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Avoid unnecessary single-child Append nodes. 

Before commit d3cc37f1d8, an inheritance parent
whose only children were temp tables of other sessions would end up
as a simple scan of the parent; but with that commit, we end up with
an Append node, per a report from Ashutosh Bapat.  Tweak the logic
so that we go back to the old way, and update the function header
comment for partitioning while we're at it.

Ashutosh Bapat, reviewed by Amit Langote and adjusted by me.

Discussion: http://postgr.es/m/CAFjFpReWJr1yTkHU=OqiMBmcYCMoSW3VPR39RBuQ_ovwDFBT5Q@mail.gmail.com
This commit is contained in:
Robert Haas 2017-08-15 09:16:33 -04:00
parent 1295a77788
commit d57929afc7
1 changed files with 13 additions and 6 deletions
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19
src/backend/optimizer/prep/prepunion.c
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@ -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. For
* 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 to the actual children.
* Since a partitioned table is not scanned, it might have only one associated
* 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;