-> Index Scan using C_Z_IDX on C

Index Condition: C.Z = A.X

If all joins are plain inner joins then this is unnecessary, because

it's always possible to reorder the joins so that a parameter is used

If all joins are plain inner joins then this isusuallyunnecessary,

becauseit's possible to reorder the joins so that a parameter is used

immediately below the nestloop node that provides it. But in the

presence of outer joins, join reordering may not be possible, and then

this option can be critical. Before version 9.2, Postgres used ad-hoc

methods for planning and executing such queries, and those methods could

not handle passing parameters down through multiple join levels.

presence of outer joins, such join reordering may not be possible.

Also, the bottom-level scan might require parameters from more than one

other relation. In principle we could join the other relations first

so that all the parameters are supplied from a single nestloop level.

But if those other relations have no join clause in common (which is

common in star-schema queries for instance), the planner won't consider

joining them directly to each other. In such a case we need to be able

to create a plan like

NestLoop

-> Seq Scan on SmallTable1 A

NestLoop

-> Seq Scan on SmallTable2 B

NestLoop

-> Index Scan using XYIndex on LargeTable C

Index Condition: C.X = A.AID and C.Y = B.BID

so we should be willing to pass down A.AID through a join even though

there is no join order constraint forcing the plan to look like this.

Before version 9.2, Postgres used ad-hoc methods for planning and

executing nestloop queries of this kind, and those methods could not

handle passing parameters down through multiple join levels.

To plan such queries, we now use a notion of a "parameterized path",

which is a path that makes use of a join clause to a relation that's not

scanned by the path. In the examplejust above, we would construct a

scanned by the path. In the exampletwo above, we would construct a

path representing the possibility of doing this:

-> Index Scan using C_Z_IDX on C

foreach(lc,root->join_info_list)

{

if (required_outer&&

!bms_overlap(required_outer,param_source_rels))

{

bms_free(required_outer);

return;

Relidsouterrelids=outer_path->parent->relids;

Relidsinnerparams=PATH_REQ_OUTER(inner_path);

if (!(bms_overlap(innerparams,outerrelids)&&

bms_nonempty_difference(innerparams,outerrelids)))

{

bms_free(required_outer);

return;

}

}

-> Seq Scan on int4_tbl

(12 rows)

--

-- test ability to generate a suitable plan for a star-schema query

--

explain (costs off)

select * from

tenk1, int8_tbl a, int8_tbl b

where thousand = a.q1 and tenthous = b.q1 and a.q2 = 1 and b.q2 = 2;

QUERY PLAN

---------------------------------------------------------------------

Nested Loop

-> Seq Scan on int8_tbl b

Filter: (q2 = 2)

-> Nested Loop

-> Seq Scan on int8_tbl a

Filter: (q2 = 1)

-> Index Scan using tenk1_thous_tenthous on tenk1

Index Cond: ((thousand = a.q1) AND (tenthous = b.q1))

(8 rows)

--

-- test placement of movable quals in a parameterized join tree

--

int4(sin(0)) q2

where thousand= (q1+ q2);

explain (costs off)

tenk1, int8_tbl a, int8_tbl b

where thousand=a.q1and tenthous=b.q1anda.q2=1andb.q2=2;