The query below seems simple and straightforward, yet it produces unexpected results.
CREATE TABLE #NUMBERS( N BIGINT);INSERT INTO #NUMBERS VALUES(1),(2),(3),(4),(5),(6),(7),(8),(9);WITHA AS( -- CHOOSE A ROW AT RANDOM SELECT TOP 1 * FROM #NUMBERS ORDER BY NewID() ),B AS( SELECT A.N AS QUANTITY, 'METERS' AS UNIT FROM A UNION ALL SELECT A.N*100 AS QUANTITY, 'CENTIMETERS' AS UNIT FROM A UNION ALL SELECT A.N*1000 AS QUANTITY, 'MILLIMETERS' AS UNIT FROM A UNION ALL SELECT A.N*1000000 AS QUANTITY, 'MICRONS' AS UNIT FROM A UNION ALL SELECT A.N*1000000000 AS QUANTITY, 'NANOMETERS' AS UNIT FROM A)SELECT *FROM BORDER BY B.QUANTITY;I would expect it to execute CTE A once, and then carry those results into CTE B to produce results something like this:
| QUANTITY | UNIT |
|---|---|
| 4 | METERS |
| 400 | CENTIMETERS |
| 4000 | MILLIMETERS |
| 4000000 | MICRONS |
| 4000000000 | NANOMETERS |
However, it produces results like this:
| QUANTITY | UNIT |
|---|---|
| 8 | METERS |
| 700 | CENTIMETERS |
| 1000 | MILLIMETERS |
| 6000000 | MICRONS |
| 3000000000 | NANOMETERS |
It means it is going back and executing CTE A five times, once for every mention of A in CTE B. Not only is this unwanted and unintuitive, but it also seems unnecessarily inefficient.
What is going on, and how would a CTE genius rewrite it to produce the desired results?
BTW, the Microsoft documentation pages on CTEs contain this cryptic statement which might or might not be related:
If more than one CTE_query_definition is defined, the query definitions must be joined by one of these set operators: UNION ALL, UNION, EXCEPT, or INTERSECT.
Finally, rewriting the query to eliminate CTE B didn't help:
WITHA AS( -- CHOOSE A ROW AT RANDOM SELECT TOP 1 * FROM #NUMBERS ORDER BY NewID() )SELECT *FROM ( SELECT A.N AS QUANTITY, 'METERS' AS UNIT FROM A UNION ALL SELECT A.N*100 AS QUANTITY, 'CENTIMETERS' AS UNIT FROM A UNION ALL SELECT A.N*1000 AS QUANTITY, 'MILLIMETERS' AS UNIT FROM A UNION ALL SELECT A.N*1000000 AS QUANTITY, 'MICRONS' AS UNIT FROM A UNION ALL SELECT A.N*1000000000 AS QUANTITY, 'NANOMETERS' AS UNIT FROM A ) AS BORDER BY B.QUANTITY;