Re: subplan resets wrong hashtable

On Sun, Feb 09, 2020 at 08:01:26PM -0800, Andres Freund wrote:
> Ugh, that indeed looks wrong. Did you check whether it can actively
> cause wrong query results? If so, did you do theoretically, or got to a
> query returning wrong results?

Actually .. I can "theoretically" prove that there's no wrong results from that
patch...since in that file it has no effect, the tested variables being zeroed
few lines earlier:

@@ -499,51 +499,60 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
* node->hashtable = NULL;
* node->hashnulls = NULL;
node->havehashrows = false;
node->havenullrows = false;

nbuckets = (long) Min(planstate->plan->plan_rows, (double) LONG_MAX);
if (nbuckets < 1)
nbuckets = 1;

- node->hashtable = BuildTupleHashTable(node->parent,
- node->descRight,
- ncols,
- node->keyColIdx,
- node->tab_eq_funcoids,
- node->tab_hash_funcs,
- nbuckets,
- 0,
- node->hashtablecxt,
- node->hashtempcxt,
- false);
*+ if (node->hashtable)
+ ResetTupleHashTable(node->hashtable);
+ else
+ node->hashtable = BuildTupleHashTableExt(node->parent,

...
*+ if (node->hashnulls)
+ ResetTupleHashTable(node->hashtable);
+ else
+ node->hashnulls = BuildTupleHashTableExt(node->parent,
+ node->descRight,