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Improve make_subplanTargetList to avoid including Vars unnecessarily.

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If a Var was used only in a GROUP BY expression, the previous
implementation would include the Var by itself (as well as the expression)
in the generated targetlist.  This wouldn't affect the efficiency of the
scan/join part of the plan at all, but it could result in passing
unnecessarily-wide rows through sorting and grouping steps.  It turns out
to take only a little more code, and not noticeably more time, to generate
a tlist without such redundancy, so let's do that.  Per a recent gripe from
HarmeekSingh Bedi.
This commit is contained in:
Tom Lane 2011-07-16 16:46:55 -04:00
parent 1af37ec96d
commit 1bc16a9460
1 changed files with 111 additions and 53 deletions
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164
src/backend/optimizer/plan/planner.c
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@ -85,6 +85,7 @@ static bool choose_hashed_distinct(PlannerInfo *root,
double dNumDistinctRows);
static List *make_subplanTargetList(PlannerInfo *root, List *tlist,
AttrNumber **groupColIdx, bool *need_tlist_eval);
static int get_grouping_column_index(Query *parse, TargetEntry *tle);
static void locate_grouping_columns(PlannerInfo *root,
List *tlist,
List *sub_tlist,
@ -2536,14 +2537,9 @@ choose_hashed_distinct(PlannerInfo *root,
* For example, given a query like
* SELECT a+b,SUM(c+d) FROM table GROUP BY a+b;
* we want to pass this targetlist to the subplan:
* a,b,c,d,a+b
* a+b,c,d
* where the a+b target will be used by the Sort/Group steps, and the
* other targets will be used for computing the final results. (In the
* above example we could theoretically suppress the a and b targets and
* pass down only c,d,a+b, but it's not really worth the trouble to
* eliminate simple var references from the subplan. We will avoid doing
* the extra computation to recompute a+b at the outer level; see
* fix_upper_expr() in setrefs.c.)
* other targets will be used for computing the final results.
*
* If we are grouping or aggregating, *and* there are no non-Var grouping
* expressions, then the returned tlist is effectively dummy; we do not
@ -2569,7 +2565,8 @@ make_subplanTargetList(PlannerInfo *root,
{
Query *parse = root->parse;
List *sub_tlist;
List *extravars;
List *non_group_cols;
List *non_group_vars;
int numCols;
*groupColIdx = NULL;
@ -2586,71 +2583,132 @@ make_subplanTargetList(PlannerInfo *root,
}
/*
* Otherwise, start with a "flattened" tlist (having just the Vars
* mentioned in the targetlist and HAVING qual). Note this includes Vars
* used in resjunk items, so we are covering the needs of ORDER BY and
* window specifications. Vars used within Aggrefs will be pulled out
* here, too.
* Otherwise, we must build a tlist containing all grouping columns,
* plus any other Vars mentioned in the targetlist and HAVING qual.
*/
sub_tlist = flatten_tlist(tlist,
PVC_RECURSE_AGGREGATES,
PVC_INCLUDE_PLACEHOLDERS);
extravars = pull_var_clause(parse->havingQual,
PVC_RECURSE_AGGREGATES,
PVC_INCLUDE_PLACEHOLDERS);
sub_tlist = add_to_flat_tlist(sub_tlist, extravars);
list_free(extravars);
sub_tlist = NIL;
non_group_cols = NIL;
*need_tlist_eval = false; /* only eval if not flat tlist */
/*
* If grouping, create sub_tlist entries for all GROUP BY expressions
* (GROUP BY items that are simple Vars should be in the list already),
* and make an array showing where the group columns are in the sub_tlist.
*/
numCols = list_length(parse->groupClause);
if (numCols > 0)
{
int keyno = 0;
/*
* If grouping, create sub_tlist entries for all GROUP BY columns, and
* make an array showing where the group columns are in the sub_tlist.
*
* Note: with this implementation, the array entries will always be
* 1..N, but we don't want callers to assume that.
