MySQL 8.0.29
Source Code Documentation
LimitOffsetIterator Class Referencefinal

Handles LIMIT and/or OFFSET; Init() eats the first "offset" rows, and Read() stops as soon as it's seen "limit" rows (including any skipped by offset). More...

#include <composite_iterators.h>

Inheritance diagram for LimitOffsetIterator:

Public Member Functions

 LimitOffsetIterator (THD *thd, unique_ptr_destroy_only< RowIterator > source, ha_rows limit, ha_rows offset, bool count_all_rows, bool reject_multiple_rows, ha_rows *skipped_rows)
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 StartPSIBatchMode () override
 Start performance schema batch mode, if supported (otherwise ignored). More...
void EndPSIBatchModeIfStarted () override
 Ends performance schema batch mode, if started. More...
void UnlockRow () override
- Public Member Functions inherited from RowIterator
 RowIterator (THD *thd)
virtual ~RowIterator ()=default
 RowIterator (const RowIterator &)=delete
 RowIterator (RowIterator &&)=default
virtual std::string TimingString () const
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_source
ha_rows m_seen_rows
bool m_needs_offset
 Whether we have OFFSET rows that we still need to skip. More...
const ha_rows m_limit
const ha_rows m_offset
const bool m_count_all_rows
const bool m_reject_multiple_rows

Additional Inherited Members

- Protected Member Functions inherited from RowIterator
THDthd () const

Detailed Description

Handles LIMIT and/or OFFSET; Init() eats the first "offset" rows, and Read() stops as soon as it's seen "limit" rows (including any skipped by offset).

Constructor & Destructor Documentation

◆ LimitOffsetIterator()

LimitOffsetIterator::LimitOffsetIterator ( THD thd,
unique_ptr_destroy_only< RowIterator source,
ha_rows  limit,
ha_rows  offset,
bool  count_all_rows,
bool  reject_multiple_rows,
ha_rows skipped_rows 
thdThread context
sourceRow source
limitMaximum number of rows to read, including the ones skipped by offset. Can be HA_POS_ERROR for no limit.
offsetNumber of initial rows to skip. Can be 0 for no offset.
count_all_rowsIf true, the query will run to completion to get more accurate numbers for skipped_rows, so you will not get any performance benefits of early end.
reject_multiple_rowsTrue if a derived table transformed from a scalar subquery needs a run-time cardinality check
skipped_rowsIf not nullptr, is incremented for each row skipped by offset or limit.

Member Function Documentation

◆ EndPSIBatchModeIfStarted()

void LimitOffsetIterator::EndPSIBatchModeIfStarted ( )

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 from RowIterator.

◆ Init()

bool LimitOffsetIterator::Init ( )

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 LimitOffsetIterator::Read ( )

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
-1End of records

Implements RowIterator.

◆ SetNullRowFlag()

void LimitOffsetIterator::SetNullRowFlag ( bool  is_null_row)

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.

◆ StartPSIBatchMode()

void LimitOffsetIterator::StartPSIBatchMode ( )

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 from RowIterator.

◆ UnlockRow()

void LimitOffsetIterator::UnlockRow ( )

Implements RowIterator.

Member Data Documentation

◆ m_count_all_rows

const bool LimitOffsetIterator::m_count_all_rows

◆ m_limit

const ha_rows LimitOffsetIterator::m_limit

◆ m_needs_offset

bool LimitOffsetIterator::m_needs_offset

Whether we have OFFSET rows that we still need to skip.

◆ m_offset

const ha_rows LimitOffsetIterator::m_offset

◆ m_reject_multiple_rows

const bool LimitOffsetIterator::m_reject_multiple_rows

◆ m_seen_rows

ha_rows LimitOffsetIterator::m_seen_rows

◆ m_skipped_rows

ha_rows* LimitOffsetIterator::m_skipped_rows

◆ m_source

unique_ptr_destroy_only<RowIterator> LimitOffsetIterator::m_source

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