Re: [HACKERS] Removing [Merge]Append nodes which contain a single subpath

On Mon, Dec 11, 2017 at 3:16 PM, David Rowley
<david.rowley@2ndquadrant.com> wrote:
>
> Sometime in the future, I'd like to see some sort of UniqueKeys List
> in RelOptInfo which is initially populated by using looking at the
> unique indexes during build_simple_rel(). The idea is that these
> UniqueKeys would get propagated into RELOPT_JOINRELs when the join
> condition matches a set of unique keys on the other side of the join.
> e.g. If the outer side of the join had a UniqueKey matching the join
> condition then we could take all of the UniqueKeys from the RelOptInfo
> for the inner side of the join (and vice versa). This infrastructure
> would allow us to no-op a DISTINCT when the RelOptInfo has targetlist
> items which completely cover at least one UniqueKey set, or not bother
> performing a sort for a GROUP BY when the GROUP BY clause is covered
> by a UniqueKey.
>

That looks neat.

> To fix the case we're discussing, we can also propagate those
> UniqueKeys to an AppendPath with a single subpath similar to how I'm
> propagating the PathKeys for the same case in this patch, that way the
> unique join code would find the UniqueKeys instead of what it does
> today and not find any unique indexes on the partitioned table.
>

Another possibility is to support partition-wise join between an
unpartitioned table and a partitioned table by joining the
unpartitioned table with each partition separately and appending the
results. That's a lot of work and quite some new infrastructure. Each
of those join will pick unique key if appropriate index exists on the
partitions.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company