question abut "order by" null fields

Hi

I have 2 table

table1:
id
column1

table2:
id
column2

I perform this query

SELECT column1, column2 FROM table1 LEFT JOIN table2 USING (id)
ORDER BY column1, column2

In the rows where column1 is not null postgres perform ORDER BY column1,
column2

Problem:
In the rows where Column1 is null, postgres don't perform ORDER BY on
the column2

Why?

Same behavior when:

SELECT column1, column2 FROM table1 LEFT JOIN table2 USING (id)
ORDER BY coalesce(column1,''), column2

Thanks in advance

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Alessandro Vincelli