Question

Does it matter which way I order the criteria in the ON clause for a JOIN?

select a.Name, b.Status from a inner join b on a.StatusID = b.ID 

versus

select a.Name, b.Status from a inner join b on b.ID = a.StatusID 

Is there any impact on performance? What if I had multiple criteria?

Is one order more maintainable than another?

Answer 1

JOIN order can be forced by putting the tables in the right order in the FROM clause:

1. MySQL has a special clause called STRAIGHT_JOIN which makes the order matter.

   This will use an index on b.id:

   SELECT  a.Name, b.Status FROM    a STRAIGHT_JOIN         b ON      b.ID = a.StatusID 

   And this will use an index on a.StatusID:

   SELECT  a.Name, b.Status FROM    b STRAIGHT_JOIN         a ON      b.ID = a.StatusID 

2. Oracle has a special hint ORDERED to enforce the JOIN order:

   This will use an index on b.id or build a hash table on b:

   SELECT  /*+ ORDERED */         * FROM    a JOIN    b ON      b.ID = a.StatusID 

   And this will use an index on a.StatusID or build a hash table on a:

   SELECT  /*+ ORDERED */         * FROM    b JOIN    a ON      b.ID = a.StatusID 

3. SQL Server has a hint called FORCE ORDER to do the same:

   This will use an index on b.id or build a hash table on b:

   SELECT  * FROM    a JOIN    b ON      b.ID = a.StatusID OPTION (FORCE ORDER) 

   And this will use an index on a.StatusID or build a hash table on a:

   SELECT  * FROM    b JOIN    a ON      b.ID = a.StatusID OPTION (FORCE ORDER) 

4. PostgreSQL guys, sorry. Your TODO list says:

   Optimizer hints (not wanted)

   Optimizer hints are used to work around problems in the optimizer. We would rather have the problems reported and fixed.

As for the order in the comparison, it doesn’t matter in any RDBMS, AFAIK.

Though I personally always try to estimate which column will be searched for and put this column in the left (for it to seem like an lvalue).

See this answer for more detail.