PostgreSQL在何处处理 sql查询之五十三

接前面。

从PostgreSQL的 log中，看到 计划树中有一个 plan_rows的东西。

分析它的来源和来龙去脉：

grouping_planner --> create_plan --> create_plan_recurse --> create_scan_plan

--> create_seqscan_plan --> copy_path_costsize

而copy_path_costsize 中：

/*
* Copy cost and size info from a Path node to the Plan node created from it.
* The executor usually won't use this info, but it's needed by EXPLAIN.
*/
static void
copy_path_costsize(Plan *dest, Path *src)
{
fprintf(stderr,"In copy_path_costsize\n");

if (src)
{
fprintf(stderr,"In src \n\n");

dest->startup_cost = src->startup_cost;
dest->total_cost = src->total_cost;
dest->plan_rows = src->rows;
dest->plan_width = src->parent->width;
}
else
{
fprintf(stderr,"In not src \n\n");

dest->startup_cost = 0;
dest->total_cost = 0;
dest->plan_rows = 0;
dest->plan_width = 0;
}
}

其中，这一段是其作用的：

if (src)
{
dest->startup_cost = src->startup_cost;
dest->total_cost = src->total_cost;
dest->plan_rows = src->rows;
dest->plan_width = src->parent->width;
}

上溯一层：create_seqscan_plan: copy_path_costsize 入口参数的 src，就是 create-seqscan_plan 入口 的best_path。

/*
* create_seqscan_plan
*     Returns a seqscan plan for the base relation scanned by 'best_path'
*     with restriction clauses 'scan_clauses' and targetlist 'tlist'.
*/
static SeqScan *
create_seqscan_plan(PlannerInfo *root, Path *best_path,
List *tlist, List *scan_clauses)
{
//fprintf(stderr,"xxx In create_seqscan_plan\n");

SeqScan    *scan_plan;
Index        scan_relid = best_path->parent->relid;

/* it should be a base rel... */
Assert(scan_relid > 0);
Assert(best_path->parent->rtekind == RTE_RELATION);

/* Sort clauses into best execution order */
scan_clauses = order_qual_clauses(root, scan_clauses);

/* Reduce RestrictInfo list to bare expressions; ignore pseudoconstants */
scan_clauses = extract_actual_clauses(scan_clauses, false);

/* Replace any outer-relation variables with nestloop params */
if (best_path->param_info)
{
scan_clauses = (List *)
replace_nestloop_params(root, (Node *) scan_clauses);
}

scan_plan = make_seqscan(tlist,
scan_clauses,
scan_relid);

copy_path_costsize(&scan_plan->plan, best_path);

return scan_plan;
}

再上溯一层：create_scan_plan: create_seqscan_plan的 best_path 来自于create_scan_plan的入口参数 best_path。

/*
* create_scan_plan
*     Create a scan plan for the parent relation of 'best_path'.
*/
static Plan *
create_scan_plan(PlannerInfo *root, Path *best_path)
{
fprintf(stderr, "xxx In create_scan_plan\n");

RelOptInfo *rel = best_path->parent;
List       *tlist;
List       *scan_clauses;
Plan       *plan;

/*
* For table scans, rather than using the relation targetlist (which is
* only those Vars actually needed by the query), we prefer to generate a
* tlist containing all Vars in order.    This will allow the executor to
* optimize away projection of the table tuples, if possible.  (Note that
* planner.c may replace the tlist we generate here, forcing projection to
* occur.)
*/
if (use_physical_tlist(root, rel))
{
if (best_path->pathtype == T_IndexOnlyScan)
{
/* For index-only scan, the preferred tlist is the index's */
tlist = copyObject(((IndexPath *) best_path)->indexinfo->indextlist);
}
else
{
tlist = build_physical_tlist(root, rel);
/* if fail because of dropped cols, use regular method */
if (tlist == NIL)
tlist = build_relation_tlist(rel);
}
}
else
tlist = build_relation_tlist(rel);

/*
* Extract the relevant restriction clauses from the parent relation. The
* executor must apply all these restrictions during the scan, except for
* pseudoconstants which we'll take care of below.
*/
scan_clauses = rel->baserestrictinfo;

