On Tue, 10 Mar 2020 at 08:56, Dmitry Dolgov <9erthalion6@gmail.com> wrote:
> Assuming we'll implement it in a way that we do not know about what kind
> of path type is that in create_distinct_path, then it can also work for
> ProjectionPath or anything else (if UniqueKeys are present). But then
> still EquivalenceMember are used only to figure out correct
> distinctPrefixKeys and do not affect whether or not skipping is applied.
> What do I miss?
I'm not sure I fully understand the question correctly, but let me
explain further.
In the 0001 patch, standard_qp_callback sets the query_uniquekeys
depending on the DISTINCT / GROUP BY clause. When building index
paths in build_index_paths(), the 0002 patch should be looking at the
root->query_uniquekeys to see if it can build any index paths that
suit those keys. Such paths should be tagged with the uniquekeys they
satisfy, basically, exactly the same as how pathkeys work. Many
create_*_path functions will need to be modified to carry forward
their uniquekeys. For example, create_projection_path(),
create_limit_path() don't do anything which would cause the created
path to violate the unique keys. This way when you get down to
create_distinct_paths(), paths other than IndexPath may have
uniquekeys. You'll be able to check which existing paths satisfy the
unique keys required by the DISTINCT / GROUP BY and select those paths
instead of having to create any HashAggregate / Unique paths.
Does that answer the question?