jihoonson commented on a change in pull request #9800:
URL: https://github.com/apache/druid/pull/9800#discussion_r421006133
##########
File path:
processing/src/main/java/org/apache/druid/query/filter/InDimFilter.java
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@@ -132,31 +145,40 @@ public FilterTuning getFilterTuning()
@Override
public byte[] getCacheKey()
{
- boolean hasNull = false;
- for (String value : values) {
- if (value == null) {
- hasNull = true;
- break;
+ if (cacheKey == null) {
+ final List<String> sortedValues = new ArrayList<>(values);
+ sortedValues.sort(Comparator.nullsFirst(Ordering.natural()));
+ final Hasher hasher = Hashing.sha256().newHasher();
Review comment:
The issue with storing all `values` in the cacheKey is that the cache
key can become huge as the size of `values` grows which can result in pollution
in the cache.
Regarding using SHA-256, I believe academical research is more trustable
than my testing. [According to the probability table, the required number of
hashed elements such that probability of at least one hash collision >= 10^−18
is 4.8×10^29 under assumption that the hash function is
perfect](https://en.wikipedia.org/wiki/Birthday_problem). This is negligible
when it comes to practice even though the SHA-256 hash function is not proven
to be perfect.
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