/*
* If this is a parameterized scan, we also need to enforce all the join
* clauses available from the outer relation(s).
*
* For paranoia's sake, don't modify the stored baserestrictinfo list.
*/
if (best_path->param_info)
scan_clauses = list_concat(list_copy(scan_clauses),
best_path->param_info->ppi_clauses);

switch (best_path->pathtype)
{
case T_SeqScan:
 plan = (Plan *) create_seqscan_plan(root,
best_path,
tlist,
scan_clauses);
break;

case T_IndexScan:
plan = (Plan *) create_indexscan_plan(root,
(IndexPath *) best_path,
tlist,
scan_clauses,
false);
break;

case T_IndexOnlyScan:
plan = (Plan *) create_indexscan_plan(root,
(IndexPath *) best_path,
tlist,
scan_clauses,
true);
break;

case T_BitmapHeapScan:
plan = (Plan *) create_bitmap_scan_plan(root,
(BitmapHeapPath *) best_path,
tlist,
scan_clauses);
break;

case T_TidScan:
plan = (Plan *) create_tidscan_plan(root,
(TidPath *) best_path,
tlist,
scan_clauses);
break;

case T_SubqueryScan:
plan = (Plan *) create_subqueryscan_plan(root,
best_path,
tlist,
scan_clauses);
break;

case T_FunctionScan:
plan = (Plan *) create_functionscan_plan(root,
best_path,
tlist,
scan_clauses);
break;

case T_ValuesScan:
plan = (Plan *) create_valuesscan_plan(root,
best_path,
tlist,
scan_clauses);
break;

case T_CteScan:
plan = (Plan *) create_ctescan_plan(root,
best_path,
tlist,
scan_clauses);
break;

case T_WorkTableScan:
plan = (Plan *) create_worktablescan_plan(root,
best_path,
tlist,
scan_clauses);
break;

case T_ForeignScan:
plan = (Plan *) create_foreignscan_plan(root,
(ForeignPath *) best_path,
tlist,
scan_clauses);
break;

default:
elog(ERROR, "unrecognized node type: %d",
(int) best_path->pathtype);
plan = NULL;        /* keep compiler quiet */
break;
}

/*
* If there are any pseudoconstant clauses attached to this node, insert a
* gating Result node that evaluates the pseudoconstants as one-time
* quals.
*/
if (root->hasPseudoConstantQuals)
plan = create_gating_plan(root, plan, scan_clauses);

return plan;
}

再上溯：create_plan_recurse: 入口参数里已经带入了 best_path

/*
* create_plan_recurse
*      Recursive guts of create_plan().
*/
static Plan *
create_plan_recurse(PlannerInfo *root, Path *best_path)
{

fprintf(stderr,"xxx In create_plan_recurse\n");

Plan       *plan;

switch (best_path->pathtype)
{
case T_SeqScan:
case T_IndexScan:
case T_IndexOnlyScan:
case T_BitmapHeapScan:
case T_TidScan:
case T_SubqueryScan:
case T_FunctionScan:
case T_ValuesScan:
case T_CteScan:
case T_WorkTableScan:
case T_ForeignScan:
plan = create_scan_plan(root, best_path);
break;
case T_HashJoin:
case T_MergeJoin:
case T_NestLoop:
plan = create_join_plan(root,
(JoinPath *) best_path);
break;
case T_Append:
plan = create_append_plan(root,
(AppendPath *) best_path);
break;
case T_MergeAppend:
plan = create_merge_append_plan(root,
(MergeAppendPath *) best_path);
break;
case T_Result:
plan = (Plan *) create_result_plan(root,
(ResultPath *) best_path);
break;
case T_Material:
plan = (Plan *) create_material_plan(root,
(MaterialPath *) best_path);
break;
case T_Unique:
plan = create_unique_plan(root,
(UniquePath *) best_path);
break;
default:
elog(ERROR, "unrecognized node type: %d",
(int) best_path->pathtype);
plan = NULL;        /* keep compiler quiet */
break;
}

return plan;
}

再次上溯：create_plan: 入口参数里已经带入了 best_path

/*
* create_plan
*      Creates the access plan for a query by recursively processing the
*      desired tree of pathnodes, starting at the node 'best_path'.    For
*      every pathnode found, we create a corresponding plan node containing
*      appropriate id, target list, and qualification information.
*
*      The tlists and quals in the plan tree are still in planner format,
*      ie, Vars still correspond to the parser's numbering.  This will be
*      fixed later by setrefs.c.
*
*      best_path is the best access path
*
*      Returns a Plan tree.
*/
Plan *
create_plan(PlannerInfo *root, Path *best_path)
{

fprintf(stderr,"xxx In create_plan \n");

Plan       *plan;

/* plan_params should not be in use in current query level */
Assert(root->plan_params == NIL);

/* Initialize this module's private workspace in PlannerInfo */
root->curOuterRels = NULL;
root->curOuterParams = NIL;

/* Recursively process the path tree */
plan = create_plan_recurse(root, best_path);

/* Check we successfully assigned all NestLoopParams to plan nodes */
if (root->curOuterParams != NIL)
elog(ERROR, "failed to assign all NestLoopParams to plan nodes");

