This abstraction is used to merge the output of multiple readers, each
opened for a single partition query, into a non-decreasing stream
of mutation_fragments.
It is similar to `mutation_reader_merger`,
but an important difference is that the new merger may select new readers
in the middle of a partition after it already returned some fragments
from that partition. It uses the new `position_reader_queue` abstraction
to select new readers. It doesn't support multi-partition (ring range) queries.
The new merger will be later used when reading from sstable sets created
by TimeWindowCompactionStrategy. This strategy creates many sstables
that are mostly disjoint w.r.t the contained clustering keys, so we can
delay opening sstable readers when querying a partition until after we have
processed all mutation fragments with positions before the keys
contained by these sstables.
A microbenchmark was added that compares the existing combining reader
(which uses `mutation_reader_merger` underneath) with a new combining reader
built using the new `clustering_order_reader_merger` and a simple queue of readers
that returns readers from some supplied set. The used set of readers is built from the following
ranges of keys (each range corresponds to a single reader):
`[0, 31]`, `[30, 61]`, `[60, 91]`, `[90, 121]`, `[120, 151]`.
The microbenchmark runs the reader and divides the result by the number of mutation fragments.
The results on my laptop were:
```
$ build/release/test/perf/perf_mutation_readers -t clustering_combined.* -r 10
single run iterations: 0
single run duration: 1.000s
number of runs: 10
test iterations median mad min max
clustering_combined.ranges_generic 2911678 117.598ns 0.685ns 116.175ns 119.482ns
clustering_combined.ranges_specialized 3005618 111.015ns 0.349ns 110.063ns 111.840ns
```
`ranges_generic` denotes the existing combining reader, `ranges_specialized` denotes the new reader.
Split from https://github.com/scylladb/scylla/pull/7437.
Closes#7688
* github.com:scylladb/scylla:
tests: mutation_source_test for clustering_order_reader_merger
perf: microbenchmark for clustering_order_reader_merger
mutation_reader_test: test clustering_order_reader_merger in memory
test: generalize `random_subset` and move to header
mutation_reader: introduce clustering_order_reader_merger
a95c2a946c