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Data Management

Oracle Tip: Understand the difference between IN and EXISTS in subqueries

When using rule-based optimization, you can determine the performance of a rule-based query by understanding which table is the driving table and how many rows each part returns. Find out how with this hands-on solution.

When coding a SQL statement with tables in master-detail relationships, it's common to have to decide whether to write the query using the WHERE EXISTS (. . .) clause or the WHERE value IN (. . .) clause. You may resist using WHERE EXISTS because it has the awkward syntax of returning a value, which you always ignore.

However, there's a difference when using rule-based optimization. You can determine the performance of a rule-based query by understanding which table is the driving table and how many rows each part returns.

When you write a query using the IN clause, you're telling the rule-based optimizer that you want the inner query to drive the outer query (think: IN = inside to outside). For example, to query the 14-row EMP table for the direct reports to the employee KING, you could write the following:

select ename from emp e
    where mgr in (select empno from emp where ename = 'KING');

Here's the EXPLAIN PLAN for this query:

OBJECT     OPERATION
————— ————————————————————
                 SELECT STATEMENT()
                  NESTED LOOPS()
EMP                TABLE ACCESS(FULL)
EMP                 TABLE ACCESS(BY INDEX ROWID)
PK_EMP               INDEX(UNIQUE SCAN)

This query is virtually equivalent to this:

select e1.ename from emp e1,(select empno from emp where ename = 'KING') e2
    where e1.mgr = e2.empno;

You can write the same query using EXISTS by moving the outer query column to a subquery condition, like this:

select ename from emp e
    where exists (select 0 from emp where e.mgr = empno and ename = 'KING');

When you write EXISTS in a where clause, you're telling the optimizer that you want the outer query to be run first, using each value to fetch a value from the inner query (think: EXISTS = outside to inside).

The EXPLAIN PLAN result for the query is:

OBJECT     OPERATION
————— ————————————————————
                 SELECT STATEMENT()
                  FILTER()
EMP                TABLE ACCESS(FULL)
EMP                 TABLE ACCESS(BY INDEX ROWID)
PK_EMP               INDEX(UNIQUE SCAN)

This is virtually similar to the PL/SQL code:

set serveroutput on;
declare
    l_count integer;
begin
    for e in (select mgr,ename from emp) loop
        select count(*) into l_count from emp
         where e.mgr = empno and ename = 'KING';
        if l_count != 0 then
            dbms_output.put_line(e.ename);
        end if;
    end loop;
end;

To determine which clause offers better performance in rule-based optimization, consider how many rows the inner query will return in comparison to the outer query. In many cases, EXISTS is better because it requires you to specify a join condition, which can invoke an INDEX scan. However, IN is often better if the results of the subquery are very small. You usually want to run the query that returns the smaller set of results first.

Some people avoid the EXISTS clause because of the requirement to return a result from the query—even though the result is never used. Depending on personal style, people often use 'X,' 1, 0, or null. From looking at the EXPLAIN PLAN output, it appears that the optimizer throws out whatever value you enter and uses 0 all the time. Many developers get into the habit of always entering some constant value.

If you want to run your own tests, or see other examples, here are the two scripts I used:

REM — explain.sql - view plan from PLAN_TABLE
set feedback off
set verify off
set pages 2000
column operation format a40
column object format a10

TTITLE * STATEMENT_ID = '&1' *
select object_name object,
       lpad(' ',level-1)||operation||'('||options||')' operation
  from plan_table
 start with id = 0 and statement_id = '&1'
 connect by prior id = parent_id and statement_id = '&1';

REM — exists.sql - examples with EXPLAIN PLAN
REM — IN vs. EXISTS

REM — if you don't have a PLAN_TABLE, run ...
REM — @?/rdbms/admin/xplan
alter session set optimizer_goal = rule;
truncate table plan_table;

REM — find direct reports to KING
explain plan set statement_id = 'IN' for
select ename from emp e
    where mgr in (select empno from emp where ename = 'KING');

explain plan set statement_id = 'JOIN-IN' for
select e1.ename from emp e1,(select empno from emp where ename = 'KING') e2
    where e1.mgr = e2.empno;

explain plan set statement_id = 'EXISTS' for
select ename from emp e
    where exists (select 0 from emp where e.mgr = empno and ename = 'KING');

explain plan set statement_id = '=' for
select ename from emp e
    where mgr = (select empno from emp where ename = 'KING');

explain plan set statement_id = 'JOIN1' for
select e1.ename from emp e1,emp e2
 where e1.mgr = e2.empno
   and e2.ename = 'KING';

REM — find employees with greater than average salaries
explain plan set statement_id = '>' for
select ename from emp e where e.sal > (select avg(sal) from emp);

explain plan set statement_id = 'JOIN2' for
select e1.ename from emp e1,(select avg(sal) sal from emp) e2
 where e1.sal > e2.sal;

@@explain IN
@@explain JOIN-IN
@@explain EXISTS
@@explain =
@@explain JOIN1
@@explain >
@@explain JOIN2

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