/*
* Reset plan_params to ensure param IDs used for nestloop params are not
* re-used later
*/
root->plan_params = NIL;

return plan;
}

再上溯：grouping_planner:

/*--------------------
* grouping_planner
*      Perform planning steps related to grouping, aggregation, etc.
*      This primarily means adding top-level processing to the basic
*      query plan produced by query_planner.
*
* tuple_fraction is the fraction of tuples we expect will be retrieved
*
* tuple_fraction is interpreted as follows:
*      0: expect all tuples to be retrieved (normal case)
*      0 < tuple_fraction < 1: expect the given fraction of tuples available
*        from the plan to be retrieved
*      tuple_fraction >= 1: tuple_fraction is the absolute number of tuples
*        expected to be retrieved (ie, a LIMIT specification)
*
* Returns a query plan.  Also, root->query_pathkeys is returned as the
* actual output ordering of the plan (in pathkey format).
*--------------------
*/
static Plan *
grouping_planner(PlannerInfo *root, double tuple_fraction)
{

fprintf(stderr,"xxx In grouping_planner\n");

Query       *parse = root->parse;
List       *tlist = parse->targetList;
int64        offset_est = 0;
int64        count_est = 0;
double        limit_tuples = -1.0;
Plan       *result_plan;
List       *current_pathkeys;
double        dNumGroups = 0;
bool        use_hashed_distinct = false;
bool        tested_hashed_distinct = false;

/* Tweak caller-supplied tuple_fraction if have LIMIT/OFFSET */
if (parse->limitCount || parse->limitOffset)
{
tuple_fraction = preprocess_limit(root, tuple_fraction,
&offset_est, &count_est);

/*
* If we have a known LIMIT, and don't have an unknown OFFSET, we can
* estimate the effects of using a bounded sort.
*/
if (count_est > 0 && offset_est >= 0)
limit_tuples = (double) count_est + (double) offset_est;
}

if (parse->setOperations)
{

List       *set_sortclauses;

/*
* If there's a top-level ORDER BY, assume we have to fetch all the
* tuples.    This might be too simplistic given all the hackery below
* to possibly avoid the sort; but the odds of accurate estimates here
* are pretty low anyway.
*/
if (parse->sortClause)
tuple_fraction = 0.0;

/*
* Construct the plan for set operations.  The result will not need
* any work except perhaps a top-level sort and/or LIMIT.  Note that
* any special work for recursive unions is the responsibility of
* plan_set_operations.
*/
result_plan = plan_set_operations(root, tuple_fraction,
&set_sortclauses);

/*
* Calculate pathkeys representing the sort order (if any) of the set
* operation's result.  We have to do this before overwriting the sort
* key information...
*/
current_pathkeys = make_pathkeys_for_sortclauses(root,
set_sortclauses,
result_plan->targetlist,
true);

/*
* We should not need to call preprocess_targetlist, since we must be
* in a SELECT query node.    Instead, use the targetlist returned by
* plan_set_operations (since this tells whether it returned any
* resjunk columns!), and transfer any sort key information from the
* original tlist.
*/
Assert(parse->commandType == CMD_SELECT);

tlist = postprocess_setop_tlist(copyObject(result_plan->targetlist),
tlist);

/*
* Can't handle FOR UPDATE/SHARE here (parser should have checked
* already, but let's make sure).
*/
if (parse->rowMarks)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("SELECT FOR UPDATE/SHARE is not allowed with UNION/INTERSECT/EXCEPT")));

/*
* Calculate pathkeys that represent result ordering requirements
*/
Assert(parse->distinctClause == NIL);
root->sort_pathkeys = make_pathkeys_for_sortclauses(root,
parse->sortClause,
tlist,
true);
}
else
{

/* No set operations, do regular planning */
List       *sub_tlist;
double        sub_limit_tuples;
AttrNumber *groupColIdx = NULL;
bool        need_tlist_eval = true;
Path       *cheapest_path;
Path       *sorted_path;
Path       *best_path;
long        numGroups = 0;
AggClauseCosts agg_costs;
int            numGroupCols;
double        path_rows;
int            path_width;
bool        use_hashed_grouping = false;
WindowFuncLists *wflists = NULL;
List       *activeWindows = NIL;

MemSet(&agg_costs, 0, sizeof(AggClauseCosts));

/* A recursive query should always have setOperations */
Assert(!root->hasRecursion);

/* Preprocess GROUP BY clause, if any */
if (parse->groupClause)
preprocess_groupclause(root);
numGroupCols = list_length(parse->groupClause);

/* Preprocess targetlist */
tlist = preprocess_targetlist(root, tlist);

