Question

Do I have to create an index on columns referenced in Joins?
E.g.

SELECT
  *
FROM
  left_table
INNER JOIN
  right_table
ON
  left_table.foo = right_table.bar
WHERE
  ...

Should I create indexes on left_table(foo), right_table(bar), or both?

I noticed different results when I used EXPLAIN (Postgresql) with and without indexes and switching around the order of the comparison

(right_table.bar = left_table.foo)

I know for sure that indexes are used for the left of the WHERE clause but I am wondering whether I need indexes for columns listed in ON clauses.

Answer 1

It depends on your where condition, but brief answer is yes.

Usually left table would have this column as primary key which is an index by default.
The second column would be a foreign key and it is likely your queries will benefit from the index added for this column. You need to analyze your query to tell for sure if you need the index or not.

Let say the query is like this:

SELECT  *
FROM  left_table
   INNER JOIN right_table ON
      left_table.foo = right_table.bar
WHERE left_table.SomeField = 1

Index on right_table.bar will definitely be used.

On the other side:

SELECT  *
FROM  left_table
   INNER JOIN right_table ON
      left_table.foo = right_table.bar
WHERE right_table.SomeField = 1

In this case index on right_table.bar will not be used at all, but index on left_table.foo will.