database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-28 23:42:10 +03:00
Code Activity

Add a concept of "placeholder" variables to the planner. These are variables

Browse Source 
that represent some expression that we desire to compute below the top level
of the plan, and then let that value "bubble up" as though it were a plain
Var (ie, a column value).

The immediate application is to allow sub-selects to be flattened even when
they are below an outer join and have non-nullable output expressions.
Formerly we couldn't flatten because such an expression wouldn't properly
go to NULL when evaluated above the outer join.  Now, we wrap it in a
PlaceHolderVar and arrange for the actual evaluation to occur below the outer
join.  When the resulting Var bubbles up through the join, it will be set to
NULL if necessary, yielding the correct results.  This fixes a planner
limitation that's existed since 7.1.

In future we might want to use this mechanism to re-introduce some form of
Hellerstein's "expensive functions" optimization, ie place the evaluation of
an expensive function at the most suitable point in the plan tree.
This commit is contained in:
Tom Lane
2008-10-21 20:42:53 +00:00
parent 831abae506
commit e6ae3b5dbf
28 changed files with 1135 additions and 226 deletions
Download Patch File Download Diff File Expand all files Collapse all files

34
src/backend/optimizer/plan/createplan.c
View File

@ -10,7 +10,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.250 2008/10/07 19:27:04 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.251 2008/10/21 20:42:53 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -37,7 +37,7 @@
static Plan *create_scan_plan(PlannerInfo *root, Path *best_path);
static List *build_relation_tlist(RelOptInfo *rel);
static bool use_physical_tlist(RelOptInfo *rel);
static bool use_physical_tlist(PlannerInfo *root, RelOptInfo *rel);
static void disuse_physical_tlist(Plan *plan, Path *path);
static Plan *create_gating_plan(PlannerInfo *root, Plan *plan, List *quals);
static Plan *create_join_plan(PlannerInfo *root, JoinPath *best_path);
@ -212,7 +212,7 @@ create_scan_plan(PlannerInfo *root, Path *best_path)
* planner.c may replace the tlist we generate here, forcing projection to
* occur.)
*/
if (use_physical_tlist(rel))
if (use_physical_tlist(root, rel))
{
tlist = build_physical_tlist(root, rel);
/* if fail because of dropped cols, use regular method */
@ -325,9 +325,9 @@ build_relation_tlist(RelOptInfo *rel)
foreach(v, rel->reltargetlist)
{
/* Do we really need to copy here? Not sure */
Var *var = (Var *) copyObject(lfirst(v));
Node *node = (Node *) copyObject(lfirst(v));
tlist = lappend(tlist, makeTargetEntry((Expr *) var,
tlist = lappend(tlist, makeTargetEntry((Expr *) node,
resno,
NULL,
false));
@ -342,9 +342,10 @@ build_relation_tlist(RelOptInfo *rel)
* rather than only those Vars actually referenced.
*/
static bool
use_physical_tlist(RelOptInfo *rel)
use_physical_tlist(PlannerInfo *root, RelOptInfo *rel)
{
int i;
ListCell *lc;
/*
* We can do this for real relation scans, subquery scans, function scans,
@ -365,9 +366,9 @@ use_physical_tlist(RelOptInfo *rel)
return false;
/*
* Can't do it if any system columns or whole-row Vars are requested,
* either. (This could possibly be fixed but would take some fragile
* assumptions in setrefs.c, I think.)
* Can't do it if any system columns or whole-row Vars are requested.
* (This could possibly be fixed but would take some fragile assumptions
* in setrefs.c, I think.)
*/
for (i = rel->min_attr; i <= 0; i++)
{
@ -375,6 +376,19 @@ use_physical_tlist(RelOptInfo *rel)
return false;
}
/*
* Can't do it if the rel is required to emit any placeholder expressions,
* either.
*/
foreach(lc, root->placeholder_list)
{
PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
if (bms_nonempty_difference(phinfo->ph_needed, rel->relids) &&
bms_is_subset(phinfo->ph_eval_at, rel->relids))
return false;
}
return true;
}
@ -2925,7 +2939,7 @@ make_sort_from_pathkeys(PlannerInfo *root, Plan *lefttree, List *pathkeys,
if (em->em_is_const || em->em_is_child)
continue;
sortexpr = em->em_expr;
exprvars = pull_var_clause((Node *) sortexpr, false);
exprvars = pull_var_clause((Node *) sortexpr, true);
foreach(k, exprvars)
{
if (!tlist_member_ignore_relabel(lfirst(k), tlist))