Question

Let’s say I have two tables:

Table A
ProdID | PartNumber | Data...
1      | ABC-a      | "Data A"
2      | (null)     | "Data B"
3      | ABC-c      | "Data C"
...

and

Table B
ProdID | PartNumber | DataB
(null) | ABC-a      | "Data D"
2      | (null)     | "Data E"
3      | (null)     | "Data F"
(null) | ABC-z      | "Data G"
...

Not ideal, but anyway. I want

ProdID | PartNumber | Data     | DataB...
1      | ABC-a      | "Data A" |  "Data D"
2      | (null)     | "Data B" |  "Data E"
3      | ABC-c      | "Data C" |  "Data F"
(null) | ABC-z      | (null)   |  "Data G"

So I use

SELECT * 
FROM Table1 T1
     RIGHT JOIN Table2 T2 ON
          T1.ProdID = T2.ProdID OR T1.PartNumber = T2.PartNumber

Which does exactly what I want, but is seems to take about 100 times as long as either side of the or individually. As part of a more complex query it takes 2 minutes for the OR compared to <1 second for just the int and 1 second for just the nvarchar(50). Table “A” has ~13k rows, table “b” has ~35k and the whole query returns ~40k.

Query Plans

I think this “Table Spool” may be the problem.

SQL Server 2008 R2 Express. Thoughts?

Answer 1

Join each way separately, then combine the results:

SELECT T1.ProdID, T1.PartNumber, T1.Data, ISNULL(tprodid.DataB, tpartno.DataB) as DataB
FROM Table1 T1
LEFT JOIN Table2 tprodid ON T1.ProdID = tprodid.ProdID
LEFT JOIN Table2 tpartno ON T1.PartNumber = tpartno.PartNumber;

This will use both indexes and will perform well. You may want to tweak the ISNULL logic to your liking.