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Guid.Comb

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In ultimele zile am început sa lucrez cu Fluent NHibernate si am descoperit GuidComb. M-am documentat sa vad ce înseamna de fapt. Pe scurt:
One of the better alternatives that i kinda like is the guid.comb strategy. Using regular guids as a primary key value leads to fragmented indexes (due to the randomness of the guid’s value) which leads to bad performance. This is a problem that the guid.comb strategy can solve quite easily for you.
Nu are rost sa traduc și să explic din nou acelasi lucru. O să găsiți pe Google destule materiale. Partea interesanta este modul in care se generează aceste guiduri.
Din cei 32 de biti pe care un guid ii conține, 16 sunt generați in funcție de configurația hardware a mașinii( începînd cu Windows 2000, seria plăcii de rețea nu se mai folosește).
Iar următorii 16 sunt generați in forma următoare:
  • 0-9: se generează pe baza unui uniqueidentifier( depinde de framework);
  • 10-15: pe baza la Datetime;
Acesta este un algoritm de generate de Guid destul de simplu si care isi face treaba foarte bine cînd avem de lucru cu DB. Este foarte folositor mai ales cand trebe sa generam PK. Începînd de la Sql Server 2005 exista Sequential Guid care ii foarte asemănător.
In general se cîștiga un spor de performanta de 25%-30% pe tabele care conține PK ca și guid.In cazul in care tabela pe care lucrați nu are PK, sporul de performanta poate sa fie 0% sau chiar negativ in unele situații. Pentru mai multe informații despre sporul de performanta puteti sa gasiti aici.

PS: Cînd ne ajuta un Guid? Un exemplu este cînd se lucrează cu o aplicația și offline, caz in care la sincronizare nu mai este nevoie de regenerare de PK.
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Comments

  1. IoanAugust 13, 2010 at 3:39 PM

    Se pare ca pierderea de performanta e numai daca acel PK are un clustered index, altfel nu.

    ReplyDelete

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