Without join optimization, the following join plan is generated independently of the type of index created on the collection of join columns (color, size, and options) of the large table:
|
Operation |
Joined Tables |
Total Processing Time (seconds) |
|
Spool 4: Product Join |
duplicated options, direct size |
2.67 |
|
Spool 5: Product Join |
duplicated color, direct widgets |
6 660.00 This equates to 1 hour, 51 minutes. |
|
Spool 1: Merge Join |
duplicated 4, local 5 |
7.43 |
Note that the total estimated completion time, including time for two product joins and a merge join, is 1 hour 52 minutes.
The following EXPLAIN output is generated without star join optimization, independently of the type of index created on the collection of join columns (color, size, and options) of the large table:
Explanation
--------------------------------------------------------------------
1) First, we lock TEST.Color for read, we lock TEST.Options for read,
we lock TEST.Size for read, and we lock TEST.Widgets for read.
2) Next, we execute the following steps in parallel.
a) We do an all-AMPs RETRIEVE step from TEST.Color by way of
an all-rows scan with no residual conditions into Spool 2, which
is duplicated on all AMPs. The size of Spool 2 is estimated to be
4 rows. The estimated time for this step is 0.08 seconds.
b) We do an all-AMPs RETRIEVE step from TEST.Options by way of an
all-rows scan with no residual conditions into Spool 3, which is
duplicated on all AMPs. The size of Spool 3 is estimated to be 20
rows. The estimated time for this step is 0.24 seconds.
3) We execute the following steps in parallel.
a) We do an all-AMPs JOIN step from TEST.Size by way of an all-rows
scan with no residual conditions, which is joined to Spool 3
(Last Use). TEST.Size and Spool 3 are joined using a product
join. The result goes into Spool 4, which is duplicated on all
AMPs. Then we do a SORT to order Spool 4 by row hash. The size
of Spool 4 is estimated to be 200 rows. The estimated time for
this step is 2.43 seconds.
b) We do an all-AMPs JOIN step from Spool 2 (Last Use) by way of
an all-rows scan, which is joined to TEST.Widgets. Spool 2 and
TEST.Widgets are joined using a product join, with a join
condition of (“TEST.Widgets.color = Spool_2.code”). The result
goes into Spool 5, which is built locally on the AMPs. Then we do
a SORT to order Spool 5 by row hash. The size of Spool 5 is
estimated to be 200,000 rows. The estimated time for this step is
1 hour and 51 minutes.
4) We do an all-AMPs JOIN step from Spool 4 (Last Use) by way of an
all-rows scan, which is joined to Spool 5 (Last Use). Spool 4 and
Spool 5 are joined using a merge join, with a join condition of
(“(Spool_5.size=Spool_4.code) AND (Spool_5.options=Spool_4.code)”).
The result goes into Spool 1, which is built locally on the AMPs. The
size of Spool 1 is estimated to be 200 rows. The estimated time for
this step is 7.43 seconds.
5) Finally, we send out an END TRANSACTION step to all AMPs involved in
processing the request.
-> The contents of spool 1 are sent back to the user as the result of
statement 1. The total estimated time is 1 hour and 52 seconds.