Make entirely-dummy appendrels get marked as such in set_append_rel_size.

The planner generally expects that the estimated rowcount of any relation
is at least one row, *unless* it has been proven empty by constraint
exclusion or similar mechanisms, which is marked by installing a dummy path
as the rel's cheapest path (cf. IS_DUMMY_REL).  When I split up
allpaths.c's processing of base rels into separate set_base_rel_sizes and
set_base_rel_pathlists steps, the intention was that dummy rels would get
marked as such during the "set size" step; this is what justifies an Assert
in indxpath.c's get_loop_count that other relations should either be dummy
or have positive rowcount.  Unfortunately I didn't get that quite right
for append relations: if all the child rels have been proven empty then
set_append_rel_size would come up with a rowcount of zero, which is
correct, but it didn't then do set_dummy_rel_pathlist.  (We would have
ended up with the right state after set_append_rel_pathlist, but that's
too late, if we generate indexpaths for some other rel first.)

In addition to fixing the actual bug, I installed an Assert enforcing this
convention in set_rel_size; that then allows simplification of a couple
of now-redundant tests for zero rowcount in set_append_rel_size.

Also, to cover the possibility that third-party FDWs have been careless
about not returning a zero rowcount estimate, apply clamp_row_est to
whatever an FDW comes up with as the rows estimate.

Per report from Andreas Seltenreich.  Back-patch to 9.2.  Earlier branches
did not have the separation between set_base_rel_sizes and
set_base_rel_pathlists steps, so there was no intermediate state where an
appendrel would have had inconsistent rowcount and pathlist.  It's possible
that adding the Assert to set_rel_size would be a good idea in older
branches too; but since they're not under development any more, it's likely
not worth the trouble.

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 52722bf4cd3965b8897ceceaefa0134cff512ee4..149e2babbfa1e170493d9613edbc3123d5866234 100644 (file)
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -284,6 +284,11 @@ set_rel_size(PlannerInfo *root, RelOptInfo *rel,
                break;
        }
    }
+
+   /*
+    * We insist that all non-dummy rels have a nonzero rowcount estimate.
+    */
+   Assert(rel->rows > 0 || IS_DUMMY_REL(rel));
 }
 
 /*
@@ -407,6 +412,9 @@ set_foreign_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 
    /* Let FDW adjust the size estimates, if it can */
    rel->fdwroutine->GetForeignRelSize(root, rel, rte->relid);
+
+   /* ... but do not let it set the rows estimate to zero */
+   rel->rows = clamp_row_est(rel->rows);
 }
 
 /*
@@ -439,6 +447,7 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
                    Index rti, RangeTblEntry *rte)
 {
    int         parentRTindex = rti;
+   bool        has_live_children;
    double      parent_rows;
    double      parent_size;
    double     *parent_attrsizes;
@@ -459,6 +468,7 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
     * Note: if you consider changing this logic, beware that child rels could
     * have zero rows and/or width, if they were excluded by constraints.
     */
+   has_live_children = false;
    parent_rows = 0;
    parent_size = 0;
    nattrs = rel->max_attr - rel->min_attr + 1;
@@ -586,69 +596,79 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
        if (IS_DUMMY_REL(childrel))
            continue;
 
+       /* We have at least one live child. */
+       has_live_children = true;
+
        /*
         * Accumulate size information from each live child.
         */
-       if (childrel->rows > 0)
+       Assert(childrel->rows > 0);
+
+       parent_rows += childrel->rows;
+       parent_size += childrel->width * childrel->rows;
+
+       /*
+        * Accumulate per-column estimates too.  We need not do anything for
+        * PlaceHolderVars in the parent list.  If child expression isn't a
+        * Var, or we didn't record a width estimate for it, we have to fall
+        * back on a datatype-based estimate.
+        *
+        * By construction, child's reltargetlist is 1-to-1 with parent's.
+        */
+       forboth(parentvars, rel->reltargetlist,
+               childvars, childrel->reltargetlist)
        {
-           parent_rows += childrel->rows;
-           parent_size += childrel->width * childrel->rows;
+           Var        *parentvar = (Var *) lfirst(parentvars);
+           Node       *childvar = (Node *) lfirst(childvars);
 
