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MySQL 8.0.39
Source Code Documentation
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MySQL 8.0.39
Source Code Documentation
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An iterator that is semantically equivalent to a semijoin NestedLoopIterator immediately followed by a RemoveDuplicatesOnIndexIterator. More...
#include <composite_iterators.h>
Public Member Functions | |
| NestedLoopSemiJoinWithDuplicateRemovalIterator (THD *thd, unique_ptr_destroy_only< RowIterator > source_outer, unique_ptr_destroy_only< RowIterator > source_inner, const TABLE *table, KEY *key, size_t key_len) | |
| 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 | 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 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 RowIterator * | real_iterator () |
| If this iterator is wrapping a different iterator (e.g. More... | |
| virtual const RowIterator * | real_iterator () const |
Private Attributes | |
| unique_ptr_destroy_only< RowIterator > const | m_source_outer |
| unique_ptr_destroy_only< RowIterator > const | m_source_inner |
| const TABLE * | m_table_outer |
| KEY * | m_key |
| uchar * | m_key_buf |
| const size_t | m_key_len |
| bool | m_deduplicate_against_previous_row |
Additional Inherited Members | |
| Protected Member Functions inherited from RowIterator | |
| THD * | thd () const |
An iterator that is semantically equivalent to a semijoin NestedLoopIterator immediately followed by a RemoveDuplicatesOnIndexIterator.
It is used to implement the “loose scan” strategy in queries with multiple tables on the inside of a semijoin, like
... FROM t1 WHERE ... IN ( SELECT ... FROM t2 JOIN t3 ... )
In this case, the query tree without this iterator would ostensibly look like
-> Nested loop join -> Table scan on t1 -> Remove duplicates on t2_idx -> Nested loop semijoin -> Index scan on t2 using t2_idx -> Filter (e.g. t3.a = t2.a) -> Table scan on t3
(t3 will be marked as “first match” on t2 when implementing loose scan, thus the semijoin.)
First note that we can't put the duplicate removal directly on t2 in this case, as the first t2 row doesn't necessarily match anything in t3, so it needs to be above. However, this is wasteful, because once we find a matching t2/t3 pair, we should stop scanning t3 until we have a new t2.
NestedLoopSemiJoinWithDuplicateRemovalIterator solves the problem by doing exactly this; it gets a row from the outer side, gets exactly one row from the inner side, and then skips over rows from the outer side (without scanning the inner side) until its keypart changes.
| NestedLoopSemiJoinWithDuplicateRemovalIterator::NestedLoopSemiJoinWithDuplicateRemovalIterator | ( | THD * | thd, |
| unique_ptr_destroy_only< RowIterator > | source_outer, | ||
| unique_ptr_destroy_only< RowIterator > | source_inner, | ||
| const TABLE * | table, | ||
| KEY * | key, | ||
| size_t | key_len | ||
| ) |
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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.
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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.
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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].
| 0 | OK |
| -1 | End of records |
| 1 | Error |
Implements RowIterator.
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inlineoverridevirtual |
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.
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Implements RowIterator.
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1.9.2