MySQL  8.0.27
Source Code Documentation
EQRefIterator Class Referencefinal

Like RefIterator, but used in situations where we're guaranteed to have exactly zero or one rows for each reference (due to e.g. More...

#include <ref_row_iterators.h>

Inheritance diagram for EQRefIterator:
TableRowIterator RowIterator

Public Member Functions

 EQRefIterator (THD *thd, TABLE *table, TABLE_REF *ref, bool use_order, ha_rows *examined_rows)
 
bool Init () override
 Read row using unique key: eq_ref access method implementation. More...
 
int Read () override
 Read row using unique key: eq_ref access method implementation. More...
 
void UnlockRow () override
 Since EQRefIterator may buffer a record, do not unlock it if it was not used in this invocation of EQRefIterator::Read(). More...
 
void StartPSIBatchMode () override
 Start performance schema batch mode, if supported (otherwise ignored). 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
 
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

TABLE_REF *const m_ref
 
const bool m_use_order
 
bool m_first_record_since_init
 
ha_rows *const m_examined_rows
 

Additional Inherited Members

- Protected Member Functions inherited from TableRowIterator
int HandleError (int error)
 
void PrintError (int error)
 
TABLEtable () const
 
- Protected Member Functions inherited from RowIterator
THDthd () const
 

Detailed Description

Like RefIterator, but used in situations where we're guaranteed to have exactly zero or one rows for each reference (due to e.g.

unique constraints). It adds extra buffering to reduce the number of calls to the storage engine in the case where many consecutive rows on the left side contain the same value.

Member Function Documentation

◆ StartPSIBatchMode()

void EQRefIterator::StartPSIBatchMode ( )
inlineoverridevirtual

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.

Member Data Documentation

◆ m_examined_rows

ha_rows* const EQRefIterator::m_examined_rows
private

◆ m_first_record_since_init

bool EQRefIterator::m_first_record_since_init
private

◆ m_ref

TABLE_REF* const EQRefIterator::m_ref
private

◆ m_use_order

const bool EQRefIterator::m_use_order
private

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