Re: surprising query optimisation

Greetings,

* Chris Withers (chris(at)withers(dot)org) wrote:
> On 30/11/2018 15:33, Stephen Frost wrote:
> >* Chris Withers (chris(at)withers(dot)org) wrote:
> >>On 28/11/2018 22:49, Stephen Frost wrote:
> >For this, specifically, it's because you end up with exactly what you
> >have: a large index with tons of duplicate values. Indexes are
> >particularly good when you have high-cardinality fields. Now, if you
> >have a skewed index, where there's one popular value and a few much less
> >popular ones, then that's where you really want a partial index (as I
> >suggest earlier) so that queries against the non-popular value(s) is
> >able to use the index and the index is much smaller.
>
> Interesting! In my head, for some reason, I'd always assumed a btree index
> would break down a char field based on the characters within it. Does that
> never happen?

Not sure what you mean by 'break down a char field'.

> If I changed this to be an enum field, would != still perform poorly or can
> the query optimiser spot that it's an enum and just look for the other
> options?

I don't believe we've got any kind of optimization like that today for
enums.

Thanks!

Stephen