MySQL 8.0.31
Source Code Documentation
StreamingIterator Class Referencefinal

StreamingIterator is a minimal version of MaterializeIterator that does not actually materialize; instead, every Read() just forwards the call to the subquery iterator and does the required copying from one set of fields to another. More...

#include <composite_iterators.h>

Inheritance diagram for StreamingIterator:
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Public Member Functions

 StreamingIterator (THD *thd, unique_ptr_destroy_only< RowIterator > subquery_iterator, Temp_table_param *temp_table_param, TABLE *table, bool provide_rowid, JOIN *join, int ref_slice)
 
bool Init () override
 Initialize or reinitialize the iterator. More...
 
int Read () override
 Read a single 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 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 IteratorProfilerGetProfiler () const
 Get profiling data for this iterator (for 'EXPLAIN ANALYZE'). More...
 
virtual void SetOverrideProfiler ([[maybe_unused]] const IteratorProfiler *profiler)
 
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_subquery_iterator
 
Temp_table_paramm_temp_table_param
 
ha_rows m_row_number
 
JOIN *const m_join
 
const int m_output_slice
 
int m_input_slice
 
const bool m_provide_rowid
 

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

StreamingIterator is a minimal version of MaterializeIterator that does not actually materialize; instead, every Read() just forwards the call to the subquery iterator and does the required copying from one set of fields to another.

It is used for when the optimizer would normally set up a materialization, but you don't actually need one, ie. you don't want to read the rows multiple times after writing them, and you don't want to access them by index (only a single table scan). It also takes care of setting the NULL row flag on the temporary table.

Constructor & Destructor Documentation

◆ StreamingIterator()

StreamingIterator::StreamingIterator ( THD thd,
unique_ptr_destroy_only< RowIterator subquery_iterator,
Temp_table_param temp_table_param,
TABLE table,
bool  provide_rowid,
JOIN join,
int  ref_slice 
)
Parameters
thdThread handle.
subquery_iteratorThe iterator to read rows from.
temp_table_paramParameters for the temp table.
tableThe table we are streaming through. Will never actually be written to, but its fields will be used.
provide_rowidIf true, generate a row ID for each row we stream. This is used if the parent needs row IDs for deduplication, in particular weedout.
joinSee MaterializeIterator.
ref_sliceSee MaterializeIterator.

Member Function Documentation

◆ EndPSIBatchModeIfStarted()

void StreamingIterator::EndPSIBatchModeIfStarted ( )
inlineoverridevirtual

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 StreamingIterator::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 TABLE_REF) and allow you to read the records anew.

Implements RowIterator.

◆ Read()

int StreamingIterator::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
0OK
-1End of records
1Error

Implements RowIterator.

◆ StartPSIBatchMode()

void StreamingIterator::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.

◆ UnlockRow()

void StreamingIterator::UnlockRow ( )
inlineoverridevirtual

Implements RowIterator.

Member Data Documentation

◆ m_input_slice

int StreamingIterator::m_input_slice
private

◆ m_join

JOIN* const StreamingIterator::m_join
private

◆ m_output_slice

const int StreamingIterator::m_output_slice
private

◆ m_provide_rowid

const bool StreamingIterator::m_provide_rowid
private

◆ m_row_number

ha_rows StreamingIterator::m_row_number
private

◆ m_subquery_iterator

unique_ptr_destroy_only<RowIterator> StreamingIterator::m_subquery_iterator
private

◆ m_temp_table_param

Temp_table_param* StreamingIterator::m_temp_table_param
private

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