I've started work on a patch that allows to combine covering and unique functionality.
Great to hear someone is working on this!
Next issue is pg_index changes. Now there's only a boolean flag
bool indisunique; /* is this a unique index? */
But new algorithm requires to store a single number
unit16 n_unique_columns; /* number of first columns of index which has unique constrains. */
I think, that numbers of all attributes themselves are not needed. Is it right?
I think the total number of attributes is already in indnatts.
I imagine you're planning to keep the indexed columns at the start of the indkey[] array, and just use n_unique_columns to mark how many of the indkey[] attributes to check as indexed columns. I'd imagine the change would be fairly simple from a planner point of view as you'd just need to check columns 1 to n_unique_columns instead of 1 to indnatts. Although I have a tendency to under estimate these things :(
I imagine you don't want to name the new column n_unique_columns, since it does not fit too well with non-unique indexes.
Perhaps just indindexedatts, or something slightly along those lines. But perhaps it would be a good idea to also rename "ncolumns" in code, to ensure that any non-updated code does not even compile. Perhaps including "tot" or "total" in there might help indicate it's new meaning.
