DataBlockHashIndex: Remove the division from EstimateSize() (#4293)
Summary: `BlockBasedTableBuilder::Add()` eventually calls `DataBlockHashIndexBuilder::EstimateSize()`. The previous implementation divides the `num_keys` by the `util_ratio_` to get an estimizted `num_buckets`. Such division is expensive as it happens in every `BlockBasedTableBuilder::Add()`. This diff estimates the `num_buckets` by double addition instead of double division. Specifically, in each `Add()`, we add `bucket_per_key_` (inverse of `util_ratio_`) to the current `estimiated_num_buckets_`. The cost is that we are gonna have the `estimated_num_buckets_` stored as one extra field in the DataBlockHashIndexBuilder. Pull Request resolved: https://github.com/facebook/rocksdb/pull/4293 Differential Revision: D9412472 Pulled By: fgwu fbshipit-source-id: 2925c8509a401e7bd3c1ab1d9e9c7244755c277a
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