MySQL  8.0.20
Source Code Documentation
SortingIterator Class Referencefinal

An adapter that takes in another RowIterator and produces the same rows, just in sorted order. More...

#include <sorting_iterator.h>

Inheritance diagram for SortingIterator:
RowIterator

Classes

union  IteratorHolder
 

Public Member Functions

 SortingIterator (THD *thd, QEP_TAB *qep_tab, Filesort *filesort, unique_ptr_destroy_only< RowIterator > source, ha_rows *examined_rows)
 
 ~SortingIterator () override
 
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 UnlockRow () override
 
std::vector< Childchildren () const override
 List of zero or more iterators which are direct children of this one. More...
 
std::vector< std::string > DebugString () const override
 Returns a short string (used for EXPLAIN FORMAT=tree) with user-readable information for this iterator. More...
 
void CleanupAfterQuery ()
 Optional (when JOIN::destroy() runs, the iterator and its buffers will be cleaned up anyway); used to clean up the buffers a little bit earlier. More...
 
- Public Member Functions inherited from RowIterator
 RowIterator (THD *thd)
 
virtual ~RowIterator ()
 
virtual std::string TimingString () const
 
JOINjoin_for_explain () const
 
void set_join_for_explain (JOIN *join)
 
virtual void StartPSIBatchMode ()
 Start performance schema batch mode, if supported (otherwise ignored). More...
 
virtual void EndPSIBatchModeIfStarted ()
 Ends performance schema batch mode, if started. More...
 
void set_estimated_cost (double estimated_cost)
 
double estimated_cost () const
 
void set_expected_rows (double expected_rows)
 
double expected_rows () const
 
virtual RowIteratorreal_iterator ()
 If this iterator is wrapping a different iterator (e.g. More...
 
virtual const RowIteratorreal_iterator () const
 

Private Member Functions

int DoSort ()
 
void ReleaseBuffers ()
 

Private Attributes

Filesortm_filesort
 
QEP_TABm_qep_tab
 
unique_ptr_destroy_only< RowIteratorm_source_iterator
 
unique_ptr_destroy_only< RowIteratorm_result_iterator
 
Filesort_info m_fs_info
 
Sort_result m_sort_result
 
ha_rowsm_examined_rows
 
union SortingIterator::IteratorHolder m_result_iterator_holder
 

Additional Inherited Members

- Protected Member Functions inherited from RowIterator
THDthd () const
 

Detailed Description

An adapter that takes in another RowIterator and produces the same rows, just in sorted order.

(The actual sort happens in Init().) Unfortunately, it is still bound to working off a TABLE object, which means that you can't use it to e.g. sort the output of a join without materializing into a temporary table first (ignoring that we currently have no Iterators for joins).

The primary reason for this is that we currently have no way of communicating read sets through Iterators, and SortingIterator needs to add fields used in ORDER BY to the read set for the appropriate tables. This could be mitigated by e.g. sending in an unordered_set<Field *>, but we don't currently have such a mechanism.

Constructor & Destructor Documentation

◆ SortingIterator()

SortingIterator::SortingIterator ( THD thd,
QEP_TAB qep_tab,
Filesort filesort,
unique_ptr_destroy_only< RowIterator source,
ha_rows examined_rows 
)

◆ ~SortingIterator()

SortingIterator::~SortingIterator ( )
override

Member Function Documentation

◆ children()

std::vector<Child> SortingIterator::children ( ) const
inlineoverridevirtual

List of zero or more iterators which are direct children of this one.

By convention, if there are multiple ones (ie., we're doing a join), the outer iterator is listed first. So for a LEFT JOIN b, we'd list a before b.

Reimplemented from RowIterator.

◆ CleanupAfterQuery()

void SortingIterator::CleanupAfterQuery ( )

Optional (when JOIN::destroy() runs, the iterator and its buffers will be cleaned up anyway); used to clean up the buffers a little bit earlier.

When we get cached JOIN objects (prepare/optimize once) that can live for a long time between queries, calling this will become more important.

◆ DebugString()

vector< string > SortingIterator::DebugString ( ) const
overridevirtual

Returns a short string (used for EXPLAIN FORMAT=tree) with user-readable information for this iterator.

When implementing these, try to avoid internal jargon (e.g. “eq_ref”); prefer things that read like normal, technical English (e.g. “single-row index lookup”).

For certain complex operations, such as MaterializeIterator, there can be multiple strings. If so, they are interpreted as nested operations, with the outermost, last-done operation first and the other ones indented as if they were child iterators.

Callers should use FullDebugString() below, which adds costs (see set_estimated_cost() etc.) if present.

Implements RowIterator.

◆ DoSort()

int SortingIterator::DoSort ( )
private

◆ Init()

bool SortingIterator::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 SortingIterator::Read ( )
inlineoverridevirtual

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.

◆ ReleaseBuffers()

void SortingIterator::ReleaseBuffers ( )
private

◆ SetNullRowFlag()

void SortingIterator::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 SortingIterator::UnlockRow ( )
inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_examined_rows

ha_rows* SortingIterator::m_examined_rows
private

◆ m_filesort

Filesort* SortingIterator::m_filesort
private

◆ m_fs_info

Filesort_info SortingIterator::m_fs_info
private

◆ m_qep_tab

QEP_TAB* SortingIterator::m_qep_tab
private

◆ m_result_iterator

unique_ptr_destroy_only<RowIterator> SortingIterator::m_result_iterator
private

◆ m_result_iterator_holder

union SortingIterator::IteratorHolder SortingIterator::m_result_iterator_holder
private

◆ m_sort_result

Sort_result SortingIterator::m_sort_result
private

◆ m_source_iterator

unique_ptr_destroy_only<RowIterator> SortingIterator::m_source_iterator
private

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