Years ago, it was common to write
where exists (Select * from some_table where some_condition)
Last year I noticed that many t-sql scripts switched to using the number 1 instead of the star
where exists (Select 1 from some_table where some_condition)
just on SO I saw this Oracle example
WHERE EXISTS (SELECT NULL FROM ...
Is this a common pattern with Oracle? And which are the performance arguments to use something like this.
Short answer: I would hope not!
Even the SELECT 1 idiom may have outlived its usefulness, as database engines have advanced enough to recognize that you don't really want the rows, you just want to know if the rows exist. I've looked at execution plans in SQL 2005, and have generally seen these re-written as joins, even where SELECT * is used.
In Oracle it doesn't matter performance wise. You can even put 1/0 there and the statement won't return a divide by zero error because the expression isn't evaluated.
