jtuglu1 commented on code in PR #20815:
URL: https://github.com/apache/druid/pull/20815#discussion_r4504360086
##########
server/src/main/java/org/apache/druid/query/ResultLevelCachingQueryRunner.java:
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@@ -95,7 +98,7 @@ public Sequence<T> run(QueryPlus queryPlus, ResponseContext
responseContext)
{
if (useResultCache || populateResultCache) {
- final byte[] queryCacheKey = strategy.computeResultLevelCacheKey(query);
+ final byte[] queryCacheKey = computeResultLevelCacheKey();
final Cache.NamedKey cacheKey =
CacheUtil.computeResultLevelCacheKey(queryCacheKey);
final byte[] cachedResultSet =
fetchResultsFromResultLevelCache(cacheKey);
Review Comment:
something I noticed while looking over
[this](https://github.com/maytasm/druid/blob/f577780e1c7d044cddda8e629c3e8a1dadbae678/server/src/main/java/org/apache/druid/query/ResultLevelCachingQueryRunner.java#L366)
code is that looks like there might? be a further way to increase hit rate.
Consider 3 query instances that map to the same query cache key:
```
q1: read cache, calculate E1 from old segments, start execution
q2: read cache, calculate E2 from new segments, start execution
q2: finish, write (key, {E2, results2})
q1: finish, write (key, {E1, results1})
q3: read cached {E1, results1}
q3: calculate E2 from current segments
q3: reject E1, execute query, write (key, {E2, results3})
```
In order to prevent q3 from re-executing, we might be able to either:
- Do some sort of cache.putIfAbsentOrOlder() which sets the result if it is
not there yet or the etag represents a segment set that is "fresher" than the
prior cached result.
- Alternatively, can maybe think about storing the etag in the cache key
itself which would blow up the cache size potentially (and you'd need eviction
policy which is complicated)
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