Restated: "Fields are expensive, records are cheap"
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From: David W. Fenton on 28 Feb 2010 21:37 =?Utf-8?B?RGVubmlz?= <Dennis(a)discussions.microsoft.com> wrote in news:557B9E51-F983-483F-91D9-DFA2B8FEB700(a)microsoft.com: > Also, as I stated in the other discussion on this subject (which I > surprised you missed as you are commenting in that discussion > also), I� Tve worked on 12 different vendor� Ts insurance systems > over the years. Those system have been written DECADES apart with > totally different technology and totally different development > groups I have looked at the data structures that have become common practice for a lot of different application types and have repeatedly seen bad designs become the norm for those creating those types of applications. That many people have used the same data structure doesn't make it a good one. Likewise, lots of applications have to support legacy applications that can't easily handle more normalized relational structures and thus a lot of structures that were required 15-20 years ago by the available technology are still in use. This is quite understandable, of course, but if you're developing a new application using modern technology, there's no reason to maintain the old data structures unless they are really the best model of the entities involved. I see this kind of thing all the time. A 200-field table is just not a good model and for Access is way too close to the 255-field limit for me to be comfortable with it. While it's theoretically possible that the structure is not denormalized, I think it's extremely unlikely, and that there are better structures to store and manipulate the very same data. -- David W. Fenton http://www.dfenton.com/ usenet at dfenton dot com http://www.dfenton.com/DFA/
From: David W. Fenton on 28 Feb 2010 21:44 =?Utf-8?B?RGVubmlz?= <Dennis(a)discussions.microsoft.com> wrote in news:0112D48C-5157-4FB6-937B-AAE2CB87A260(a)microsoft.com: > So once again, how does all this caching and b-tree traversal > speed up the physical reading of a record that is not in memory. > the database engine still has to go to disk or worse yet - over > the network to get the desired record. Well, it doesn't, but I thought your question implied that what I was proposing was somehow going to require reading the entire table into memory, which is patently not the case. Only as much as is needed will be retrieved, and once retrieved that will be cached (so that if it's needed again, there won't be another trip to disk/server). This is EXACTLY the way any alternative is going to work, so I just don't see your point in disputing the question. Jet is a random-access engine, in that it doesn't have to read the file sequentially to get to the data, nor does it use record numbers and offsets (as in a fixed-length record). It retrieves the minimum data pages needed to fulfill the request, having looked up in the indexes which are the needed the data pages. It will make the trip to disk only once and cache the result, and go back to disk for a read only when the record has been updated (according to the locking data, rather than checking the data page itself). Your question seems to me to be based on assumptions that don't take account of how modern random-access database access works, or how modern OS's and database engines cache and lock data. But maybe I'm just misunderstanding your basic question. -- David W. Fenton http://www.dfenton.com/ usenet at dfenton dot com http://www.dfenton.com/DFA/
From: David W. Fenton on 28 Feb 2010 22:16 =?Utf-8?B?RGVubmlz?= <Dennis(a)discussions.microsoft.com> wrote in news:72685400-255F-46E9-B79F-0BE0CBBAAE30(a)microsoft.com: > Your comment: Schema design should be as independent of the > database engine as possible, so Access is *not* different in any > way, shape or form. I would recommend *as a starting point* the > same normalized design for any database engine. > > Response: Your comment is self contradictory. Instead of saying > “Schema design should be independent of the database engine” > you stated “Schema design should be as independent of the > database engine as possible.” The “as possible” statement > by definition states that thing will be different between database > engines. Which, throws you whole argument out the door. Only in edge cases, which you don't encounter until implementation time. > Yes, Access is different from DB/400, and Oracle, and D3. It is > very similar, but it is not the same. Well, 99% of the differences between Jet/ACE (Access is not a database) and the mentioned databases is that Jet/ACE is a file-based database engine, and the ones I recognize (D3 is unknown to me) are server-based databases. > From what I've read Access had not > implemented the entire SQL language. There is no such thing as "entire SQL language." There are only SQL standards, e.g., SQL 89, SQL 92, etc., and each establishes a baseline meaning for particular SQL commands. No database engine is required to implement 100% of a SQL spec, though from reading the fanboy arguments about which db engine is better you'd think that was the only thing that mattered. Jet/ACE being a relatively old SQL implementation and predating the first major SQL standard (SQL 89) is rather idiosyncratic in its SQL implementation, and has a lot of legacy aspects (some of which would be quite nice if they were implemented in other SQL dialects, e.g., DISTINCTROW). Jet/ACE SQL is also different in that it tends not to support DDL as well as most other SQL dialects, but I would argue that this is because Jet/ACE very early on got a very fine high-level database interface, i.e., DAO, that was closely tied to Jet/ACE and allowed full control of the database engine without needing to write DDL SQL. There are still many things that you can do in DAO that you can't do in Jet/ACE DDL, and not just things that are Jet/ACE-specific. > Also, Access does not support blobs > very well where Oracle does. Oh, come on. Who cares? Secondly, Oracle didn't always support BLOB data well -- it's something that has been added into the engine as its users needed it. Jet/ACE's BLOB support has not been as important to its users so it doesn't perhaps support the same level of features as Oracle's implementation. But really, there about a million things that Oracle offers that Jet/ACE lacks, but that's because the purposes of the two database engines are completely different. > From what I've read, it is highly recommended > that we not store blobs in Access database. Yes. And I think it's mostly advisable in other database engines, too. > Rather, we store the path and > file name to the blob and let DOS/Windows store the binary file in > the specified directory. I think you're actually confusing the advice regarding a specific type of BLOB field, i.e., OLE fields, and BLOB fields in general. OLE fields have a lot of overhead that makes