Inner join vs Where

Question

Is there a difference in performance (in oracle) between

Select * from Table1 T1 
Inner Join Table2 T2 On T1.ID = T2.ID

And

Answer 1

In PostgreSQL, there's definitely no difference - they both equate to the same query plan. I'm 99% sure that's also the case for Oracle.

Answer 2

i had this conundrum today when inspecting one of our sp's timing out in production, changed an inner join on a table built from an xml feed to a 'where' clause instead....average exec time is now 80ms over 1000 executions, whereas before average exec was 2.2 seconds...major difference in the execution plan is the dissapearance of a key lookup... The message being you wont know until youve tested using both methods.

cheers.

Answer 3

I have an addition to that good answer:

That's what is defined as SQL92 and SQL89 respectively, there is no performance difference between them although you can omit the word INNER (using just JOIN is clear enough and in the simplest query you save 5 keyboard strokes now imagine how many strokes there are in big ones).

Answer 4

In a scenario where tables are in 3rd normal form, joins between tables shouldn't change. I.e. join CUSTOMERS and PAYMENTS should always remain the same.

However, we should distinguish joins from filters. Joins are about relationships and filters are about partitioning a whole.

Some authors, referring to the standard (i.e. Jim Melton; Alan R. Simon (1993). Understanding The New SQL: A Complete Guide. Morgan Kaufmann. pp. 11–12. ISBN 978-1-55860-245-8.), wrote about benefits to adopt JOIN syntax over comma-separated tables in FROM clause.

I totally agree with this point of view.

There are several ways to write SQL and achieve the same results but for many of those who do teamwork, source code legibility is an important aspect, and certainly separate how tables relate to each other from specific filters was a big leap in sense of clarifying source code.

Answer 5

[For a bonus point...]

Using the JOIN syntax allows you to more easily comment out the join as its all included on one line. This can be useful if you are debugging a complex query

As everyone else says, they are functionally the same, however the JOIN is more clear of a statement of intent. It therefore may help the query optimiser either in current oracle versions in certain cases (I have no idea if it does), it may help the query optimiser in future versions of Oracle (no-one has any idea), or it may help if you change database supplier.

Answer 6

They're logically identical, but in the earlier versions of Oracle that adopted ANSI syntax there were often bugs with it in more complex cases, so you'll sometimes encounter resistance from Oracle developers when using it.