FollowTailIterator Class Reference

FollowTailIterator is a special version of TableScanIterator that is used as part of WITH RECURSIVE queries. More...

#include <basic_row_iterators.h>

Inheritance diagram for FollowTailIterator:

Public Member Functions
	FollowTailIterator (THD *thd, TABLE *table, double expected_rows, ha_rows *examined_rows)
	~FollowTailIterator () override
bool	Init () override
	Initialize or reinitialize the iterator. More...
int	Read () override
	Read a single row. More...
void	set_stored_rows_pointer (ha_rows *stored_rows)
	Signal where we can expect to find the number of generated rows for this materialization (this points into the MaterializeIterator's data). More...
bool	RepositionCursorAfterSpillToDisk ()
	Signal to the iterator that the underlying table was closed and replaced with an InnoDB table with the same data, due to a spill-to-disk (e.g. More...
Public Member Functions inherited from TableRowIterator
	TableRowIterator (THD *thd, TABLE *table)
void	UnlockRow () override
	The default implementation of unlock-row method of RowIterator, used in all access methods except EQRefIterator. 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	StartPSIBatchMode () override
	Start performance schema batch mode, if supported (otherwise ignored). More...
void	EndPSIBatchModeIfStarted () override
	Ends performance schema batch mode, if started. More...
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 ([[maybe_unused]] const IteratorProfiler *profiler)
virtual RowIterator *	real_iterator ()
	If this iterator is wrapping a different iterator (e.g. More...
virtual const RowIterator *	real_iterator () const

Private Attributes
bool	m_inited = false
uchar *const	m_record
const double	m_expected_rows
ha_rows *const	m_examined_rows
ha_rows	m_read_rows
ha_rows	m_end_of_current_iteration
unsigned	m_recursive_iteration_count
ha_rows *	m_stored_rows = nullptr

Additional Inherited Members
Protected Member Functions inherited from TableRowIterator
int	HandleError (int error)
void	PrintError (int error)
TABLE *	table () const
Protected Member Functions inherited from RowIterator
THD *	thd () const

Detailed Description

FollowTailIterator is a special version of TableScanIterator that is used as part of WITH RECURSIVE queries.

It is designed to read from a temporary table at the same time as MaterializeIterator writes to the same table, picking up new records in the order they come in – it follows the tail, much like the UNIX tool “tail -f”.

Furthermore, when materializing a recursive query expression consisting of multiple query blocks, MaterializeIterator needs to run each block several times until convergence. (For a single query block, one iteration suffices, since the iterator sees new records as they come in.) Each such run, the recursive references should see only rows that were added since the last iteration, even though Init() is called anew. FollowTailIterator is thus different from TableScanIterator in that subsequent calls to Init() do not move the cursor back to the start.

In addition, FollowTailIterator implements the WITH RECURSIVE iteration limit. This is not specified in terms of Init() calls, since one run can encompass many iterations. Instead, it keeps track of the number of records in the table at the start of iteration, and when it has read all of those records, the next iteration is deemed to have begun. If the iteration counter is above the user-set limit, it raises an error to stop runaway queries with infinite recursion.

Constructor & Destructor Documentation

FollowTailIterator()

FollowTailIterator::FollowTailIterator	(	THD *	thd,
		TABLE *	table,
		double	expected_rows,
		ha_rows *	examined_rows
	)

~FollowTailIterator()

FollowTailIterator::~FollowTailIterator ( )

override

Member Function Documentation

Init()

bool FollowTailIterator::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 FollowTailIterator::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.

RepositionCursorAfterSpillToDisk()

bool FollowTailIterator::RepositionCursorAfterSpillToDisk

(

)

Signal to the iterator that the underlying table was closed and replaced with an InnoDB table with the same data, due to a spill-to-disk (e.g.

the table used to be MEMORY and now is InnoDB). This is required so that Read() can continue scanning from the right place. Called by MaterializeIterator::MaterializeRecursive().

set_stored_rows_pointer()

void FollowTailIterator::set_stored_rows_pointer ( ha_rows *  stored_rows )

inline

Signal where we can expect to find the number of generated rows for this materialization (this points into the MaterializeIterator's data).

This must be called when we start materializing the CTE, before Init() runs.

Member Data Documentation

m_end_of_current_iteration

ha_rows FollowTailIterator::m_end_of_current_iteration

private

m_examined_rows

ha_rows* const FollowTailIterator::m_examined_rows

private

m_expected_rows

const double FollowTailIterator::m_expected_rows

private

m_inited

bool FollowTailIterator::m_inited = false

private

m_read_rows

ha_rows FollowTailIterator::m_read_rows

private

m_record

uchar* const FollowTailIterator::m_record

private

m_recursive_iteration_count

unsigned FollowTailIterator::m_recursive_iteration_count

private

m_stored_rows

ha_rows* FollowTailIterator::m_stored_rows = nullptr

private

---

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

• sql/iterators/basic_row_iterators.h
• sql/iterators/basic_row_iterators.cc