While doing experiments with rather long FROM-lists, I looked closely at the
logic related to from_collapse_limit.
I noticed that - unlike join_collapse_limit - the from_collapse_limit does not
enforce maximum length of the top-level list. Shouldn't it do? Too long
FROM-list can obviously lead to excessive planning time.
Also, the order of FROM-list items seems to affect the way RTEs are grouped
into (sub)lists. In this example, the join of tab_0, tab_1, tab_2, tab_3 gets
expanded into 4 separate RTE refs:
SET from_collapse_limit TO 5;
SELECT *
FROM
(
(
tab_0
JOIN
tab_1
ON tab_0.id = tab_1.id
)
JOIN
(
tab_2
JOIN
tab_3
ON tab_2.id = tab_3.id
)
ON tab_1.id = tab_2.id
),
tab_4
JOIN
tab_5
ON tab_4.id = tab_5.id
WHERE tab_3.id = tab_4.id;
However, in the next example (the JOIN of tab_4 and tab_5 moved to the
beginning of the FROM list), the "bigger join" (tab_0 through tab_3) "comes
too late", so it's inserted as a sub-list.
SET from_collapse_limit TO 5;
SELECT *
FROM
tab_4
JOIN
tab_5
ON tab_4.id = tab_5.id,
(
(
tab_0
JOIN
tab_1
ON tab_0.id = tab_1.id
)
JOIN
(
tab_2
JOIN
tab_3
ON tab_2.id = tab_3.id
)
ON tab_1.id = tab_2.id
)
WHERE tab_3.id = tab_4.id;
Is anything wrong about the idea not to estimate the total length of the FROM
list in deconstruct_recurse and to do additional collapsing later instead? The
patch attached here tries to do so.
I wonder if change of the logic behind from_collapse_limit should be
considered acceptable for users or not: although it improves control over
planning of queries having long FROM-list, it can make some plans of existing
applications worse, unless from_collapse_limit is increased accordingly.
diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index f88e493..fffc3aa 100644
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -51,6 +51,7 @@ static List *deconstruct_recurse(PlannerInfo *root, Node *jtnode,
bool below_outer_join,
Relids *qualscope, Relids *inner_join_rels,
List **postponed_qual_list);
+static List *collapse_fromlist(List *fromlist);
static SpecialJoinInfo *make_outerjoininfo(PlannerInfo *root,
Relids left_rels, Relids right_rels,
Relids inner_join_rels,
@@ -659,6 +660,9 @@ deconstruct_jointree(PlannerInfo *root)
/* Shouldn't be any leftover quals */
Assert(postponed_qual_list == NIL);
+ /* Create sub-list(s) if the FROM-list appears to be too long. */
+ result = collapse_fromlist(result);
+
return result;
}
@@ -710,7 +714,6 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
{
FromExpr *f = (FromExpr *) jtnode;
List *child_postponed_quals = NIL;
- int remaining;
ListCell *l;
/*
@@ -722,12 +725,10 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
*qualscope = NULL;
*inner_join_rels = NULL;
joinlist = NIL;
- remaining = list_length(f->fromlist);
foreach(l, f->fromlist)
{
Relids sub_qualscope;
List *sub_joinlist;
- int sub_members;
sub_joinlist = deconstruct_recurse(root, lfirst(l),
below_outer_join,
@@ -735,13 +736,14 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
inner_join_rels,
&child_postponed_quals);
*qualscope = bms_add_members(*qualscope, sub_qualscope);
- sub_members = list_length(sub_joinlist);
- remaining--;
- if (sub_members <= 1 ||
-list_length(joinlist) + sub_members + remaining <= from_collapse_limit)
-joinlist = list_concat(joinlist, sub_joinlist);
- else
-joinlist = lappend(joinlist, sub_joinlist);
+
+ /*
+ * Sub-lists are not created at this stage, as we can't predict how
+ * many RTEs the possibly following join nodes may contain. Instead,
+ * length of the top-level list is checked later when
+ * deconstruct_recurse has finished.
+ */
+ joinlist = list_concat(joinlist, sub_joinlist);
}
/*
@@ -1013,6 +1015,65 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
return joinlist;
}
+
+/*
+ * Ensure that neither the top-level FROM-list nor any its sublist exceeds
+ * from_collapse_limit.
+ */
+static List *
+collapse_fromlist(List *fromlist)
+{
+ List *result = f
