sql execution plan - Performance of SQL "EXISTS" usage variants

Question

Is there any difference in the performance of the following three SQL statements?

SELECT * FROM tableA WHERE EXISTS (SELECT * FROM tableB WHERE tableA.x = tableB.y)

SELECT * FROM tableA WHERE EXISTS (SELECT y FROM tableB WHERE tableA.x = tableB.y)

SELECT * FROM tableA WHERE EXISTS (SELECT 1 FROM tableB WHERE tableA.x = tableB.y)

They all should work and return the same result set. But does it matter if the inner SELECT selects all fields of tableB, one field, or just a constant?

Is there any best practice when all statements behave equal?

Answer 1

The truth about the EXISTS clause is that the SELECT clause is not evaluated in an EXISTS clause - you could try:

SELECT * 
  FROM tableA 
 WHERE EXISTS (SELECT 1/0 
                 FROM tableB 
                WHERE tableA.x = tableB.y)

...and should expect a divide by zero error, but you won't because it's not evaluated. This is why my habit is to specify NULL in an EXISTS to demonstrate that the SELECT can be ignored:

SELECT * 
  FROM tableA 
 WHERE EXISTS (SELECT NULL
                 FROM tableB 
                WHERE tableA.x = tableB.y)

All that matters in an EXISTS clause is the FROM and beyond clauses - WHERE, GROUP BY, HAVING, etc.

This question wasn't marked with a database in mind, and it should be because vendors handle things differently -- so test, and check the explain/execution plans to confirm. It is possible that behavior changes between versions...