/*
* Locate any window functions in the tlist.  (We don't need to look
* anywhere else, since expressions used in ORDER BY will be in there
* too.)  Note that they could all have been eliminated by constant
* folding, in which case we don't need to do any more work.
*/
if (parse->hasWindowFuncs)
{
wflists = find_window_functions((Node *) tlist,
list_length(parse->windowClause));
if (wflists->numWindowFuncs > 0)
activeWindows = select_active_windows(root, wflists);
else
parse->hasWindowFuncs = false;
}

/*
* Generate appropriate target list for subplan; may be different from
* tlist if grouping or aggregation is needed.
*/
sub_tlist = make_subplanTargetList(root, tlist,
&groupColIdx, &need_tlist_eval);

/*
* Do aggregate preprocessing, if the query has any aggs.
*
* Note: think not that we can turn off hasAggs if we find no aggs. It
* is possible for constant-expression simplification to remove all
* explicit references to aggs, but we still have to follow the
* aggregate semantics (eg, producing only one output row).
*/
if (parse->hasAggs)
{
/*
* Collect statistics about aggregates for estimating costs. Note:
* we do not attempt to detect duplicate aggregates here; a
* somewhat-overestimated cost is okay for our present purposes.
*/
count_agg_clauses(root, (Node *) tlist, &agg_costs);
count_agg_clauses(root, parse->havingQual, &agg_costs);

/*
* Preprocess MIN/MAX aggregates, if any.  Note: be careful about
* adding logic between here and the optimize_minmax_aggregates
* call.  Anything that is needed in MIN/MAX-optimizable cases
* will have to be duplicated in planagg.c.
*/
preprocess_minmax_aggregates(root, tlist);
}

/*
* Calculate pathkeys that represent grouping/ordering requirements.
* Stash them in PlannerInfo so that query_planner can canonicalize
* them after EquivalenceClasses have been formed.    The sortClause is
* certainly sort-able, but GROUP BY and DISTINCT might not be, in
* which case we just leave their pathkeys empty.
*/
if (parse->groupClause &&
grouping_is_sortable(parse->groupClause))
root->group_pathkeys =
make_pathkeys_for_sortclauses(root,
parse->groupClause,
tlist,
false);
else
root->group_pathkeys = NIL;

/* We consider only the first (bottom) window in pathkeys logic */
if (activeWindows != NIL)
{
WindowClause *wc = (WindowClause *) linitial(activeWindows);

root->window_pathkeys = make_pathkeys_for_window(root,
wc,
tlist,
false);
}
else
root->window_pathkeys = NIL;

if (parse->distinctClause &&
grouping_is_sortable(parse->distinctClause))
root->distinct_pathkeys =
make_pathkeys_for_sortclauses(root,
parse->distinctClause,
tlist,
false);
else
root->distinct_pathkeys = NIL;

root->sort_pathkeys =
make_pathkeys_for_sortclauses(root,
parse->sortClause,
tlist,
false);

/*
* Figure out whether we want a sorted result from query_planner.
*
* If we have a sortable GROUP BY clause, then we want a result sorted
* properly for grouping.  Otherwise, if we have window functions to
* evaluate, we try to sort for the first window.  Otherwise, if
* there's a sortable DISTINCT clause that's more rigorous than the
* ORDER BY clause, we try to produce output that's sufficiently well
* sorted for the DISTINCT.  Otherwise, if there is an ORDER BY
* clause, we want to sort by the ORDER BY clause.
*
* Note: if we have both ORDER BY and GROUP BY, and ORDER BY is a
* superset of GROUP BY, it would be tempting to request sort by ORDER
* BY --- but that might just leave us failing to exploit an available
* sort order at all.  Needs more thought.    The choice for DISTINCT
* versus ORDER BY is much easier, since we know that the parser
* ensured that one is a superset of the other.
*/
if (root->group_pathkeys)
root->query_pathkeys = root->group_pathkeys;
else if (root->window_pathkeys)
root->query_pathkeys = root->window_pathkeys;
else if (list_length(root->distinct_pathkeys) >
list_length(root->sort_pathkeys))
root->query_pathkeys = root->distinct_pathkeys;
else if (root->sort_pathkeys)
root->query_pathkeys = root->sort_pathkeys;
else
root->query_pathkeys = NIL;

/*
* Figure out whether there's a hard limit on the number of rows that
* query_planner's result subplan needs to return.  Even if we know a
* hard limit overall, it doesn't apply if the query has any
* grouping/aggregation operations.
*/
if (parse->groupClause ||
parse->distinctClause ||
parse->hasAggs ||
parse->hasWindowFuncs ||
root->hasHavingQual)
sub_limit_tuples = -1.0;
else
sub_limit_tuples = limit_tuples;

