Add error check for lossy distance functions in index-only scans.

Maybe we should actually support this, but for the moment let's just
throw an error if the opclass tries it.

diff --git a/src/backend/executor/nodeIndexonlyscan.c b/src/backend/executor/nodeIndexonlyscan.c
index 61bd644ab7135c446e3b467c455f415a917f5cde..976c77b76c653108791ba469d38b12504bf37fe7 100644 (file)
--- a/src/backend/executor/nodeIndexonlyscan.c
+++ b/src/backend/executor/nodeIndexonlyscan.c
@@ -165,6 +165,19 @@ IndexOnlyNext(IndexOnlyScanState *node)
                        }
                }
 
+               /*
+                * We don't currently support rechecking ORDER BY distances.  (In
+                * principle, if the index can support retrieval of the originally
+                * indexed value, it should be able to produce an exact distance
+                * calculation too.  So it's not clear that adding code here for
+                * recheck/re-sort would be worth the trouble.  But we should at least
+                * throw an error if someone tries it.)
+                */
+               if (scandesc->numberOfOrderBys > 0 && scandesc->xs_recheckorderby)
+                       ereport(ERROR,
+                                       (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+                                        errmsg("lossy distance functions are not supported in index-only scans")));
+
                /*
                 * Predicate locks for index-only scans must be acquired at the page
                 * level when the heap is not accessed, since tuple-level predicate