CASTING in JOIN or WHERE

Hi All,

I have written some SQL and have noticed that PostgreSQL seems to be modifying
the syntax slightly. I do not care per se but if it impacts performance I
want to understand how to write the SQL in the more efficient form. I am
recalling posts commenting on columns with dissimilar types and the lack of
index usage.

I have a column of type varchar
id | character varying(20) | not null

I use this in a WHERE clause
WHERE id = 'SN'
PostgreSQL changes the statement to
WHERE id::text = 'SN'::text

I notice a similar behavior when doing JOINs
Given in table 1
item_id | character varying(20) | not null
and in table 2
id | character varying(20) | not null

The JOIN
ON ( tbl_line_item.item_id = tbl_item.id )
is changed to
ON ( tbl_line_item.item_id::text = tbl_item.id::text )

Kind Regards,
Keith