PostgreSQL: Ordering, NULLs, and indexes

When using ORDER BY "column" in PostgreSQL, NULL values will come last.

When using ORDER BY "column" DESC, NULLs will come first. This is often not useful.

Luckily, you can tell PostgeSQL where you want your NULLs, by saying

Copy
... ORDER BY "column" DESC NULLS LAST
... ORDER BY "column" ASC NULLS FIRST

Your indexes will have to specify this as well. In Rails, declare them using

Copy
add_index :table, :column, order: { column: 'DESC NULLS LAST' }

Multiple columns

When sorting by multiple columns, you can do the same thing:

Copy
SELECT * FROM "posts" ORDER BY "modified_at" DESC NULLS LAST, "title";

PostgreSQL will only use your index, if it includes all columns, and all columns are order correctly (or if all columns are ordered reversely). So, possible indexes are:

Copy
add_index :posts, [:modified_at, :title], order: { modified_at: 'DESC NULLS LAST' }

and

Copy
add_index :posts, [:modified_at, :title], order: { modified_at: 'ASC NULLS FIRST', :title => 'DESC' }