Constant DEFAULT_BLOCK_CACHE_SIZE 

pub(crate) const DEFAULT_BLOCK_CACHE_SIZE: usize = _; // 67_108_864usize

Default block cache size: 64 MiB (RocksDB default is 8 MiB). Index/filter blocks are placed in this cache too (set_cache_index_and_filter_blocks), so everything is bounded. With long keys a table’s index block reaches several MiB, and every lookup needs it, so the cache must be able to hold the index blocks of the tables being read or each lookup reads and decodes them again.