RowIterator Class Reference

A context for reading through a single table using a chosen access method: index read, scan, etc, use of cache, etc. More...

#include <row_iterator.h>

Inheritance diagram for RowIterator:

Public Member Functions
	RowIterator (THD *thd)
virtual	~RowIterator ()=default
	RowIterator (const RowIterator &)=delete
	RowIterator (RowIterator &&)=default
virtual bool	Init ()=0
	Initialize or reinitialize the iterator. More...
virtual int	Read ()=0
	Read a single row. More...
virtual void	SetNullRowFlag (bool is_null_row)=0
	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...
virtual void	UnlockRow ()=0
virtual const IteratorProfiler *	GetProfiler () 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 void	EndPSIBatchModeIfStarted ()
	Ends performance schema batch mode, if started. More...
virtual RowIterator *	real_iterator ()
	If this iterator is wrapping a different iterator (e.g. More...
virtual const RowIterator *	real_iterator () const

Protected Member Functions
THD *	thd () const

Private Attributes
THD *const	m_thd

Detailed Description

A context for reading through a single table using a chosen access method: index read, scan, etc, use of cache, etc.

. It is mostly meant as an interface, but also contains some private member functions that are useful for many implementations, such as error handling.

A RowIterator is a simple iterator; you initialize it, and then read one record at a time until Read() returns EOF. A RowIterator can read from other Iterators if you want to, e.g., SortingIterator, which takes in records from another RowIterator and sorts them.

The abstraction is not completely tight. In particular, it still leaves some specifics to TABLE, such as which columns to read (the read_set). This means it would probably be hard as-is to e.g. sort a join of two tables.

Use by:

unique_ptr<RowIterator> iterator(new ...);
if (iterator->Init())
  return true;
while (iterator->Read() == 0) {
  ...
}

Constructor & Destructor Documentation

RowIterator() [1/3]

RowIterator::RowIterator ( THD *  thd )

inlineexplicit

~RowIterator()

virtual RowIterator::~RowIterator ( )

virtualdefault

RowIterator() [2/3]

RowIterator::RowIterator ( const RowIterator &  )

delete

RowIterator() [3/3]

RowIterator::RowIterator ( RowIterator &&  )

default

Member Function Documentation

EndPSIBatchModeIfStarted()

virtual void RowIterator::EndPSIBatchModeIfStarted ( )

inlinevirtual

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 in BKAIterator, MaterializeIterator< Profiler >, TemptableAggregateIterator< Profiler >, FilterIterator, LimitOffsetIterator, AggregateIterator, NestedLoopIterator, StreamingIterator, MaterializedTableFunctionIterator, WeedoutIterator, RemoveDuplicatesIterator, RemoveDuplicatesOnIndexIterator, NestedLoopSemiJoinWithDuplicateRemovalIterator, MaterializeInformationSchemaTableIterator, AppendIterator, DeleteRowsIterator, HashJoinIterator, TableRowIterator, TimingIterator< RealIterator >, UpdateRowsIterator, WindowIterator, and BufferingWindowIterator.

GetProfiler()

virtual const IteratorProfiler * RowIterator::GetProfiler ( ) const

inlinevirtual

Get profiling data for this iterator (for 'EXPLAIN ANALYZE').

Valid for TimingIterator, MaterializeIterator and TemptableAggregateIterator only.

Reimplemented in MaterializeIterator< Profiler >, TemptableAggregateIterator< Profiler >, and TimingIterator< RealIterator >.

Init()

virtual bool RowIterator::Init ( )

pure virtual

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.