/*
* Generate the best unsorted and presorted paths for this Query (but
* note there may not be any presorted path).  query_planner will also
* estimate the number of groups in the query, and canonicalize all
* the pathkeys.
*/
query_planner(root, sub_tlist, tuple_fraction, sub_limit_tuples,
&cheapest_path, &sorted_path, &dNumGroups);

/*
* Extract rowcount and width estimates for possible use in grouping
* decisions.  Beware here of the possibility that
* cheapest_path->parent is NULL (ie, there is no FROM clause).
*/
if (cheapest_path->parent)
{
path_rows = cheapest_path->parent->rows;
path_width = cheapest_path->parent->width;
}
else
{
path_rows = 1;        /* assume non-set result */
path_width = 100;    /* arbitrary */
}

if (parse->groupClause)
{
/*
* If grouping, decide whether to use sorted or hashed grouping.
*/
use_hashed_grouping =
choose_hashed_grouping(root,
tuple_fraction, limit_tuples,
path_rows, path_width,
cheapest_path, sorted_path,
dNumGroups, &agg_costs);
/* Also convert # groups to long int --- but 'ware overflow! */
numGroups = (long) Min(dNumGroups, (double) LONG_MAX);
}
else if (parse->distinctClause && sorted_path &&
!root->hasHavingQual && !parse->hasAggs && !activeWindows)
{
/*
* We'll reach the DISTINCT stage without any intermediate
* processing, so figure out whether we will want to hash or not
* so we can choose whether to use cheapest or sorted path.
*/
use_hashed_distinct =
choose_hashed_distinct(root,
tuple_fraction, limit_tuples,
path_rows, path_width,
cheapest_path->startup_cost,
cheapest_path->total_cost,
sorted_path->startup_cost,
sorted_path->total_cost,
sorted_path->pathkeys,
dNumGroups);
tested_hashed_distinct = true;
}

/*
* Select the best path.  If we are doing hashed grouping, we will
* always read all the input tuples, so use the cheapest-total path.
* Otherwise, trust query_planner's decision about which to use.
*/
if (use_hashed_grouping || use_hashed_distinct || !sorted_path)
best_path = cheapest_path;
else
best_path = sorted_path;

/*
* Check to see if it's possible to optimize MIN/MAX aggregates. If
* so, we will forget all the work we did so far to choose a "regular"
* path ... but we had to do it anyway to be able to tell which way is
* cheaper.
*/
result_plan = optimize_minmax_aggregates(root,
tlist,
&agg_costs,
best_path);
if (result_plan != NULL)
{
/*
* optimize_minmax_aggregates generated the full plan, with the
* right tlist, and it has no sort order.
*/
current_pathkeys = NIL;
}
else
{
/*
* Normal case --- create a plan according to query_planner's
* results.
*/
bool        need_sort_for_grouping = false;

result_plan = create_plan(root, best_path);
current_pathkeys = best_path->pathkeys;

/* Detect if we'll need an explicit sort for grouping */
if (parse->groupClause && !use_hashed_grouping &&
!pathkeys_contained_in(root->group_pathkeys, current_pathkeys))
{
need_sort_for_grouping = true;

/*
* Always override create_plan's tlist, so that we don't sort
* useless data from a "physical" tlist.
*/
need_tlist_eval = true;
}

/*
* create_plan returns a plan with just a "flat" tlist of required
* Vars.  Usually we need to insert the sub_tlist as the tlist of
* the top plan node.  However, we can skip that if we determined
* that whatever create_plan chose to return will be good enough.
*/
if (need_tlist_eval)
{
/*
* If the top-level plan node is one that cannot do expression
* evaluation, we must insert a Result node to project the
* desired tlist.
*/
if (!is_projection_capable_plan(result_plan))
{
result_plan = (Plan *) make_result(root,
sub_tlist,
NULL,
result_plan);
}
else
{
/*
* Otherwise, just replace the subplan's flat tlist with
* the desired tlist.
*/
result_plan->targetlist = sub_tlist;
}

/*
* Also, account for the cost of evaluation of the sub_tlist.
* See comments for add_tlist_costs_to_plan() for more info.
*/
add_tlist_costs_to_plan(root, result_plan, sub_tlist);
}
else
{
/*
* Since we're using create_plan's tlist and not the one
* make_subplanTargetList calculated, we have to refigure any
* grouping-column indexes make_subplanTargetList computed.
*/
locate_grouping_columns(root, tlist, result_plan->targetlist,
groupColIdx);
}

