Tweak hash join code to use an additional heuristic for deciding whether

it's worth probing the outer relation for emptiness before building the
hash table.  To wit, if we're rescanning a join previously performed,
remember whether we found it nonempty the previous time, and don't bother
with the probe if it was nonempty.  This buys back the performance lost
in examples like Mario Weilguni's.

diff --git a/src/backend/executor/nodeHashjoin.c b/src/backend/executor/nodeHashjoin.c
index 01c67e8c43580889b716839d413d610f5ec43a9c..2dc9d6e91cd66eef6bbf3bb72c7fcfbe8833a5ce 100644 (file)
--- a/src/backend/executor/nodeHashjoin.c
+++ b/src/backend/executor/nodeHashjoin.c
@@ -120,16 +120,28 @@ ExecHashJoin(HashJoinState *node)
                 * since we aren't going to be able to skip the join on the strength
                 * of an empty inner relation anyway.)
                 *
+                * If we are rescanning the join, we make use of information gained
+                * on the previous scan: don't bother to try the prefetch if the
+                * previous scan found the outer relation nonempty.  This is not
+                * 100% reliable since with new parameters the outer relation might
+                * yield different results, but it's a good heuristic.
+                *
                 * The only way to make the check is to try to fetch a tuple from the
                 * outer plan node.  If we succeed, we have to stash it away for later
                 * consumption by ExecHashJoinOuterGetTuple.
                 */
-               if (outerNode->plan->startup_cost < hashNode->ps.plan->total_cost ||
-                       node->js.jointype == JOIN_LEFT)
+               if (node->js.jointype == JOIN_LEFT ||
+                       (outerNode->plan->startup_cost < hashNode->ps.plan->total_cost &&
+                        !node->hj_OuterNotEmpty))
                {
                        node->hj_FirstOuterTupleSlot = ExecProcNode(outerNode);
                        if (TupIsNull(node->hj_FirstOuterTupleSlot))
+                       {
+                               node->hj_OuterNotEmpty = false;
                                return NULL;
+                       }
+                       else
+                               node->hj_OuterNotEmpty = true;
                }
                else
                        node->hj_FirstOuterTupleSlot = NULL;
@@ -159,6 +171,13 @@ ExecHashJoin(HashJoinState *node)
                 * scanning the outer relation
                 */
                hashtable->nbatch_outstart = hashtable->nbatch;
+
+               /*
+                * Reset OuterNotEmpty for scan.  (It's OK if we fetched a tuple
+                * above, because ExecHashJoinOuterGetTuple will immediately
+                * set it again.)
+                */
+               node->hj_OuterNotEmpty = false;
        }
 
        /*
@@ -454,6 +473,7 @@ ExecInitHashJoin(HashJoin *node, EState *estate)
        hjstate->js.ps.ps_TupFromTlist = false;
        hjstate->hj_NeedNewOuter = true;
        hjstate->hj_MatchedOuter = false;
+       hjstate->hj_OuterNotEmpty = false;
 
        return hjstate;
 }
@@ -546,6 +566,9 @@ ExecHashJoinOuterGetTuple(PlanState *outerNode,
                        *hashvalue = ExecHashGetHashValue(hashtable, econtext,
                                                                                          hjstate->hj_OuterHashKeys);
 
+                       /* remember outer relation is not empty for possible rescan */
+                       hjstate->hj_OuterNotEmpty = true;
+
                        return slot;
                }
 
@@ -810,7 +833,19 @@ ExecReScanHashJoin(HashJoinState *node, ExprContext *exprCtxt)
                if (node->hj_HashTable->nbatch == 1 &&
                        ((PlanState *) node)->righttree->chgParam == NULL)
                {
-                       /* okay to reuse the hash table; needn't rescan inner, either */
+                       /*
+                        * okay to reuse the hash table; needn't rescan inner, either.
+                        *
+                        * What we do need to do is reset our state about the emptiness
+                        * of the outer relation, so that the new scan of the outer will
+                        * update it correctly if it turns out to be empty this time.
+                        * (There's no harm in clearing it now because ExecHashJoin won't
+                        * need the info.  In the other cases, where the hash table
+                        * doesn't exist or we are destroying it, we leave this state
+                        * alone because ExecHashJoin will need it the first time
+                        * through.)
+                        */
+                       node->hj_OuterNotEmpty = false;
                }
                else
                {

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 0d4397f96488d4552ef75e743b9c7cbbb1c37d95..3fe2886107e7b583864dc809aedb806d990bbd55 100644 (file)
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1101,6 +1101,7 @@ typedef struct MergeJoinState
  *             hj_FirstOuterTupleSlot  first tuple retrieved from outer plan
  *             hj_NeedNewOuter                 true if need new outer tuple on next call
  *             hj_MatchedOuter                 true if found a join match for current outer
+ *             hj_OuterNotEmpty                true if outer relation known not empty
  * ----------------
  */
 
@@ -1125,6 +1126,7 @@ typedef struct HashJoinState
        TupleTableSlot *hj_FirstOuterTupleSlot;
        bool            hj_NeedNewOuter;
        bool            hj_MatchedOuter;
+       bool            hj_OuterNotEmpty;
 } HashJoinState;