Re: index-only scans

On Wed, Oct 12, 2011 at 12:35 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Hm. I had been supposing that lossless compress functions would just be
> no-ops. If that's not necessarily the case then we might need something
> different from the opclass's decompress function to get back the
> original data. However, that doesn't really solve the problem I'm
> concerned about, because the existence and use of such a function would
> be entirely internal to GiST. There still needs to be a way for the
> planner to know which opclasses support data retrieval. And I do *not*
> want to see us hard-wire "the presence of opclass function 8 means a
> GiST opclass can return data" into the planner.
>
> Maybe, instead of a simple constant amcanreturn column, we need an AM
> API function that says whether the index can return data.
>
I like idea of such AM API function. Since single multicolumn index can use
multiple opclasses, AM API function should also say *what* data index can
return.

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With best regards,
Alexander Korotkov.