them problematic, and BLOB fields lack that overhead (because they are just a bucket in which to store binary data). OLE fields are easy to use because of the wrapper, but are inefficient because of it. BLOB fields are more efficient but harder to use. And Oracle doesn't support OLE fields at all. This is because it has a different intended user than Jet/ACE, and that's as it should be. > From what I've read Oracle has no problems storing > blobs in their records.. Nor does Jet/ACE. > I don't know if DB/400 stores blobs. I know D3 > does not store blobs, but jBase might. I know D3 support > multi-valued list and I think Oracle does also, which are very > useful in exploding part on a fully assembled unit (ie car door). > Access does not support multi-valued list. So much for “Access > *not* different in any way, shape, or form”. You don't seem to be paying close attention, as multi-vield fields were added in Access 2007 in the ACE in ACCDB format. This was done for Sharepoint comatibility, not because multi-value fields are a good idea -- they most categorically are *not* a good idea, in fact, and support for them is not an indication of a good database engine. > Even as a newbie, I knew that statement was false. You're arguing against something I never asserted. You seem to have some sort of reading comprehension problem, as it is patently obvious that when I said this: Schema design should be as independent of the database engine as possible, so Access is *not* different in any way, shape or form. ....the second phrase applies to the first. That is, in regard to the ideal independence of schema design and database engine capabilities, Access is not different from other databases. It is entirely perverse to divorce the second clause from the first and insist that I was saying there were no differences of any kind between Access/Jet/ACE and other databases. No reasonable person would count that as a valid or good-faith interpretation of what was written. > Even a prima facia > examination of that statement indicates it is false. Are you > saying Access is not different in any way, shape of from DB/400, > Oracle, My SQL or SQL server? No, I didn't say that. And you know perfectly well that I didn't, unless you are blazingly stupid. > I give you one difference. .... Who cares? You're off on a crazy tangent that has nothing to do with what I wrote. I didn't make any assertion that all databases were alike in all respects, and you know I didn't make that assertion. Yet, took several paragraphs to provide evidence to contradict something that was never asserted. I deleted it. I didn't read it. I didn't need to, as it doesn't have anything to do with what I actually assered. > Each database engine has different capabilities, enhancements, > different levels of SQL implementation, and limitations than the > next. What it appears that you are saying is we should design our > schema to the lowest common denominator and ignore any additional > capability offered by the particular database. No, I'm not saying that at all. I'm saying that schema design is a logical operation, not a physical one. You model the entities according to their characteristics and logical relationships and then you design tables that implement those relationships and that can store the characteristics and do all of that in a way that maximizes data integrity and consistency. Minor details may differ (you may choose different data types in different engines to store a particular attribute), but the logical structure and relationships is determined not by the database engine, but by the characteristics and structure of the real-world entities being modelled for the purpose of the application at hand. The last statement has a couple of corollaries: 1. the same entity may be modelled differently in different applications because the purposes of the applications and role of the entity in each application may be different. 2. two different structures can be logically equivalent, but in terms of database performance one or the other may be more efficient because of the real-world realities of the way database engines store and retrieve data. In some cases, the particular features of a particular database engine may make a difference as to which structure you choose. For instance, if you are putting business rules in the database schema, an engine that supports triggers is likely to be a better choice, and this will have an impact on how you implement the structure. Too often people think of data normalization as a process of taking a flat table and breaking it into constituent parts. But that gets things backwards, because you're starting from the denormalized structure and working backwards to the normalized structure. In my opinion, you should begin with the normalized structure and only denormalize when the application and/or database engine forces it on you. The design of an application is often going to force you to denormalize some things for practical reasons (usually performance), but that doesn't mean you start from the denormalized structure (that would be a form of premature optimization). > As soon as you > move away from this position, you then have to design different > schema for different engines. Different in details, but usually not in terms of entity definitions and logical structure. That is, you might have different data types for certain fields, but the tables and their relationships will be more or less the same. Now, I already outlined above a case where this might not be the case, and that's the one where business rules are incorporated into the schema design, which very often necessitates the use of triggers. In that case, your design will likely be very different structurally for a database that lacks triggers as opposed to one that supports them. Of course, modern practices deprecate the whole idea of incorporating business rules at the engine level. The usual modern preference is to put those in a layer between the application and the database so that the business rules can be altered independent of both the database structure and the application implementation. Historically this has mostly not been possible with Access, as it's designed around direct communcation between Access/Jet/ACE and the database. This changes in A2010 with Sharepoint integration, because Sharepoint segregates certain things into is own separate layers so that things you'd normally put in the application or in the database end up in the layer in between. > Granted, those changes might be > slight. But as soon you design something different for the > different engines, you violated your statement that Access is not > different. This is not a statement that I made, so I really see no reason that I should have to defend it. -- David W. Fenton http://www.dfenton.com/ usenet at dfenton dot com http://www.dfenton.com/DFA/
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