Re: Index Skip Scan

On Wed, Jul 03, 2019 at 12:27:09AM +1200, David Rowley wrote:
> On Tue, 2 Jul 2019 at 21:00, Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> The more I think about these UniqueKeys, the more I think they need to
> be a separate concept to PathKeys. For example, UniqueKeys: { x, y }
> should be equivalent to { y, x }, but with PathKeys, that's not the
> case, since the order of each key matters. UniqueKeys equivalent
> version of pathkeys_contained_in() would not care about the order of
> individual keys, it would say things like, { a, b, c } is contained in
> { b, a }, since if the path is unique on columns { b, a } then it must
> also be unique on { a, b, c }.

Is that actually true, though? I can see unique {a, b, c} => unique
{a, b}, but for example:

a | b | c
--|---|--
1 | 2 | 3
1 | 2 | 4
1 | 2 | 5

is unique on {a, b, c} but not on {a, b}, at least as I understand the
way "unique" is used here, which is 3 distinct {a, b, c}, but only one
{a, b}.

Or I could be missing something obvious, and in that case, please
ignore.

Best,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

Remember to vote!
Consider donating to Postgres: http://www.postgresql.org/about/donate