On Fri, Aug 11, 2017 at 9:08 PM, Robert Haas <robertmh...@gmail.com> wrote:
>
> rhaas=# create table parent (a int) partition by list (a);
> CREATE TABLE
> rhaas=# create temp table child partition of parent for values in (1);
> CREATE TABLE
> rhaas=# explain verbose select * from parent;
> QUERY PLAN
> -------------------------------------------------------------------------
> Append (cost=0.00..35.50 rows=2550 width=4)
> -> Seq Scan on pg_temp_3.child (cost=0.00..35.50 rows=2550 width=4)
> Output: child.a
> (3 rows)
>
> But the comments say:
>
> * A childless table is never considered to be an inheritance set; therefore
> * a parent RTE must always have at least two associated AppendRelInfos.
>
> Yet, not. So at least the comments need to be updated;
A partitioned table with at least a single partition is not childless.
So, above comment doesn't apply here. In a regular inheritance there
will be at least two AppendRelInfos one representing a scan on the
parent table and others representing the scan on children. The comment
needs to be distinguish between regular inheritance and partitioned
inheritance. I have modified the comments that way.
> not sure if we
> want to try to eliminate the Append node in this case also.
>
The rest of the query tree and plan tree, expects parent's targetlist
which may be different from that of the child. A notable difference is
whole row expressions getting translated using ConvertRowExpr, which
are not expected to pop up in a scan's targetlist. So we can't
eliminate the Append node.
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index cf46b74..23c66f2 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -1360,8 +1360,12 @@ expand_inherited_tables(PlannerInfo *root)
* table, but with inh = false, to represent the parent table in its role
* as a simple member of the inheritance set.
*
- * A childless table is never considered to be an inheritance set; therefore
- * a parent RTE must always have at least two associated AppendRelInfos.
+ * A childless table is never considered to be an inheritance set. In case of
+ * regular inheritance a parent RTE must always have at least two associated
+ * AppendRelInfos: one corresponding to the parent table as a simple member of
+ * inheritance set and one or more corresponding the actual children. We do not
+ * scan a partitioned table, and hence do not create AppendRelInfo for it.
+ * Hence a partitioned table must have at least one asscociated AppendRelInfo.
*/
static void
expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
@@ -1374,7 +1378,7 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
List *inhOIDs;
List *appinfos;
ListCell *l;
- bool need_append;
+ bool has_child;
PartitionedChildRelInfo *pcinfo;
List *partitioned_child_rels = NIL;
@@ -1448,7 +1452,7 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
/* Scan the inheritance set and expand it */
appinfos = NIL;
- need_append = false;
+ has_child = false;
foreach(l, inhOIDs)
{
Oid childOID = lfirst_oid(l);
@@ -1502,7 +1506,10 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
*/
if (childrte->relkind != RELKIND_PARTITIONED_TABLE)
{
- need_append = true;
+ /* Remember if we saw a real child. */
+ if (childOID != parentOID)
+ has_child = true;
+
appinfo = makeNode(AppendRelInfo);
appinfo->parent_relid = rti;
appinfo->child_relid = childRTindex;
@@ -1582,7 +1589,7 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
* the parent table is harmless, so we don't bother to get rid of it;
* ditto for the useless PlanRowMark node.
*/
- if (!need_append)
+ if (!has_child)
{
/* Clear flag before returning */
rte->inh = false;
--
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