Re: Querying distinct values from a large table

["Luke Lonergan" <llonergan(at)greenplum(dot)com>]

Chad,

On 1/30/07 6:13 AM, "Chad Wagner" <chad(dot)wagner(at)gmail(dot)com> wrote:

> Sounds like an opportunity to implement a "Sort Unique" (sort of like a hash,
> I guess), there is no need to push 3M rows through a sort algorithm to only
> shave it down to 1848 unique records.
> 
> I am assuming this optimization just isn't implemented in PostgreSQL?

Not that it helps Igor, but we've implemented single pass sort/unique,
grouping and limit optimizations and it speeds things up to a single seqscan
over the data, from 2-5 times faster than a typical external sort.

I can't think of a way that indexing would help this situation given the
required visibility check of each tuple.

- Luke