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Commit4cc832f

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Tidy up code in get_cheapest_group_keys_order()
There are a few things that we could do a little better withinget_cheapest_group_keys_order():1. We should be using list_free() rather than pfree() on a List.2. We should use for_each_from() instead of manually coding a for loop toskip the first n elements of a List3. list_truncate(list_copy(...), n) is not a great way to copy the first nelements of a list. Let's invent list_copy_head() for that. That way wedon't need to copy the entire list just to truncate it directlyafterwards.4. We can simplify finding the cheapest cost by setting the cheapest costvariable to DBL_MAX. That allows us to skip special-casing the initialiteration of the loop.Author: David RowleyDiscussion:https://postgr.es/m/CAApHDvrGyL3ft8waEkncG9y5HDMu5TFFJB1paoTC8zi9YK97Nw@mail.gmail.comBackpatch-through: 15, where get_cheapest_group_keys_order was added.
1 parent83f1c7b commit4cc832f

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3 files changed

+40
-16
lines changed

3 files changed

+40
-16
lines changed

‎src/backend/nodes/list.c‎

Lines changed: 21 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1584,6 +1584,27 @@ list_copy(const List *oldlist)
15841584
returnnewlist;
15851585
}
15861586

1587+
/*
1588+
* Return a shallow copy of the specified list containing only the first 'len'
1589+
* elements. If oldlist is shorter than 'len' then we copy the entire list.
1590+
*/
1591+
List*
1592+
list_copy_head(constList*oldlist,intlen)
1593+
{
1594+
List*newlist;
1595+
1596+
len=Min(oldlist->length,len);
1597+
1598+
if (len <=0)
1599+
returnNIL;
1600+
1601+
newlist=new_list(oldlist->type,len);
1602+
memcpy(newlist->elements,oldlist->elements,len*sizeof(ListCell));
1603+
1604+
check_list_invariants(newlist);
1605+
returnnewlist;
1606+
}
1607+
15871608
/*
15881609
* Return a shallow copy of the specified list, without the first N elements.
15891610
*/

‎src/backend/optimizer/path/pathkeys.c‎

Lines changed: 18 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -17,6 +17,8 @@
1717
*/
1818
#include"postgres.h"
1919

20+
#include<float.h>
21+
2022
#include"miscadmin.h"
2123
#include"access/stratnum.h"
2224
#include"catalog/pg_opfamily.h"
@@ -591,7 +593,7 @@ get_cheapest_group_keys_order(PlannerInfo *root, double nrows,
591593

592594
ListCell*cell;
593595
PathkeyMutatorStatemstate;
594-
doublecheapest_sort_cost=-1.0;
596+
doublecheapest_sort_cost=DBL_MAX;
595597

596598
intnFreeKeys;
597599
intnToPermute;
@@ -620,23 +622,23 @@ get_cheapest_group_keys_order(PlannerInfo *root, double nrows,
620622
nToPermute=4;
621623
if (nFreeKeys>nToPermute)
622624
{
623-
inti;
624625
PathkeySortCost*costs=palloc(sizeof(PathkeySortCost)*nFreeKeys);
626+
PathkeySortCost*cost=costs;
625627

626-
/* skip the pre-ordered pathkeys */
627-
cell=list_nth_cell(*group_pathkeys,n_preordered);
628-
629-
/* estimate cost for sorting individual pathkeys */
630-
for (i=0;cell!=NULL;i++, (cell=lnext(*group_pathkeys,cell)))
628+
/*
629+
* Estimate cost for sorting individual pathkeys skipping the
630+
* pre-ordered pathkeys.
631+
*/
632+
for_each_from(cell,*group_pathkeys,n_preordered)
631633
{
632-
List*to_cost=list_make1(lfirst(cell));
633-
634-
Assert(i<nFreeKeys);
634+
PathKey*pathkey= (PathKey*)lfirst(cell);
635+
List*to_cost=list_make1(pathkey);
635636

636-
costs[i].pathkey=lfirst(cell);
637-
costs[i].cost=cost_sort_estimate(root,to_cost,0,nrows);
637+
cost->pathkey=pathkey;
638+
cost->cost=cost_sort_estimate(root,to_cost,0,nrows);
639+
cost++;
638640

639-
pfree(to_cost);
641+
list_free(to_cost);
640642
}
641643

642644
/* sort the pathkeys by sort cost in ascending order */
@@ -646,9 +648,9 @@ get_cheapest_group_keys_order(PlannerInfo *root, double nrows,
646648
* Rebuild the list of pathkeys - first the preordered ones, then the
647649
* rest ordered by cost.
648650
*/
649-
new_group_pathkeys=list_truncate(list_copy(*group_pathkeys),n_preordered);
651+
new_group_pathkeys=list_copy_head(*group_pathkeys,n_preordered);
650652

651-
for (i=0;i<nFreeKeys;i++)
653+
for (inti=0;i<nFreeKeys;i++)
652654
new_group_pathkeys=lappend(new_group_pathkeys,costs[i].pathkey);
653655

654656
pfree(costs);
@@ -689,7 +691,7 @@ get_cheapest_group_keys_order(PlannerInfo *root, double nrows,
689691

690692
cost=cost_sort_estimate(root,var_group_pathkeys,n_preordered,nrows);
691693

692-
if (cost<cheapest_sort_cost||cheapest_sort_cost<0)
694+
if (cost<cheapest_sort_cost)
693695
{
694696
list_free(new_group_pathkeys);
695697
new_group_pathkeys=list_copy(var_group_pathkeys);

‎src/include/nodes/pg_list.h‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -620,6 +620,7 @@ extern void list_free(List *list);
620620
externvoidlist_free_deep(List*list);
621621

622622
externpg_nodiscardList*list_copy(constList*list);
623+
externpg_nodiscardList*list_copy_head(constList*oldlist,intlen);
623624
externpg_nodiscardList*list_copy_tail(constList*list,intnskip);
624625
externpg_nodiscardList*list_copy_deep(constList*oldlist);
625626

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