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FWD-H2 has a Value.softCache that stores the values used by H2. Every bit of information stored by H2 is wrapped by such Value: integers, strings, dates, etc. I recently done a cache ratio statistic and reached ~66% (this was retrieved after #7363) 51%. The cache is very fast, but doesn't consider LRU policy. It is just some kind of fixed size hash-map (array with hash index). This way it is faster (avoiding LRU overhead), but it has a worst-case scenario (two values with the same hash being continuously used).

The default is set on 1024. My statistic shown 549.859 cache hits and 209.659 cache misses (this was retrieved after #7363) 471.874 cache hits and 230.256 cache misses on a customer application. I think these number are incomparable with the cache size, so there is room for improvement. My intend is to reach ~90% without sacrificing too much of memory (also considering #7448).

• I intend to do a test with 2048 and 4096 sizes just to check if larger caches actually produce significant performance bonuses.
• From my POV, private static SoftReference<Value[]> softCache really means one instance per JVM, right? This is inside H2. Thus, I wonder if multiple embedded in-memory connections share this same cache instance. Otherwise, maybe H2 is doing some hacks so that this softCache is in fact per-connection. Is this even possible?
  • If this is per connection, which i doubt, then increasing the size is quite dangerous.
  • If this is not per connection, then we can increase the size by much more, considering we have only one such instance.
  • Note that H2 also has OBJECT_CACHE_MAX_PER_ELEMENT_SIZE, which is the maximum size of a cached element. We we fear of caching objects that are too large, we can reduce this threshold. Currently, the default is 4096. E.g. strings with more than 4096 characters are not cached. This means that this softCache can retain only 8MB at most.
• As of #7388, we can consider making the cache size configurable.