Choice of Datatypes - Accuracy v. space! 1

[Welshbird]

I'm trying to re-design a database from scratch, to be used with OLAP services.<br><br>I've been given conflicting advice from various people as to the datatypes I should use for the measures in the main fact tables. Some say use &quot;float&quot;, some &quot;real&quot;, and some &quot;numeric&quot;. <br><br>No figures will be bigger that 9,999.99 , and on average will be about 4,500.00, and some will be monetory values, whilst others may be integers.<br><br>I suspect that &quot;real&quot; uses half the space of the others, and the table will end up haveing approx. 9,000,000 odd rows. What so you all think? - Cost of diskspace now whould make the space not an issue, but I don't want to be too shortsighted - I think I should be trying to design a database with size in mind (!). Any advice would be greatly appriciated!<br><br><br>Thanks,<br><br>Fi. <p>Fiona Williams<br><a href=mailto:willif@mosspharmacy.co.uk>willif@mosspharmacy.co.uk</a><br><a href= > </a><br>

 

[MichaelRed]

Fiona,<br><br>While space is not the issue, it does affect the access time.&nbsp;&nbsp;Since the system will actually get the records in &quot;chunks&quot; which are really &quot;Allocation Units&quot; on the Disk, the real question is what is the record size w/ respect to the &quot;allocation Unit&quot; size.&nbsp;&nbsp;In some cases, the addition of the 4 bytes per record will have NO effect on the access speed, while other cases will cause the access speed to change by a factor of two.<br><br>Since you probably have NO control of the &quot;Allocation Unit&quot; size for the equipment in the system, the issue is probably only of academic interest.&nbsp;&nbsp;But.&nbsp;&nbsp;Disc &quot;Allocation Units&quot; range from 512 Bytes to ? 8192 ? bytes in seps of 512.&nbsp;&nbsp;Record lengths are generally the sum of the Individual record element sizes plus some overhead (there is a discussion of this in MS Access documentation).&nbsp;&nbsp;If you calculate YOUR record length and divide it into the various possible allocation unit sizes, you will see how many records fit into the allocation unit.&nbsp;&nbsp;If you get the modulo: Alloc Mod RecSize for each Alloc, yuou can see how much wasted space there is for each Alloc size.&nbsp;&nbsp;If you can know or control the allocation unit size, you may set the system up to minimize the &quot;wsted&quot; space, or see if the difference in record length will be efficient.

 

[MalcolmW]

I'd recommend reading this from BOL...<br>Use the decimal data type to store numbers with decimals when the data values must be stored exactly as specified.<br><br>Using float and real Data<br>The float and real data types are known as approximate data types. The behavior of float and real follows the IEEE 754 specification on approximate numeric data types.<br><br>Approximate numeric data types do not store the exact values specified for many numbers; they store an extremely close approximation of the value. For many applications, the tiny difference between the specified value and the stored approximation is not noticeable. At times, though, the difference becomes noticeable. Because of this approximate nature of the float and real data types, do not use these data types when exact numeric behavior is required, such as in financial applications, in operations involving rounding, or in equality checks. Instead, use the integer, decimal, money, or smallmoney data types.<br><br>Avoid using float or real columns in WHERE clause search conditions, especially the = and &lt;&gt; operators. It is best to limit float and real columns to &gt; or &lt; comparisons.<br> <p>Malcolm Wynden<br><a href=mailto:wynden@island.dot.net>wynden@island.dot.net</a><br><a href= > </a><br>

 

[Waynest]

Go for accuracy every time.&nbsp;&nbsp;Servers improve all the time, but your code wont. Your database might still be in use in 10 years time, long after your current server is dead and buried.&nbsp;&nbsp;Trust me, you dont want to go back and rewrite all your code when a rounding error turns up.