“ORDER BY … USING” clause in PostgreSQL

Question

The ORDER BY clause is decribed in the PostgreSQLdocumentation as:

ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...]
         


        

Answer 1

Nice answers, but they didn't mentioned one real valuable case for ´USING´.

When you have created an index with non default operators family, for example varchar_pattern_ops ( ~>~, ~<~, ~>=~, ... ) instead of <, >, >= then if you search based on index and you want to use index in order by clause you need to specify USING with the appropriate operator.

This can be illustrated with such example:

CREATE INDEX index_words_word ON words(word text_pattern_ops); 

Lets compare this two queries:

SELECT * FROM words WHERE word LIKE 'o%' LIMIT 10;

and

SELECT * FROM words WHERE word LIKE 'o%' ORDER BY word LIMIT 10;

The difference between their executions is nearly 100 times in a 500K words DB! And also results may not be correct within non-C locale.

How this could happend?

When you making search with LIKE and ORDER BY clause, you actually make this call:

SELECT * FROM words WHERE word ~>=~ 'o' AND word ~<~'p' ORDER BY word USING < LIMIT 10;

Your index created with ~<~ operator in mind, so PG cannot use given index in a given ORDER BY clause. To get things done right query must be rewritten to this form:

SELECT * FROM words WHERE word ~>=~ 'o' AND word ~<~'p' ORDER BY word USING ~<~ LIMIT 10;

or

SELECT * FROM words WHERE word LIKE 'o%' ORDER BY word USING ~<~ LIMIT 10;