/*
* Insert AGG or GROUP node if needed, plus an explicit sort step
* if necessary.
*
* HAVING clause, if any, becomes qual of the Agg or Group node.
*/
if (use_hashed_grouping)
{
/* Hashed aggregate plan --- no sort needed */
result_plan = (Plan *) make_agg(root,
tlist,
(List *) parse->havingQual,
AGG_HASHED,
&agg_costs,
numGroupCols,
groupColIdx,
extract_grouping_ops(parse->groupClause),
numGroups,
result_plan);
/* Hashed aggregation produces randomly-ordered results */
current_pathkeys = NIL;
}
else if (parse->hasAggs)
{
/* Plain aggregate plan --- sort if needed */
AggStrategy aggstrategy;

if (parse->groupClause)
{
if (need_sort_for_grouping)
{
result_plan = (Plan *)
make_sort_from_groupcols(root,
parse->groupClause,
groupColIdx,
result_plan);
current_pathkeys = root->group_pathkeys;
}
aggstrategy = AGG_SORTED;

/*
* The AGG node will not change the sort ordering of its
* groups, so current_pathkeys describes the result too.
*/
}
else
{
aggstrategy = AGG_PLAIN;
/* Result will be only one row anyway; no sort order */
current_pathkeys = NIL;
}

result_plan = (Plan *) make_agg(root,
tlist,
(List *) parse->havingQual,
aggstrategy,
&agg_costs,
numGroupCols,
groupColIdx,
extract_grouping_ops(parse->groupClause),
numGroups,
result_plan);
}
else if (parse->groupClause)
{
/*
* GROUP BY without aggregation, so insert a group node (plus
* the appropriate sort node, if necessary).
*
* Add an explicit sort if we couldn't make the path come out
* the way the GROUP node needs it.
*/
if (need_sort_for_grouping)
{
result_plan = (Plan *)
make_sort_from_groupcols(root,
parse->groupClause,
groupColIdx,
result_plan);
current_pathkeys = root->group_pathkeys;
}

result_plan = (Plan *) make_group(root,
tlist,
(List *) parse->havingQual,
numGroupCols,
groupColIdx,
extract_grouping_ops(parse->groupClause),
dNumGroups,
result_plan);
/* The Group node won't change sort ordering */
}
else if (root->hasHavingQual)
{
/*
* No aggregates, and no GROUP BY, but we have a HAVING qual.
* This is a degenerate case in which we are supposed to emit
* either 0 or 1 row depending on whether HAVING succeeds.
* Furthermore, there cannot be any variables in either HAVING
* or the targetlist, so we actually do not need the FROM
* table at all!  We can just throw away the plan-so-far and
* generate a Result node.    This is a sufficiently unusual
* corner case that it's not worth contorting the structure of
* this routine to avoid having to generate the plan in the
* first place.
*/
result_plan = (Plan *) make_result(root,
tlist,
parse->havingQual,
NULL);
}
}                        /* end of non-minmax-aggregate case */

/*
* Since each window function could require a different sort order, we
* stack up a WindowAgg node for each window, with sort steps between
* them as needed.
*/
if (activeWindows)
{
List       *window_tlist;
ListCell   *l;

/*
* If the top-level plan node is one that cannot do expression
* evaluation, we must insert a Result node to project the desired
* tlist.  (In some cases this might not really be required, but
* it's not worth trying to avoid it.)  Note that on second and
* subsequent passes through the following loop, the top-level
* node will be a WindowAgg which we know can project; so we only
* need to check once.
*/
if (!is_projection_capable_plan(result_plan))
{
result_plan = (Plan *) make_result(root,
NIL,
NULL,
result_plan);
}

/*
* The "base" targetlist for all steps of the windowing process is
* a flat tlist of all Vars and Aggs needed in the result.  (In
* some cases we wouldn't need to propagate all of these all the
* way to the top, since they might only be needed as inputs to
* WindowFuncs.  It's probably not worth trying to optimize that
* though.)  We also add window partitioning and sorting
* expressions to the base tlist, to ensure they're computed only
* once at the bottom of the stack (that's critical for volatile
* functions).  As we climb up the stack, we'll add outputs for
* the WindowFuncs computed at each level.
*/
window_tlist = make_windowInputTargetList(root,
tlist,
activeWindows);

/*
* The copyObject steps here are needed to ensure that each plan
* node has a separately modifiable tlist.  (XXX wouldn't a
* shallow list copy do for that?)
*/
result_plan->targetlist = (List *) copyObject(window_tlist);

foreach(l, activeWindows)
{
WindowClause *wc = (WindowClause *) lfirst(l);
List       *window_pathkeys;
int            partNumCols;
AttrNumber *partColIdx;
Oid           *partOperators;
int            ordNumCols;
AttrNumber *ordColIdx;
Oid           *ordOperators;

window_pathkeys = make_pathkeys_for_window(root,
wc,
tlist,
true);

