Make NULL tuple values always advance skip arrays.

_bt_check_compare neglected to handle a case that can arise when the
scan's keys are temporarily treated as nonrequired, as an optimization:
whenever a NULL tuple value was encountered that had a skip array whose
current element wasn't already NULL, _bt_check_compare failed to advance
the array to the NULL element.  This allowed _bt_check_compare to fail
to return matching tuples containing a NULL value (though only with an
array column that came before a skip array column with NULLs, and only
during _bt_readpage calls that set pstate.forcenonrequired=true on a
page where the higher-order column also had to advance).

To fix, teach _bt_check_compare to handle this case just like any other
case where a skip array key is unsatisfied and must be advanced directly
(due to the key being considered a nonrequired key).

Oversight in commit 8a510275, which optimized nbtree search scan key
comparisons with skip arrays.

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/CAHgHdKtLFWZcjr87hMH0hYDHgcifu4Tj7iHz-xh8qsJREt5cqA@mail.gmail.com

diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c
index 8b0257961276ac5f4a34c32a6b0b6647ea3f9b3d..c899168c99563e9c5d4889214ad18918a29d1b78 100644 (file)
--- a/src/backend/access/nbtree/nbtutils.c
+++ b/src/backend/access/nbtree/nbtutils.c
@@ -2920,6 +2920,17 @@ _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
 
                if (isNull)
                {
+                       /*
+                        * Scalar scan key isn't satisfied by NULL tuple value.
+                        *
+                        * If we're treating scan keys as nonrequired, and key is for a
+                        * skip array, then we must attempt to advance the array to NULL
+                        * (if we're successful then the tuple might match the qual).
+                        */
+                       if (unlikely(forcenonrequired && key->sk_flags & SK_BT_SKIP))
+                               return _bt_advance_array_keys(scan, NULL, tuple, tupnatts,
+                                                                                         tupdesc, *ikey, false);
+
                        if (key->sk_flags & SK_BT_NULLS_FIRST)
                        {
                                /*
@@ -2958,7 +2969,7 @@ _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
                        }
 
                        /*
-                        * In any case, this indextuple doesn't match the qual.
+                        * This indextuple doesn't match the qual.
                         */
                        return false;
                }