daniel hutmacher
(imported from SE)
I have two tables with identically named, typed, and indexed key columns. One of the them has a *unique* clustered index, the other one has a *non-unique*.
## The test setup
Setup script, including some realistic statistics:
DROP TABLE IF EXISTS #left;
DROP TABLE IF EXISTS #right;
CREATE TABLE #left (
a char(4) NOT NULL,
b char(2) NOT NULL,
c varchar(13) NOT NULL,
d bit NOT NULL,
e char(4) NOT NULL,
f char(25) NULL,
g char(25) NOT NULL,
h char(25) NULL
--- and a few other columns
);
CREATE UNIQUE CLUSTERED INDEX IX ON #left (a, b, c, d, e, f, g, h)
UPDATE STATISTICS #left WITH ROWCOUNT=63800000, PAGECOUNT=186000;
CREATE TABLE #right (
a char(4) NOT NULL,
b char(2) NOT NULL,
c varchar(13) NOT NULL,
d bit NOT NULL,
e char(4) NOT NULL,
f char(25) NULL,
g char(25) NOT NULL,
h char(25) NULL
--- and a few other columns
);
CREATE CLUSTERED INDEX IX ON #right (a, b, c, d, e, f, g, h)
UPDATE STATISTICS #right WITH ROWCOUNT=55700000, PAGECOUNT=128000;
## The repro
When I join these two tables on their clustering keys, I expect a one-to-many MERGE join, like so:
SELECT *
FROM #left AS l
LEFT JOIN #right AS r ON
l.a=r.a AND
l.b=r.b AND
l.c=r.c AND
l.d=r.d AND
l.e=r.e AND
l.f=r.f AND
l.g=r.g AND
l.h=r.h
WHERE l.a='2018';
This is the query plan I want:
[![This is what I want.][1]][1]
(Never mind the warnings, they have to do with the fake statistics.)
However, if I change the order of the columns around in the join, like so:
SELECT *
FROM #left AS l
LEFT JOIN #right AS r ON
l.c=r.c AND -- used to be third
l.a=r.a AND -- used to be first
l.b=r.b AND -- used to be second
l.d=r.d AND
l.e=r.e AND
l.f=r.f AND
l.g=r.g AND
l.h=r.h
WHERE l.a='2018';
... this happens:
[![The query plan after changing the declared column order in the join.][2]][2]
The Sort operator seems to order the streams according to the declared order of the join, i.e. `c, a, b, d, e, f, g, h`, which adds a blocking operation to my query plan.
## Things I've looked at
* I've tried changing the columns to `NOT NULL`, same results.
* The original table was created with `ANSI_PADDING OFF`, but creating it with `ANSI_PADDING ON` does not affect this plan.
* I tried an `INNER JOIN` instead of `LEFT JOIN`, no change.
* I discovered it on a 2014 SP2 Enterprise, created a repro on a 2017 Developer (current CU).
* Removing the WHERE clause on the leading index column does generate the good plan, but it kind of affects the results.. :)
## Finally, we get to the question
* Is this intentional?
* Can I eliminate the sort without changing the query (which is vendor code, so I'd really rather not...). I can change the table and indexes.
[1]: https://i.stack.imgur.com/HnXXA.png
[2]: https://i.stack.imgur.com/qw7bT.png