Piotr Dulikowski 2b4ca0c562 mutation_reader: gallop mode for combined reader ...

In case when a single reader contributes a stream of fragments
and keeps winning over other readers, mutation_reader_merger will
enter gallop mode, in which it is assumed that the reader will keep
winning over other readers. Currently, a reader needs to contribute
3 fragments to enter that mode.

In gallop mode, fragments returned by the galloping reader will be
compared with the best fragment from _fragment_heap. If it wins, the
fragment is directly returned. Otherwise, gallop mode ends and
merging performed as in general case, which involves heap operations.

In current implementation, when the end of partition is encountered
while in gallop mode, the gallop mode is ended unconditionally.

Fixes #3593.

2019-10-30 09:51:18 +01:00

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