MySQL  8.0.27
Source Code Documentation
UnqualifiedCountIterator Class Referencefinal

An iterator for unqualified COUNT(*) (ie., no WHERE, no join conditions, etc.), taking a special fast path in the handler. More...

#include <basic_row_iterators.h>

Inheritance diagram for UnqualifiedCountIterator:
RowIterator

Public Member Functions

 UnqualifiedCountIterator (THD *thd, JOIN *join)
 
bool Init () override
 Initialize or reinitialize the iterator. More...
 
int Read () override
 (end of group Query_Executor) More...
 
void SetNullRowFlag (bool) 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
 
- Public Member Functions inherited from RowIterator
 RowIterator (THD *thd)
 
virtual ~RowIterator ()=default
 
virtual std::string TimingString () const
 
virtual void StartPSIBatchMode ()
 Start performance schema batch mode, if supported (otherwise ignored). More...
 
virtual void EndPSIBatchModeIfStarted ()
 Ends performance schema batch mode, if started. More...
 
virtual RowIteratorreal_iterator ()
 If this iterator is wrapping a different iterator (e.g. More...
 
virtual const RowIteratorreal_iterator () const
 

Private Attributes

bool m_has_row
 
JOIN *const m_join
 

Additional Inherited Members

- Protected Member Functions inherited from RowIterator
THDthd () const
 

Detailed Description

An iterator for unqualified COUNT(*) (ie., no WHERE, no join conditions, etc.), taking a special fast path in the handler.

It returns a single row, much like FakeSingleRowIterator; however, unlike said iterator, it actually does the counting in Read() instead of expecting all fields to already be filled out.

Constructor & Destructor Documentation

◆ UnqualifiedCountIterator()

UnqualifiedCountIterator::UnqualifiedCountIterator ( THD thd,
JOIN join 
)
inline

Member Function Documentation

◆ Init()

bool UnqualifiedCountIterator::Init ( )
inlineoverridevirtual

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 UnqualifiedCountIterator::Read ( )
overridevirtual

(end of group Query_Executor)

Implements RowIterator.

◆ SetNullRowFlag()

void UnqualifiedCountIterator::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 UnqualifiedCountIterator::UnlockRow ( )
inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_has_row

bool UnqualifiedCountIterator::m_has_row
private

◆ m_join

JOIN* const UnqualifiedCountIterator::m_join
private

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