Product Documentation

Database Administrator's Guide

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PREIMAGE_HASH <number of hash bins>

The number of hash bins available to the preimage hash algorithm. During a transaction, a preimage space is used to hold intermediate results pending an abort or a commit. To search this area, a hashing algorithm is employed. The PREIMAGE_HASH value specifies the number of four-byte bins available per user for use by this algorithm. This value should be increased only if a large volume of updates and/or additions per transaction (e.g., several thousand) is anticipated.

Default: 128

The following keywords are closely related to the lock hash bin strategy and are enumerated here for convenience.

PREIMAGE_HASH_MAX <maximum number of lock hash bins>

Default: 131072

PREIMAGE_HASH_LOADFAC <avg locks per bin>

Default: 8

PREIMAGE_HASH_REHASHFAC <fraction of hash bins reorganized>

Default: 4 (denominator - one fourth at a time)

Note: Dynamic hashing is now used for transaction entries held before commit. This results in more efficient searching of intermediate transaction contents. Dynamic hashing automatically adjusts to changing conditions of transaction and lock data.