The adaptive hash index feature enables
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 feature is enabled by the
variable, or turned off at server startup by
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 can
speed up queries by enabling direct lookup of any element, turning
the index value into a sort of pointer.
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 feature, turning it off
reduces unnecessary performance overhead. Because it is difficult
to predict in advance whether the adaptive hash index feature is
appropriate for a particular system and workload, consider running
benchmarks with it enabled and disabled. Architectural changes in
MySQL 5.6 make it more suitable to disable the adaptive hash index
feature than in earlier releases.
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
an RW-latch created in
btr0sea.c, it might be
useful to disable the adaptive hash index feature.
For information about the performance characteristics of hash indexes, see Section 8.3.8, “Comparison of B-Tree and Hash Indexes”.