Inner Join vs. WHERE Clause in Oracle: What's the Real Performance Difference?

Inner joins and WHERE clause in Oracle

In Oracle database, the difference between using inner join (INNER JOIN) and WHERE clause to join two tables is a common problem. While there may be subtle differences between the two in specific situations, the overall performance difference is usually negligible.

Example below:

<code class="language-sql">Select * from Table1 T1 
Inner Join Table2 T2 On T1.ID = T2.ID</code>

and

<code class="language-sql">Select * from Table1 T1, Table2 T2 
Where T1.ID = T2.ID</code>

Both queries perform the same operation: join the rows in Table1 with the rows in Table2 based on equality of the ID columns. To understand this better, let's create two example tables:

<code class="language-sql">CREATE TABLE table1 (
  id INT,
  name VARCHAR(20)
);

CREATE TABLE table2 (
  id INT,
  name VARCHAR(20)
);</code>

Run execution plan for queries using inner joins:

<code class="language-sql">-- 使用内连接
EXPLAIN PLAN FOR
SELECT * FROM table1 t1
INNER JOIN table2 t2 ON t1.id = t2.id;

SELECT *
FROM TABLE (DBMS_XPLAN.DISPLAY);</code>

...gets the following output:

<code>-- 0 select statement
-- 1 hash join (access("T1"."ID"="T2"."ID"))
-- 2 table access full table1
-- 3 table access full table2</code>

Similarly, the execution plan for a query using the WHERE clause:

<code class="language-sql">-- 使用 WHERE 子句
EXPLAIN PLAN FOR
SELECT * FROM table1 t1, table2 t2
WHERE t1.id = t2.id;

SELECT *
FROM TABLE (DBMS_XPLAN.DISPLAY);</code>

...returns the following output:

<code>-- 0 select statement
-- 1 hash join (access("T1"."ID"="T2"."ID"))
-- 2 table access full table1
-- 3 table access full table2</code>

As you can see, both queries use hash joins to perform join operations, and there is no significant difference in the execution plans.

Thus, the choice between joining tables using inner joins or WHERE clauses in Oracle mainly depends on personal preference or the specific needs of the database schema or query used.

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