/*
* This is a bit tricky: we build a sort node even if we don't
* really have to sort.  Even when no explicit sort is needed,
* we need to have suitable resjunk items added to the input
* plan's tlist for any partitioning or ordering columns that
* aren't plain Vars.  (In theory, make_windowInputTargetList
* should have provided all such columns, but let's not assume
* that here.)  Furthermore, this way we can use existing
* infrastructure to identify which input columns are the
* interesting ones.
*/
if (window_pathkeys)
{
Sort       *sort_plan;

sort_plan = make_sort_from_pathkeys(root,
result_plan,
window_pathkeys,
-1.0);
if (!pathkeys_contained_in(window_pathkeys,
current_pathkeys))
{
/* we do indeed need to sort */
result_plan = (Plan *) sort_plan;
current_pathkeys = window_pathkeys;
}
/* In either case, extract the per-column information */
get_column_info_for_window(root, wc, tlist,
sort_plan->numCols,
sort_plan->sortColIdx,
&partNumCols,
&partColIdx,
&partOperators,
&ordNumCols,
&ordColIdx,
&ordOperators);
}
else
{
/* empty window specification, nothing to sort */
partNumCols = 0;
partColIdx = NULL;
partOperators = NULL;
ordNumCols = 0;
ordColIdx = NULL;
ordOperators = NULL;
}

if (lnext(l))
{
/* Add the current WindowFuncs to the running tlist */
window_tlist = add_to_flat_tlist(window_tlist,
wflists->windowFuncs[wc->winref]);
}
else
{
/* Install the original tlist in the topmost WindowAgg */
window_tlist = tlist;
}

/* ... and make the WindowAgg plan node */
result_plan = (Plan *)
make_windowagg(root,
(List *) copyObject(window_tlist),
wflists->windowFuncs[wc->winref],
wc->winref,
partNumCols,
partColIdx,
partOperators,
ordNumCols,
ordColIdx,
ordOperators,
wc->frameOptions,
wc->startOffset,
wc->endOffset,
result_plan);
}
}
}                            /* end of if (setOperations) */

/*
* If there is a DISTINCT clause, add the necessary node(s).
*/
if (parse->distinctClause)
{
double        dNumDistinctRows;
long        numDistinctRows;

/*
* If there was grouping or aggregation, use the current number of
* rows as the estimated number of DISTINCT rows (ie, assume the
* result was already mostly unique).  If not, use the number of
* distinct-groups calculated by query_planner.
*/
if (parse->groupClause || root->hasHavingQual || parse->hasAggs)
dNumDistinctRows = result_plan->plan_rows;
else
dNumDistinctRows = dNumGroups;

/* Also convert to long int --- but 'ware overflow! */
numDistinctRows = (long) Min(dNumDistinctRows, (double) LONG_MAX);

/* Choose implementation method if we didn't already */
if (!tested_hashed_distinct)
{
/*
* At this point, either hashed or sorted grouping will have to
* work from result_plan, so we pass that as both "cheapest" and
* "sorted".
*/
use_hashed_distinct =
choose_hashed_distinct(root,
tuple_fraction, limit_tuples,
result_plan->plan_rows,
result_plan->plan_width,
result_plan->startup_cost,
result_plan->total_cost,
result_plan->startup_cost,
result_plan->total_cost,
current_pathkeys,
dNumDistinctRows);
}

if (use_hashed_distinct)
{
/* Hashed aggregate plan --- no sort needed */
result_plan = (Plan *) make_agg(root,
result_plan->targetlist,
NIL,
AGG_HASHED,
NULL,
list_length(parse->distinctClause),
extract_grouping_cols(parse->distinctClause,
result_plan->targetlist),
extract_grouping_ops(parse->distinctClause),
numDistinctRows,
result_plan);
/* Hashed aggregation produces randomly-ordered results */
current_pathkeys = NIL;
}
else
{
/*
* Use a Unique node to implement DISTINCT.  Add an explicit sort
* if we couldn't make the path come out the way the Unique node
* needs it.  If we do have to sort, always sort by the more
* rigorous of DISTINCT and ORDER BY, to avoid a second sort
* below.  However, for regular DISTINCT, don't sort now if we
* don't have to --- sorting afterwards will likely be cheaper,
* and also has the possibility of optimizing via LIMIT.  But for
* DISTINCT ON, we *must* force the final sort now, else it won't
* have the desired behavior.
*/
List       *needed_pathkeys;

