Question

I have this table structure:

CREATE TABLE users
(
uid bigint NOT NULL,
first_name character varying,
last_name character varying,
email character varying,
login_count integer,
CONSTRAINT users_pkey PRIMARY KEY (uid)
)

with this index:

CREATE INDEX users__login_count
ON users
USING btree
(login_count DESC NULLS LAST);

The login_count column may consists of NULL values and i need to select all users ordered descending by login_count and need NULLs to be at the end.

Unfortunately this query:

SELECT * FROM users ORDER BY login_count DESC LIMIT 30;

won’t use the index, so NULLs are at the beggining, why?

Answer 1

Your query is effectively ORDER BY login_count DESCNULLS FIRSTLIMIT 30 as explained here. On this page it describes how an index can satisfy an ordering:

An index stored in ascending order
with nulls first can satisfy either
ORDER BY x ASC NULLS FIRST or ORDER BY x DESC NULLS LAST
depending on which direction it is scanned in.

So your index is the same – it can satisfy ASC NULLS FIRST and DESC NULLS LAST, but your query is DESC NULLS FIRST.