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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Best Wishes,
Chris Travers
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