Implemented in TableScanIterator, IndexScanIterator< Reverse >, SortBufferIterator< Packed_addon_fields >, SortBufferIndirectIterator, SortFileIterator< Packed_addon_fields >, SortFileIndirectIterator, FakeSingleRowIterator, UnqualifiedCountIterator, ZeroRowsIterator, ZeroRowsAggregatedIterator, FollowTailIterator, TableValueConstructorIterator, BKAIterator, MultiRangeRowIterator, MaterializeIterator< Profiler >, TemptableAggregateIterator< Profiler >, FilterIterator, LimitOffsetIterator, AggregateIterator, NestedLoopIterator, CacheInvalidatorIterator, StreamingIterator, MaterializedTableFunctionIterator, WeedoutIterator, RemoveDuplicatesIterator, RemoveDuplicatesOnIndexIterator, NestedLoopSemiJoinWithDuplicateRemovalIterator, MaterializeInformationSchemaTableIterator, AppendIterator, DeleteRowsIterator, HashJoinIterator, RefIterator< Reverse >, RefOrNullIterator, EQRefIterator, ConstIterator, FullTextSearchIterator, DynamicRangeIterator, PushedJoinRefIterator, AlternativeIterator, SortingIterator, TimingIterator< RealIterator >, UpdateRowsIterator, WindowIterator, BufferingWindowIterator, GroupIndexSkipScanIterator, IndexMergeIterator, IndexRangeScanIterator, IndexSkipScanIterator, ReverseIndexRangeScanIterator, RowIDIntersectionIterator, and RowIDUnionIterator.

Read()

virtual int RowIterator::Read ( )

pure virtual

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

Implemented in TableScanIterator, IndexScanIterator< Reverse >, SortBufferIterator< Packed_addon_fields >, SortBufferIndirectIterator, SortFileIterator< Packed_addon_fields >, SortFileIndirectIterator, FakeSingleRowIterator, UnqualifiedCountIterator, ZeroRowsIterator, ZeroRowsAggregatedIterator, FollowTailIterator, TableValueConstructorIterator, BKAIterator, MultiRangeRowIterator, MaterializeIterator< Profiler >, TemptableAggregateIterator< Profiler >, FilterIterator, LimitOffsetIterator, AggregateIterator, NestedLoopIterator, CacheInvalidatorIterator, StreamingIterator, MaterializedTableFunctionIterator, WeedoutIterator, RemoveDuplicatesIterator, RemoveDuplicatesOnIndexIterator, NestedLoopSemiJoinWithDuplicateRemovalIterator, MaterializeInformationSchemaTableIterator, AppendIterator, DeleteRowsIterator, HashJoinIterator, RefIterator< Reverse >, RefOrNullIterator, EQRefIterator, ConstIterator, FullTextSearchIterator, DynamicRangeIterator, PushedJoinRefIterator, AlternativeIterator, SortingIterator, TimingIterator< RealIterator >, UpdateRowsIterator, WindowIterator, BufferingWindowIterator, GeometryIndexRangeScanIterator, GroupIndexSkipScanIterator, IndexMergeIterator, IndexRangeScanIterator, IndexSkipScanIterator, ReverseIndexRangeScanIterator, RowIDIntersectionIterator, and RowIDUnionIterator.

real_iterator() [1/2]

virtual RowIterator * RowIterator::real_iterator ( )

inlinevirtual

If this iterator is wrapping a different iterator (e.g.

TimingIterator<T>) and you need to down_cast<> to a specific iterator type, this allows getting at the wrapped iterator.

Reimplemented in TimingIterator< RealIterator >.

real_iterator() [2/2]

virtual const RowIterator * RowIterator::real_iterator ( ) const

inlinevirtual

Reimplemented in TimingIterator< RealIterator >.

SetNullRowFlag()

virtual void RowIterator::SetNullRowFlag ( bool  is_null_row )

pure virtual

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.

Implemented in FakeSingleRowIterator, ZeroRowsIterator, BKAIterator, MaterializeIterator< Profiler >, TemptableAggregateIterator< Profiler >, FilterIterator, LimitOffsetIterator, AggregateIterator, NestedLoopIterator, CacheInvalidatorIterator, MaterializedTableFunctionIterator, WeedoutIterator, RemoveDuplicatesIterator, RemoveDuplicatesOnIndexIterator, NestedLoopSemiJoinWithDuplicateRemovalIterator, MaterializeInformationSchemaTableIterator, AppendIterator, HashJoinIterator, AlternativeIterator, TableRowIterator, SortingIterator, TimingIterator< RealIterator >, UpdateRowsIterator, WindowIterator, BufferingWindowIterator, SortBufferIterator< Packed_addon_fields >, SortBufferIndirectIterator, SortFileIterator< Packed_addon_fields >, SortFileIndirectIterator, UnqualifiedCountIterator, ZeroRowsAggregatedIterator, TableValueConstructorIterator, and DeleteRowsIterator.

