Currently amgettuple returns one matching tuple at a time, in index order. I'm proposing two changes to add support for
- candidate matches
- partial ordering

Those two features are essential to make clustered indexes work, and in the future, binned bitmap indexes that don't have a bitmap for each distinct value but for ranges of values.

There's a third feature looming in the future, that I haven't addressed:
- returning index values, for index-only scans or at least for filtering rows before fetching heap tuples.

I'm proposing that we keep the one tuple per call nature of the interface, but add a flag to mark candidate matches. index_getnext or the executor would need to recheck the original quals for tuples marked as candidates.

Another flag would be used to mean "this tuple marks the boundary of a partial ordering". Let's call it boundary_mark for now.

For example, if an index scan returned tuples with the following keys, with tuples on same line meaning the index doesn't know their relative ordering.

3 4 2
5 8 6 7

amgettuple would return the above tuples like this:

1 3 4 2 5 8 6 7 9 10
* *     *       * *

where the tuples marked with * would have the boundary_mark-flag set. If the plan requires ordered results, index_getnext would have to sort tuples between two markers before returning them to the caller.

amgetmulti would also need to have the candidate-flag added as I proposed in the "Bitmapindexscan changes" patch I sent earlier to pgsql-patches.

This interface change would solve much of the ugliness of my GIT patch, by generalizing the index quals checking and sorting code to index_getnext.

Another source of ugliness in the patch is in inserting new tuples. Inserts need to reach to the heap to fetch heap tuples, to compare keys when splitting a group. I don't see a clean fix for that, but I don't think it's as bad as the index scan code.

  Heikki Linnakangas

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