Re: Large data and slow queries

On 2017-04-19 13:25, Martijn Tonies (Upscene Productions) wrote:
> Samuel, others,
>
> Perhaps I'm missing something, but I'd be interested in the reasoning
> behind this.
>
> For column 'what', it seems you have no index on all values, only
> indices with specific values for 'what'.
>
> How does this speed up the search? Will PostgreSQL use those indices,
> instead of using a generic index on 'what' and optionally other
> columns?
>
>
> With regards,
>
> Martijn Tonies
> Upscene Productions
> http://www.upscene.com
>

That's a "partial index", it only contains records that meet the
requirements of the index definition.

https://www.postgresql.org/docs/9.5/static/indexes-partial.html

Basically; if you create an index on records where 'name = kees' then if
your query contains "where name=kees"
the planner can just load that index and know that the records in that
index will not contain
any other names, saving the need to filter for 'name=kees'