SetOverrideProfiler()

virtual void RowIterator::SetOverrideProfiler ( [ [maybe_unused] ] const IteratorProfiler *  profiler )

inlinevirtual

See also

TimingIterator .

StartPSIBatchMode()

virtual void RowIterator::StartPSIBatchMode ( )

inlinevirtual

Start performance schema batch mode, if supported (otherwise ignored).

PFS batch mode is a mitigation to reduce the overhead of performance schema, typically applied at the innermost table of the entire join. If you start it before scanning the table and then end it afterwards, the entire set of handler calls will be timed only once, as a group, and the costs will be distributed evenly out. This reduces timer overhead.

If you start PFS batch mode, you must also take care to end it at the end of the scan, one way or the other. Do note that this is true even if the query ends abruptly (LIMIT is reached, or an error happens). The easiest workaround for this is to simply call EndPSIBatchModeIfStarted() on the root iterator at the end of the scan. See the PFSBatchMode class for a useful helper.

The rules for starting batch and ending mode are:

1. If you are an iterator with exactly one child (FilterIterator etc.), forward any StartPSIBatchMode() calls to it.
2. If you drive an iterator (read rows from it using a for loop or similar), use PFSBatchMode as described above.
3. If you have multiple children, ignore the call and do your own handling of batch mode as appropriate. For materialization, #2 would typically apply. For joins, it depends on the join type (e.g., NestedLoopIterator applies batch mode only when scanning the innermost table).

The upshot of this is that when scanning a single table, batch mode will typically be activated for that table (since we call StartPSIBatchMode() on the root iterator, and it will trickle all the way down to the table iterator), but for a join, the call will be ignored and the join iterator will activate batch mode by itself as needed.

Reimplemented in MaterializeIterator< Profiler >, FilterIterator, LimitOffsetIterator, AggregateIterator, StreamingIterator, MaterializedTableFunctionIterator, RemoveDuplicatesIterator, RemoveDuplicatesOnIndexIterator, MaterializeInformationSchemaTableIterator, AppendIterator, DeleteRowsIterator, EQRefIterator, TableRowIterator, TimingIterator< RealIterator >, UpdateRowsIterator, WindowIterator, and BufferingWindowIterator.

thd()

THD * RowIterator::thd ( ) const

inlineprotected

UnlockRow()

virtual void RowIterator::UnlockRow ( )

pure virtual

Implemented in SortBufferIterator< Packed_addon_fields >, SortBufferIndirectIterator, SortFileIterator< Packed_addon_fields >, SortFileIndirectIterator, FakeSingleRowIterator, UnqualifiedCountIterator, ZeroRowsIterator, ZeroRowsAggregatedIterator, TableValueConstructorIterator, BKAIterator, MaterializeIterator< Profiler >, TemptableAggregateIterator< Profiler >, FilterIterator, LimitOffsetIterator, AggregateIterator, NestedLoopIterator, CacheInvalidatorIterator, StreamingIterator, MaterializedTableFunctionIterator, WeedoutIterator, RemoveDuplicatesIterator, RemoveDuplicatesOnIndexIterator, NestedLoopSemiJoinWithDuplicateRemovalIterator, MaterializeInformationSchemaTableIterator, AppendIterator, DeleteRowsIterator, HashJoinIterator, EQRefIterator, ConstIterator, AlternativeIterator, TableRowIterator, SortingIterator, TimingIterator< RealIterator >, UpdateRowsIterator, WindowIterator, BufferingWindowIterator, and IndexRangeScanIterator.

Member Data Documentation

m_thd

THD* const RowIterator::m_thd

private

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

• sql/iterators/row_iterator.h