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briankind
Starting Member
17 Posts |
 Posted - 2009-07-25 : 10:37:38
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| logical vs physical layout? what is the difference? thanks |
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GilaMonster
Master Smack Fu Yak Hacker
4507 Posts |
Posted - 2009-07-25 : 15:51:59
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| Can you expand on your question? I don't fully understand what you're asking. Logical vs physical layout of what?--Gail ShawSQL Server MVP |
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dportas
Yak Posting Veteran
53 Posts |
Posted - 2009-07-26 : 09:57:25
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| You need to be careful when it comes to the terms "logical" and "physical" in database systems because they can cause a lot of confusion. In database theory and for those concerned with DBMS internals, the logical level or logical representation refers to the external representation that the database exposes to the world - such as the tables and views in a SQL DBMS. The physical representation refers to the actual way those logical objects are stored in files or on disk and the storage structures associated with them (tablespaces and indexes for example). The physical level is generally assumed to be hidden from most users of the system. The ANSI-SPARC definitions call the Logical and Physical levels the External (or Community) view and the Internal view respectively.Confusingly, the terms Logical Modelling and Physical Modelling are applied slightly differently by many database professionals, especially database designers. The Logical Data Model (LDM) usually means a preliminary design - typically somewhat abstracted from what is ultimately created in a database. The Physical Data Model (PDM) is the actual design to be implemented or already implemented. The PDM may include both logical and physical database elements - for instance tables, tablespaces and indexes may all be included in the PDM.This conventional understanding of the word "Physical" is a bit unfortunate because there is no widely understood alternative name used by the industry for the "internal only" parts of the database - ie, there is no other name for the things that database theory textbooks and DBMS engineers call the "Physical" level. The fact that industry practitioners lack such a word is the cause of a lot of misunderstandings. When someone uses the term "physical" model it may be wise to ask them to clarify just what they mean.Hopet this helps. |
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logical vs physical layout?