MySQL 8.0.31
Source Code Documentation
NestedLoopIterator Class Referencefinal

A simple nested loop join, taking in two iterators (left/outer and right/inner) and joining them together. More...

#include <composite_iterators.h>

Inheritance diagram for NestedLoopIterator:
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Public Member Functions

 NestedLoopIterator (THD *thd, unique_ptr_destroy_only< RowIterator > source_outer, unique_ptr_destroy_only< RowIterator > source_inner, JoinType join_type, bool pfs_batch_mode)
 
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 IteratorProfilerGetProfiler () 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 RowIteratorreal_iterator ()
 If this iterator is wrapping a different iterator (e.g. More...
 
virtual const RowIteratorreal_iterator () const
 

Private Types

enum  { NEEDS_OUTER_ROW , READING_FIRST_INNER_ROW , READING_INNER_ROWS , END_OF_ROWS }
 

Private Attributes

enum NestedLoopIterator:: { ... }  m_state
 
unique_ptr_destroy_only< RowIterator > const m_source_outer
 
unique_ptr_destroy_only< RowIterator > const m_source_inner
 
const JoinType m_join_type
 
const bool m_pfs_batch_mode
 Whether to use batch mode when scanning the inner iterator. More...
 

Additional Inherited Members

- Protected Member Functions inherited from RowIterator
THDthd () const
 

Detailed Description

A simple nested loop join, taking in two iterators (left/outer and right/inner) and joining them together.

This may, of course, scan the inner iterator many times. It is currently the only form of join we have.

The iterator works as a state machine, where the state records whether we need to read a new outer row or not, and whether we've seen any rows from the inner iterator at all (if not, an outer join need to synthesize a new NULL row).

The iterator takes care of activating performance schema batch mode on the right iterator if needed; this is typically only used if it is the innermost table in the entire join (where the gains from turning on batch mode is the largest, and the accuracy loss from turning it off are the least critical).

Member Enumeration Documentation

◆ anonymous enum

anonymous enum
private
Enumerator
NEEDS_OUTER_ROW 
READING_FIRST_INNER_ROW 
READING_INNER_ROWS 
END_OF_ROWS 

Constructor & Destructor Documentation

◆ NestedLoopIterator()

NestedLoopIterator::NestedLoopIterator ( THD thd,
unique_ptr_destroy_only< RowIterator source_outer,
unique_ptr_destroy_only< RowIterator source_inner,
JoinType  join_type,
bool  pfs_batch_mode 
)
inline

Member Function Documentation

◆ EndPSIBatchModeIfStarted()

void NestedLoopIterator::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 NestedLoopIterator::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 TABLE_REF) and allow you to read the records anew.

Implements RowIterator.

◆ Read()

int NestedLoopIterator::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 NestedLoopIterator::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 NestedLoopIterator::UnlockRow ( )
inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_join_type

const JoinType NestedLoopIterator::m_join_type
private

◆ m_pfs_batch_mode

const bool NestedLoopIterator::m_pfs_batch_mode
private

Whether to use batch mode when scanning the inner iterator.

◆ m_source_inner

unique_ptr_destroy_only<RowIterator> const NestedLoopIterator::m_source_inner
private

◆ m_source_outer

unique_ptr_destroy_only<RowIterator> const NestedLoopIterator::m_source_outer
private

◆ 

enum { ... } NestedLoopIterator::m_state

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