On Wed, Oct 12, 2011 at 12:35 AM, Tom Lane
<tgl@sss.pgh.pa.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.