Meant to send this to the list but hit the wrong button.
On Sun, Mar 24, 2019 at 9:45 AM Ron <ronljohnsonjr@xxxxxxxxx> wrote:
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
PostgreSQL allows _expression_ indexes
So you can:
create index foo on bar ((id % 1000));
And then use the index on:
select * from bar where id % 1000 = 45;
You could similarly
create index foo on bar (extract(day from date_field));
The left side needs to be indexed (and an immutable _expression_) but beyond that.....
>
>>
>> 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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