IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 140)

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2-46 IBM Informix OnLine Database Server Administrator’s Guide

Managing Shared-Memory Resources

If a program is using Committed Read isolation, the problem just described

cannot occur since the database server does not see a row that has been

marked for deletion. However, underCommitted Read, nolock is placed on

an undeleted row when itis read. BYTE or TEXT data isreadin a secondstep,

after the row has been fetched. During this lengthy step, it is possible for

anotherprogramtodeletetherowandcommitthedeletionandforthespace

on the diskpage tobe reused. If thespace hasbeen reused in theinterim, the

blob timestamp will have been incremented and will be greater than the

timestamp in the forward pointer. In this case, the comparison will indicate

the obsolete pointer information and the inconsistency will be reported as a

part of consistency checking.

Hash Tables and the Hashing Technique

Hashing is a technique that permits rapid lookup in tables where items are

added unpredictably. Three OnLine shared-memory tables have an

associated hash table. These three tables are the lock table, the active tblspace

table, and the buffer table.

Eachentry that istobeplaced in any OnLinetablehasa unique key. Ifahash

table is used, then the unique key is “hashed,” which means a speciﬁc

algorithm is used to map the keys onto a set of integers, which are the hash

values. The algorithm is selected so that the keys, once hashed, are fairly

evenly distributed over a range. The result is not a unique mapping; two

differentkeys maymaponto thesamehash value.Entriesarestoredby their

hash value and not solely by their unique key value.

For example, asimplehashing algorithm is“dividethe key valueby100 and

use the remainder.” With thisalgorithm, youcould expect 100different hash

values equal to each possible remainder, from 0 to 99. Each of these hash

values would correspond to an entry in the hash table.

To locate an item in the table, the item key is passed through the hashing

algorithmand its hashvalueis computed. Usingthis hash value,theentry at

that location in the hash table is examined.

Each hash-table entry contains a pointer to entries in the associated table

(lock, tblspace, or user) with the corresponding hash value. Multiple entries

with the same hash value are chained together in a linked list.

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