为什么订购时不使用索引？

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

How an index is defined does not change the meaning of a query. In order for the index to be used, the ordering of the index has to match the ordering of the query.

Having said that, it doesn't appear that MySQL supports nulls last . Try:

SELECT  * 
FROM    users
ORDER BY
        case when login_count is null then -1 else login_count end DESC 
LIMIT 30;

Answer 2

Your query is effectively ORDER BY login_count DESC NULLS FIRST LIMIT 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 .