WHERE ... IN ( SELECT ... FROM t2 JOIN t3 ... )

In this case, the query tree without this iterator would ostensibly look like

-> Nested loop join -> Table scan on t1 -> Remove duplicates on t2_idx -> Nested loop semijoin -> Index scan on t2 using t2_idx -> Filter (e.g. t3.a = t2.a) -> Table scan on t3

(t3 will be marked as “first match” on t2 when implementing loose scan, thus the semijoin.)

First note that we can't put the duplicate removal directly on t2 in this case, as the first t2 row doesn't necessarily match anything in t3, so it needs to be above. However, this is wasteful, because once we find a matching t2/t3 pair, we should stop scanning t3 until we have a new t2.

NestedLoopSemiJoinWithDuplicateRemovalIterator solves the problem by doing exactly this; it gets a row from the outer side, gets exactly one row from the inner side, and then skips over rows from the outer side (without scanning the inner side) until its keypart changes.

Constructor & Destructor Documentation

NestedLoopSemiJoinWithDuplicateRemovalIterator()

NestedLoopSemiJoinWithDuplicateRemovalIterator::NestedLoopSemiJoinWithDuplicateRemovalIterator	(	THD *	thd,
		unique_ptr_destroy_only< RowIterator >	source_outer,
		unique_ptr_destroy_only< RowIterator >	source_inner,
		const TABLE *	table,
		KEY *	key,
		size_t	key_len
	)

Member Function Documentation

EndPSIBatchModeIfStarted()

void NestedLoopSemiJoinWithDuplicateRemovalIterator::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 NestedLoopSemiJoinWithDuplicateRemovalIterator::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 NestedLoopSemiJoinWithDuplicateRemovalIterator::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 NestedLoopSemiJoinWithDuplicateRemovalIterator::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 NestedLoopSemiJoinWithDuplicateRemovalIterator::UnlockRow ( )

inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

m_deduplicate_against_previous_row

bool NestedLoopSemiJoinWithDuplicateRemovalIterator::m_deduplicate_against_previous_row

private

m_key

KEY* NestedLoopSemiJoinWithDuplicateRemovalIterator::m_key

private

m_key_buf

uchar* NestedLoopSemiJoinWithDuplicateRemovalIterator::m_key_buf

private

m_key_len

const size_t NestedLoopSemiJoinWithDuplicateRemovalIterator::m_key_len

private

m_source_inner

unique_ptr_destroy_only<RowIterator> const NestedLoopSemiJoinWithDuplicateRemovalIterator::m_source_inner

private

m_source_outer

unique_ptr_destroy_only<RowIterator> const NestedLoopSemiJoinWithDuplicateRemovalIterator::m_source_outer

private

m_table_outer

const TABLE* NestedLoopSemiJoinWithDuplicateRemovalIterator::m_table_outer

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