*/
AttrNumber *grpColIdx;
ListCell *gl;
ListCell *tl;
grpColIdx = (AttrNumber *) palloc(sizeof(AttrNumber) * numCols);
grpColIdx = (AttrNumber *) palloc0(sizeof(AttrNumber) * numCols);
*groupColIdx = grpColIdx;
foreach(gl, parse->groupClause)
foreach(tl, tlist)
{
SortGroupClause *grpcl = (SortGroupClause *) lfirst(gl);
Node *groupexpr = get_sortgroupclause_expr(grpcl, tlist);
TargetEntry *te;
TargetEntry *tle = (TargetEntry *) lfirst(tl);
int colno;
/*
* Find or make a matching sub_tlist entry. If the groupexpr
* isn't a Var, no point in searching. (Note that the parser
* won't make multiple groupClause entries for the same TLE.)
*/
if (groupexpr && IsA(groupexpr, Var))
te = tlist_member(groupexpr, sub_tlist);
else
te = NULL;
if (!te)
colno = get_grouping_column_index(parse, tle);
if (colno >= 0)
{
te = makeTargetEntry((Expr *) groupexpr,
list_length(sub_tlist) + 1,
NULL,
false);
sub_tlist = lappend(sub_tlist, te);
*need_tlist_eval = true; /* it's not flat anymore */
}
/*
* It's a grouping column, so add it to the result tlist and
* remember its resno in grpColIdx[].
*/
TargetEntry *newtle;
/* and save its resno */
grpColIdx[keyno++] = te->resno;
newtle = makeTargetEntry(tle->expr,
list_length(sub_tlist) + 1,
NULL,
false);
sub_tlist = lappend(sub_tlist, newtle);
Assert(grpColIdx[colno] == 0); /* no dups expected */
grpColIdx[colno] = newtle->resno;
if (!(newtle->expr && IsA(newtle->expr, Var)))
*need_tlist_eval = true; /* tlist contains non Vars */
}
else
{
/*
* Non-grouping column, so just remember the expression
* for later call to pull_var_clause. There's no need for
* pull_var_clause to examine the TargetEntry node itself.
*/
non_group_cols = lappend(non_group_cols, tle->expr);
}
}
}
else
{
/*
* With no grouping columns, just pass whole tlist to pull_var_clause.
* Need (shallow) copy to avoid damaging input tlist below.
*/
non_group_cols = list_copy(tlist);
}
/*
* If there's a HAVING clause, we'll need the Vars it uses, too.
*/
if (parse->havingQual)
non_group_cols = lappend(non_group_cols, parse->havingQual);
/*
* Pull out all the Vars mentioned in non-group cols (plus HAVING), and
* add them to the result tlist if not already present. (A Var used
* directly as a GROUP BY item will be present already.) Note this
* includes Vars used in resjunk items, so we are covering the needs of
* ORDER BY and window specifications. Vars used within Aggrefs will be
* pulled out here, too.
*/
non_group_vars = pull_var_clause((Node *) non_group_cols,
PVC_RECURSE_AGGREGATES,
PVC_INCLUDE_PLACEHOLDERS);
sub_tlist = add_to_flat_tlist(sub_tlist, non_group_vars);
/* clean up cruft */
list_free(non_group_vars);
list_free(non_group_cols);
return sub_tlist;
}
/*
* get_grouping_column_index
* Get the GROUP BY column position, if any, of a targetlist entry.
*
* Returns the index (counting from 0) of the TLE in the GROUP BY list, or -1
* if it's not a grouping column. Note: the result is unique because the
* parser won't make multiple groupClause entries for the same TLE.
*/
static int
get_grouping_column_index(Query *parse, TargetEntry *tle)
{
int colno = 0;
Index ressortgroupref = tle->ressortgroupref;
ListCell *gl;
/* No need to search groupClause if TLE hasn't got a sortgroupref */
if (ressortgroupref == 0)
return -1;
foreach(gl, parse->groupClause)
{
SortGroupClause *grpcl = (SortGroupClause *) lfirst(gl);
if (grpcl->tleSortGroupRef == ressortgroupref)
return colno;
colno++;
}
return -1;
}
/*
* locate_grouping_columns
* Locate grouping columns in the tlist chosen by create_plan.