Re: When to store data that could be derived

On 3/24/19 3:05 AM, Frank wrote:
>
>
> On 2019-03-24 9:25 AM, Ron wrote:
>> On 3/24/19 1:42 AM, Frank wrote:
>>> Hi all
>>>
>>> As I understand it, a  general rule of thumb is that you should never 
>>> create a physical column if the data could be derived from existing 
>>> columns. A possible reason for breaking this rule is for performance 
>>> reasons.
>>>
>>> I have a situation where I am considering breaking the rule, but I am 
>>> not experienced enough in SQL to know if my reason is valid. I would 
>>> appreciate it if someone could glance at my 'before' and 'after' 
>>> scenarios and see if, from a 'gut-feel' point of view, I should proceed.
>>>
>
> [snip]
>
>>
>> Sure the second query joins a lot of tables, but is pretty straightforward.
>>
>> What REALLY worries me is whether or not the query optimiser would look 
>> at the WHERE CASE, run away screaming and then make it use sequential 
>> scans. Thus, even query #1 would be slow.
>>
>
> I had not realised that. I hope someone else chimes in on this.

In every DBMS that I've used, the lside (left side) needs to be static (not 
"a" static) instead of variable (like a function).

For example, this always leads to a sequential scan:
    WHERE EXTRACT(DAY FROM DATE_FIELD) = 5

>
>>
>> Is this a historical data set that's never updated, or current data 
>> that's constantly added to?
>>
>
> It is the latter - current data constantly added to.
>
> Frank
>

-- 
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