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InnoDB 1.1 for MySQL 5.5 User's Guide
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8.2.4. Dynamically Changing innodb_adaptive_hash_index

As described in Section 7.5, “Controlling Adaptive Hash Indexing”, it may be desirable, depending on your workload, to dynamically enable or disable the adaptive hash indexing scheme InnoDB uses to improve query performance.

The configuration option innodb_adaptive_hash_index lets you disable the adaptive hash index. It is enabled by default. You can modify this parameter through the SET GLOBAL statement, without restarting the server. Changing the setting requires the SUPER privilege.

Disabling the adaptive hash index empties the hash table immediately. Normal operations can continue while the hash table is emptied, and executing queries that were using the hash table access the index B-trees directly instead. When the adaptive hash index is re-enabled, the hash table is populated again during normal operation.

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