if (parse->hasDistinctOn &&
list_length(root->distinct_pathkeys) <
list_length(root->sort_pathkeys))
needed_pathkeys = root->sort_pathkeys;
else
needed_pathkeys = root->distinct_pathkeys;

if (!pathkeys_contained_in(needed_pathkeys, current_pathkeys))
{
if (list_length(root->distinct_pathkeys) >=
list_length(root->sort_pathkeys))
current_pathkeys = root->distinct_pathkeys;
else
{
current_pathkeys = root->sort_pathkeys;
/* Assert checks that parser didn't mess up... */
Assert(pathkeys_contained_in(root->distinct_pathkeys,
current_pathkeys));
}

result_plan = (Plan *) make_sort_from_pathkeys(root,
result_plan,
current_pathkeys,
-1.0);
}

result_plan = (Plan *) make_unique(result_plan,
parse->distinctClause);
result_plan->plan_rows = dNumDistinctRows;
/* The Unique node won't change sort ordering */
}
}

/*
* If ORDER BY was given and we were not able to make the plan come out in
* the right order, add an explicit sort step.
*/
if (parse->sortClause)
{
if (!pathkeys_contained_in(root->sort_pathkeys, current_pathkeys))
{
result_plan = (Plan *) make_sort_from_pathkeys(root,
result_plan,
root->sort_pathkeys,
limit_tuples);
current_pathkeys = root->sort_pathkeys;
}
}

/*
* If there is a FOR UPDATE/SHARE clause, add the LockRows node. (Note: we
* intentionally test parse->rowMarks not root->rowMarks here. If there
* are only non-locking rowmarks, they should be handled by the
* ModifyTable node instead.)
*/
if (parse->rowMarks)
{
result_plan = (Plan *) make_lockrows(result_plan,
root->rowMarks,
SS_assign_special_param(root));

/*
* The result can no longer be assumed sorted, since locking might
* cause the sort key columns to be replaced with new values.
*/
current_pathkeys = NIL;
}

/*
* Finally, if there is a LIMIT/OFFSET clause, add the LIMIT node.
*/
if (parse->limitCount || parse->limitOffset)
{
result_plan = (Plan *) make_limit(result_plan,
parse->limitOffset,
parse->limitCount,
offset_est,
count_est);
}

/*
* Return the actual output ordering in query_pathkeys for possible use by
* an outer query level.
*/
root->query_pathkeys = current_pathkeys;

return result_plan;
}

上面的 grouping_planner简化一下：

static Plan *
grouping_planner(PlannerInfo *root, double tuple_fraction)
{
...
if (parse->setOperations)
{
...
}
else
{
...
Path       *cheapest_path;
Path       *sorted_path;
Path       *best_path;
...
/*
* Select the best path.  If we are doing hashed grouping, we will
* always read all the input tuples, so use the cheapest-total path.
* Otherwise, trust query_planner's decision about which to use.
*/
if (use_hashed_grouping || use_hashed_distinct || !sorted_path)
best_path = cheapest_path;
else
best_path = sorted_path;
...
}
...
}

以我的最简单的查询而言，是不会有 sorted_path的。

而cheapest_path 的来源是，上述代码中：

/*
* Generate the best unsorted and presorted paths for this Query (but
* note there may not be any presorted path).  query_planner will also
* estimate the number of groups in the query, and canonicalize all
* the pathkeys.
*/
query_planner(root, sub_tlist, tuple_fraction, sub_limit_tuples,
&cheapest_path, &sorted_path, &dNumGroups);

...

if (parse->groupClause)
{
/*
* If grouping, decide whether to use sorted or hashed grouping.
*/
use_hashed_grouping =
choose_hashed_grouping(root,
tuple_fraction, limit_tuples,
path_rows, path_width,
cheapest_path, sorted_path,
dNumGroups, &agg_costs);
/* Also convert # groups to long int --- but 'ware overflow! */
numGroups = (long) Min(dNumGroups, (double) LONG_MAX);
}
else if (parse->distinctClause && sorted_path &&
!root->hasHavingQual && !parse->hasAggs && !activeWindows)
{
/*
* We'll reach the DISTINCT stage without any intermediate
* processing, so figure out whether we will want to hash or not
* so we can choose whether to use cheapest or sorted path.
*/
use_hashed_distinct =
choose_hashed_distinct(root,
tuple_fraction, limit_tuples,
path_rows, path_width,
cheapest_path->startup_cost,
cheapest_path->total_cost,
sorted_path->startup_cost,
sorted_path->total_cost,
sorted_path->pathkeys,
dNumGroups);
tested_hashed_distinct = true;
}

对我的简单查询，只关心 query_planner 函数就行了。