Fix pfree issue in presorted DISTINCT aggregate code

The logic in this area was recently changed in 7da51590e, however, in that
commit, I neglected to consider that the conditions in which we should
pfree the old Datum needed to be updated after that change.  This could
result in trying to pfree a NULL value, as was demonstrated by Alexander
Lakhin.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/4103db46-d888-6d1d-e88d-87c21ed99472@gmail.com

diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index 827c65cc852a2d300baa22b516c2d71abb53fd9e..19351fe34bf44f9599e2b23528893aa913ea192d 100644 (file)
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -4254,7 +4254,8 @@ ExecEvalPreOrderedDistinctSingle(AggState *aggstate, AggStatePerTrans pertrans)
                                                                                                        pertrans->aggCollation,
                                                                                                        pertrans->lastdatum, value))))
        {
-               if (pertrans->haslast && !pertrans->inputtypeByVal)
+               if (pertrans->haslast && !pertrans->inputtypeByVal &&
+                       !pertrans->lastisnull)
                        pfree(DatumGetPointer(pertrans->lastdatum));
 
                pertrans->haslast = true;