SQL Tip of the Day: Be Wary of SELECT COUNT(*)
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Recently, I’ve encountered this sort of query all over the place at a customer site:
DECLARE v_var NUMBER(10); BEGIN SELECT COUNT(*) INTO v_var FROM table1 JOIN table2 ON table1.t1_id = table2.t1_id JOIN table3 ON table2.t2_id = table3.t2_id ... WHERE some_predicate; IF (v_var = 1) THEN do_something ELSE do_something_else END IF; END;
COUNT(*) is often the first solution that comes to mind when we want to check our relations for some predicate. But
COUNT() is expensive, especially if all we’re doing is checking our relations for existence. Does the word ring a bell? Yes, we should use the
EXISTS predicate, because if we don’t care about the exact number of records that return true for a given predicate, we shouldn’t go through the complete data set to actually count the exact number. The above PL/SQL block can be rewritten trivially to this one:
DECLARE v_var NUMBER(10); BEGIN SELECT CASE WHEN EXISTS ( SELECT 1 FROM table1 JOIN table2 ON table1.t1_id = table2.t1_id JOIN table3 ON table2.t2_id = table3.t2_id ... WHERE some_predicate ) THEN 1 ELSE 0 END INTO v_var FROM dual; IF (v_var = 1) THEN do_something ELSE do_something_else END IF; END;
Query 1 yields this execution plan:
Query 2 yields this execution plan:
You can ignore the
TABLE ACCESS FULL operations, the actual query was executed on a trivial database with no indexes.
What’s essential, however, are the much improved
E-Rows values (E = Estimated) and even more importantly the optimal
A-Rows values (A = Actual). As you can see, the
predicate could be aborted early, as soon as the first record that
matches the predicate is encountered – in this case immediately.
Whenever you encounter a
COUNT(*) operation, you should ask yourself if it is really needed. Do you really need to know the exact number of records that match a predicate? Or are you already happy knowing that any record matches the predicate?
Answer: It’s probably the latter.