Ranges on t.key > const are not handled efficiently

[spetrunia]

Ranges on t.key > const are not handled efficiently

[spetrunia]

It is not possible to observe the issue via counters or status variables exposed by MyRocks. Only through performance measurements or in debugger.

[hermanlee]

Should we instrument Apply_changes_iter on invocations to Next() and Prev() to track the number of rows we examine internally during the query or export out the rest of the perf_context stats somehow? How useful would this be in general to track down these types of performance issues?

[spetrunia]

I think it is general useful to know how many rocksdb::Iterator::Next()/Prev() calls were made.

Instrumenting Apply_changes_iter's invocations of rocksdb::Iterator::Next()/Prev() is a good choice, because it will not count operations made by the background threads (and maybe, it is bad for the same reason - iterator scans that are not made through Apply_changes_iter are not counted)

[spetrunia]

https://reviews.facebook.net/D35103