Perf: Remove redundant map lookups in CustomTierSelectorStrategy comparator

[abhishekrb19]

Avoid repeated containsKey() and get() calls by retrieving each map value once and using null checks instead. This reduces redundant lookups and simplifies the comparator logic.

This PR has:

• been self-reviewed.
• been tested in a test Druid cluster.

[GWphua]

LGTM, regardless of the answer to the question below.

How much of a performance boost is this, given that it's looking up a hashmap?

[abhishekrb19]

Thanks for taking a look, @GWphua!

How much of a performance boost is this, given that it's looking up a hashmap?

I didn’t benchmark this change. It should remove about 2–3 unnecessary map lookups per query.

[abhishekrb19]

I got curious about the performance improvement and ran a quick benchmark. With this lookup access pattern, we see a good ~18–20% speed up on average - CustomTierSelectorComparatorBenchmark.java. I did a quick spot check in other areas of the codebase and there seems to be a few other occurrences where we can simply do map.get(key) instead of map.containsKey(key) + map.get(key).

Benchmark                                                Mode  Cnt    Score   Error  Units
CustomTierSelectorComparatorBenchmark.newImplementation  avgt   10  456.969 ± 5.175  ns/op
CustomTierSelectorComparatorBenchmark.oldImplementation  avgt   10  571.750 ± 4.194  ns/op