add benchmarks for multivalued fastfield merge

[PSeitz]

OLD CODE

test fastfield::bench::bench_multi_value_ff_merge_few_segments                                                           ... bench:  46,103,960 ns/iter (+/- 2,066,083)
test fastfield::bench::bench_multi_value_ff_merge_many_segments                                                          ... bench:  83,073,036 ns/iter (+/- 4,373,615)
test fastfield::bench::bench_multi_value_ff_merge_many_segments_log_merge                                                ... bench:  64,178,576 ns/iter (+/- 1,466,700)

NEW CODE

running 3 tests
test fastfield::multivalued::bench::bench_multi_value_ff_merge_few_segments                                              ... bench:  57,379,523 ns/iter (+/- 3,220,787)
test fastfield::multivalued::bench::bench_multi_value_ff_merge_many_segments                                             ... bench:  90,831,688 ns/iter (+/- 1,445,486)
test fastfield::multivalued::bench::bench_multi_value_ff_merge_many_segments_log_merge                                   ... bench: 158,313,264 ns/iter (+/- 28,823,250)

[fulmicoton]

Approved...

As far as I can tell the multivalued stuff always only use the
two following adhoc columns

• MultivalueStartIndex
• SortedDocIdMultiValueColumn

Do you know which one is the most likely culprit?

[PSeitz]

It's SortedDocIdMultiValueColumn::get_val via Line::train