I've been trying to troubleshoot the cause of this PostGIS recheck bug we
have reported by two people so far.  The last test was a nice simple
repeatable one that triggered the issue:


from what I have seen this only affects cases where we are doing a distance
check between two points, which we actually don't need to enable recheck for
anyway, but trying to disable that seems like just shoving the real problem
under the covers.
Where it errors is this line 272 in src/backend/executor/nodeIndexscan


 259              * Was the ORDER BY value returned by the index accurate?
 260              * recheck flag means that the index can return inaccurate
 261              * but then again, the value returned for any particular
 262              * could also be exactly correct.  Compare the value
returned by
 263              * the index with the recalculated value.  (If the value
 264              * by the index happened to be exact right, we can often
 265              * pushing the tuple to the queue, just to pop it back out
 266              */
 267             cmp = cmp_orderbyvals(node->iss_OrderByValues,
 268                                   node->iss_OrderByNulls,
 269                                   scandesc->xs_orderbyvals,
 270                                   scandesc->xs_orderbynulls,
 271                                   node);
 272             if (cmp < 0)
 273                 elog(ERROR, "index returned tuples in wrong order");
 274             else if (cmp == 0)
 275                 was_exact = true;
 276             else
 277                 was_exact = false;

If things are out of order, why isn't just going to was_exact = false good

I'm not sure if the mistake is in our PostGIS code or something in
PostgreSQL recheck logic.
If I change the elog(ERROR ...) to a elog(NOTICE, the answers  are correct
and sort order is right.

Under what conditions would cmp return less than 0?  I tried following the
code in cmp_orderbyvals, but got lost
and trying to put elog notices in to see what the distance is returning (I
probably did it wrong), just ended up crashing by backend.

Thanks for any thoughts,

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