As ermouth said the best approach is to use the explain feature of
Fauxton / Photon.
Couch tries to guess what's the best index to use for any case so you
don't necessarily need to build complex indexes, if you ask for more
selectors than the indexed ones usually it picks an index and then scan
the documents to apply the subselectors.
Yet I must admit I feel the logic behind it a bit entropic and
cumbersome.
Just remember to ask the selector in the same order as they are indexed,
even for partial indexes.
Like a select for { fieldA, fieldB, fieldC } and an index for { fieldA,
fieldB } would work as you expect, but if the index was on { fieldA,
fieldC } it won't get used.
On the contrary I don't think Couch is (yet?) able to to Union between
indexes, so if you have index { fieldA } index { fieldB } and ask for
select { fieldA, fieldB } only index { fieldA } will be used and then
all the documents will be processed to find the subset.
That's my empiric deduction from some usage, but someone with the hands
on the code can probably correct me.
---
Andrea Brancatelli
On 2020-06-09 13:21, Piotr Zarzycki wrote:
> Hi Aurélien,
>
> It is not actually about any specific case. My questions are just helpers
> to truly understand what approach we should take in our database. I just
> have hope that I will get more understanding to get answer to some cases.
>
> Answer to your questions inline.
>
> wt., 9 cze 2020 o 13:04 Aurélien Bénel <[email protected]> napisał(a):
>
> Hi Piotr,
>
> Let's say that I have created two indexes for fields "docType" and "age":
> (...)
> My question is - How CouchDb will behave in case when I'm searching my db
> using both fields when there are two indexes which contains the same
> fields?
> To answer your question (at least with js views, I don't use Mango but I
> suppose it to be similar),
> we need more details:
>
> - Are both fields variable parameters ? Or is one of them a constant?
I think both cases I would consider. However the first one would be that
I'm searching using both fields as a variable parameters. I'm not sure
about constant question. I think non of those fields are constant - I
mean
from the database point of view docType won't change in the document
that's
for sure.
> - Will both parameters be always set ? If not, which one might be unset?
>
> Let's assume that it always will, but like I said what if I'm looking only
and searching/querying using docType only ?
> Regards,
>
> Aurélien
