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由 Fenggang Wu 提交于
Summary: Add hash index support to data blocks, which helps to reduce the CPU utilization of point-lookup operations. This feature is backward compatible with the data block created without the hash index. It is disabled by default unless `BlockBasedTableOptions::data_block_index_type` is set to `data_block_index_type = kDataBlockBinaryAndHash.` The DB size would be bigger with the hash index option as a hash table is added at the end of each data block. If the hash utilization ratio is 1:1, the space overhead is one byte per key. The hash table utilization ratio is adjustable using `BlockBasedTableOptions::data_block_hash_table_util_ratio`. A lower utilization ratio will improve more on the point-lookup efficiency, but take more space too. Pull Request resolved: https://github.com/facebook/rocksdb/pull/4174 Differential Revision: D8965914 Pulled By: fgwu fbshipit-source-id: 1c6bae5d1fc39c80282d8890a72e9e67bc247198
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