-           /*
-            * Accumulate per-column estimates too.  We need not do anything
-            * for PlaceHolderVars in the parent list.  If child expression
-            * isn't a Var, or we didn't record a width estimate for it, we
-            * have to fall back on a datatype-based estimate.
-            *
-            * By construction, child's reltargetlist is 1-to-1 with parent's.
-            */
-           forboth(parentvars, rel->reltargetlist,
-                   childvars, childrel->reltargetlist)
+           if (IsA(parentvar, Var))
            {
-               Var        *parentvar = (Var *) lfirst(parentvars);
-               Node       *childvar = (Node *) lfirst(childvars);
+               int         pndx = parentvar->varattno - rel->min_attr;
+               int32       child_width = 0;
 
-               if (IsA(parentvar, Var))
+               if (IsA(childvar, Var))
                {
-                   int         pndx = parentvar->varattno - rel->min_attr;
-                   int32       child_width = 0;
+                   int         cndx = ((Var *) childvar)->varattno - childrel->min_attr;
 
-                   if (IsA(childvar, Var))
-                   {
-                       int         cndx = ((Var *) childvar)->varattno - childrel->min_attr;
-
-                       child_width = childrel->attr_widths[cndx];
-                   }
-                   if (child_width <= 0)
-                       child_width = get_typavgwidth(exprType(childvar),
-                                                     exprTypmod(childvar));
-                   Assert(child_width > 0);
-                   parent_attrsizes[pndx] += child_width * childrel->rows;
+                   child_width = childrel->attr_widths[cndx];
                }
+               if (child_width <= 0)
+                   child_width = get_typavgwidth(exprType(childvar),
+                                                 exprTypmod(childvar));
+               Assert(child_width > 0);
+               parent_attrsizes[pndx] += child_width * childrel->rows;
            }
        }
    }
 
-   /*
-    * Save the finished size estimates.
-    */
-   rel->rows = parent_rows;
-   if (parent_rows > 0)
+   if (has_live_children)
    {
+       /*
+        * Save the finished size estimates.
+        */
        int         i;
 
+       Assert(parent_rows > 0);
+       rel->rows = parent_rows;
        rel->width = rint(parent_size / parent_rows);
        for (i = 0; i < nattrs; i++)
            rel->attr_widths[i] = rint(parent_attrsizes[i] / parent_rows);
+
+       /*
+        * Set "raw tuples" count equal to "rows" for the appendrel; needed
+        * because some places assume rel->tuples is valid for any baserel.
+        */
+       rel->tuples = parent_rows;
    }
    else
-       rel->width = 0;         /* attr_widths should be zero already */
-
-   /*
-    * Set "raw tuples" count equal to "rows" for the appendrel; needed
-    * because some places assume rel->tuples is valid for any baserel.
-    */
-   rel->tuples = parent_rows;
+   {
+       /*
+        * All children were excluded by constraints, so mark the whole
+        * appendrel dummy.  We must do this in this phase so that the rel's
+        * dummy-ness is visible when we generate paths for other rels.
+        */
+       set_dummy_rel_pathlist(rel);
+   }
 
    pfree(parent_attrsizes);
 }

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 97a6a6101530ea51540fa2e018ab8a6122e3dfd3..84fffe306572c5b8ba41f29c7c8934c2d3cec430 100644 (file)
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -2164,6 +2164,26 @@ select count(*) from tenk1 x where
 (1 row)
 
 rollback;
+--
+-- regression test: be sure we cope with proven-dummy append rels
+--
+explain (costs off)
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+        QUERY PLAN        
+--------------------------
+ Result
+   One-Time Filter: false
+(2 rows)
+
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+ aa | bb | unique1 | unique1 
+----+----+---------+---------
+(0 rows)
+
 --
 -- Clean up
 --

diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index fb6981f54d3c4cf9ee459e4c3bd47e82c0b75531..cfccb13ba8535ea21cff79c412fad2ae45cf4857 100644 (file)
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -353,6 +353,17 @@ select count(*) from tenk1 x where
   x.unique1 in (select aa.f1 from int4_tbl aa,float8_tbl bb where aa.f1=bb.f1);
 rollback;
 
+--
+-- regression test: be sure we cope with proven-dummy append rels
+--
+explain (costs off)
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
 
 --
 -- Clean up