Why is Select 1 faster than Select count(*)?

Question

In Oracle, when querying for row existence, why is Select 1 fast than Select count(*)?

Solution 1

Since Oracle doesn't support IF EXISTS in PL/SQL, CodeByMidnight's suggestion to use EXISTS would normally be done with something like

SELECT 1 
  INTO l_local_variable 
  FROM dual 
 WHERE EXISTS( 
    SELECT 1 
      FROM some_table 
     WHERE some_column = some_condition ); 

Oracle knows that it can stop processing the WHERE EXISTS clause as soon as one row is found, so it doesn't have to potentially count a large number of rows that match the criteria. This is less of a concern, of course, if you are checking to see whether a row with a particular key exists than if you are checking a condition involving unindexed columns or checking a condition that might result in a large number of rows being returned.

(Note: I wish I could post this as a comment on CodeByMidnight's post, but comments can't include formatted code).

UPDATE: Given the clarification the original poster made in their comment, the short, definitive answer is that a SELECT 1 or SELECT COUNT(1) is no faster than a SELECT COUNT(*). Contrary to whatever coding guidelines you are looking at, COUNT(*) is the preferred way of counting all the rows. There was an old myth that a COUNT(1) was faster. At a minimum, that hasn't been true in any version of Oracle released in the past decade and it is unlikely that it was ever true. It was a widely held belief, however. Today, code that does a COUNT(1) rather than a COUNT(*) generally makes me suspect that the author is prone to believe various Oracle myths which is why I would suggest using COUNT(*).

Solution 2

It is better still to use EXISTS where the RDBMS supports it or an equivalent, as this will stop processing rows as soon as it finds a match.

Solution 3

I'd be suprised if select count(*) wasn't properly optimised, there is no need to load in all the columns as there will be no column related processing.

Solution 4

http://www.dbasupport.com/forums/archive/index.php/t-28741.html

For Oracle at least.

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Comments

[Glen]

without knowing which RDBMS engine you're using there's no way to answer correctly. Different engines behave differently

[Tony Andrews]

Do you mean "why is select count(1) faster than Select count(*)"?

[E_the_Shy]

Which question can you answer fastest. (a) Is there someone called "Smith" in the phone book? (b) How many called Smith are there in the phone book?

[APC]

+1 We should only use COUNT() we need to know the actual number of records involved.

[Jeffrey Kemp]

yes. Oracle treats count(*) exactly the same as count(1), count(null), count('any atomic value you like').