MySQL  8.0.27
Source Code Documentation
ZeroRowsIterator Class Referencefinal

A simple iterator that takes no input and produces zero output rows. More...

#include <basic_row_iterators.h>

Inheritance diagram for ZeroRowsIterator:
RowIterator

Public Member Functions

 ZeroRowsIterator (THD *thd, unique_ptr_destroy_only< RowIterator > child_iterator)
 
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
 
- 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

unique_ptr_destroy_only< RowIteratorm_child_iterator
 

Additional Inherited Members

- Protected Member Functions inherited from RowIterator
THDthd () const
 

Detailed Description

A simple iterator that takes no input and produces zero output rows.

Used when the optimizer has figured out ahead of time that a given table can produce no output (e.g. SELECT ... WHERE 2+2 = 5).

The child iterator is optional (can be nullptr) if SetNullRowFlag() is not to be called. It is used when a subtree used on the inner side of an outer join is found to be never executable, and replaced with a ZeroRowsIterator; in that case, we need to forward the SetNullRowFlag call to it. This child is not printed as part of the iterator tree.

Constructor & Destructor Documentation

◆ ZeroRowsIterator()

ZeroRowsIterator::ZeroRowsIterator ( THD thd,
unique_ptr_destroy_only< RowIterator child_iterator 
)
inline

Member Function Documentation

◆ Init()

bool ZeroRowsIterator::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 ZeroRowsIterator::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.

◆ SetNullRowFlag()

void ZeroRowsIterator::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 ZeroRowsIterator::UnlockRow ( )
inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_child_iterator

unique_ptr_destroy_only<RowIterator> ZeroRowsIterator::m_child_iterator
private

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