The adaptive hash index enables InnoDB to perform more like an in-memory database on systems with appropriate combinations of workload and sufficient memory for the buffer pool without sacrificing transactional features or reliability. The adaptive hash index is disabled by the innodb_adaptive_hash_index variable, or turned on at server startup by --innodb-adaptive-hash-index.

Based on the observed pattern of searches, a hash index is built using a prefix of the index key. The prefix can be any length, and it may be that only some values in the B-tree appear in the hash index. Hash indexes are built on demand for the pages of the index that are accessed often.

If a table fits almost entirely in main memory, a hash index speeds up queries by enabling direct lookup of any element, turning the index value into a sort of pointer. InnoDB has a mechanism that monitors index searches. If InnoDB notices that queries could benefit from building a hash index, it does so automatically.

With some workloads, the speedup from hash index lookups greatly outweighs the extra work to monitor index lookups and maintain the hash index structure. Access to the adaptive hash index can sometimes become a source of contention under heavy workloads, such as multiple concurrent joins. Queries with LIKE operators and % wildcards also tend not to benefit. For workloads that do not benefit from the adaptive hash index, turning it off reduces unnecessary performance overhead. Because it is difficult to predict in advance whether the adaptive hash index is appropriate for a particular system and workload, consider running benchmarks with it enabled and disabled.

The adaptive hash index feature is partitioned. Each index is bound to a specific partition, and each partition is protected by a separate latch. Partitioning is controlled by the innodb_adaptive_hash_index_parts variable. The innodb_adaptive_hash_index_parts variable is set to 8 by default. The maximum setting is 512.

You can monitor adaptive hash index use and contention in the SEMAPHORES section of SHOW ENGINE INNODB STATUS output. If there are numerous threads waiting on rw-latches created in btr0sea.c, consider increasing the number of adaptive hash index partitions or disabling the adaptive hash index.

For information about the performance characteristics of hash indexes, see Section 10.3.9, “Comparison of B-Tree and Hash Indexes”.