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Update DistinctSortedBlockInputStream.h

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alexey-milovidov 2017-04-14 23:59:37 +04:00
parent e62769190e
commit f03f32e2ab
1 changed files with 7 additions and 2 deletions
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9
dbms/src/DataStreams/DistinctSortedBlockInputStream.h
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@ -10,6 +10,9 @@ namespace DB
/** This class is intended for implementation of SELECT DISTINCT clause and /** This class is intended for implementation of SELECT DISTINCT clause and
* leaves only unique rows in the stream. * leaves only unique rows in the stream.
* *
* Implementation for case, when input stream has rows for same DISTINCT key or at least its prefix,
* grouped together (going consecutively).
*
* To optimize the SELECT DISTINCT ... LIMIT clause we can * To optimize the SELECT DISTINCT ... LIMIT clause we can
* set limit_hint to non zero value. So we stop emitting new rows after * set limit_hint to non zero value. So we stop emitting new rows after
* count of already emitted rows will reach the limit_hint. * count of already emitted rows will reach the limit_hint.
@ -46,12 +49,14 @@ private:
size_t rows, size_t rows,
ClearableSetVariants & variants) const; ClearableSetVariants & variants) const;
const SortDescription& description; const SortDescription & description;
struct PreviousBlock struct PreviousBlock
{ {
Block block; Block block;
ConstColumnPlainPtrs clearing_hint_columns; ConstColumnPlainPtrs clearing_hint_columns;
} prev_block; };
PreviousBlock prev_block;
Names columns_names; Names columns_names;
ClearableSetVariants data; ClearableSetVariants data;