Question

I have a table containing the runtimes for generators on different sites, and I want to select the most recent entry for each site. Each generator is run once or twice a week.

I have a query that will do this, but I wonder if it’s the best option. I can’t help thinking that using WHERE x IN (SELECT …) is lazy and not the best way to formulate the query – any query.

The table is as follows:

CREATE TABLE generator_logs (     id integer NOT NULL,     site_id character varying(4) NOT NULL,     start timestamp without time zone NOT NULL,     'end' timestamp without time zone NOT NULL,     duration integer NOT NULL ); 

And the query:

SELECT id, site_id, start, 'end', duration  FROM generator_logs WHERE start IN (SELECT MAX(start) AS start                  FROM generator_logs                  GROUP BY site_id)  ORDER BY start DESC 

There isn’t a huge amount of data, so I’m not worried about optimizing the query. However, I do have to do similar things on tables with 10s of millions of rows, (big tables as far as I’m concerned!) and there optimisation is more important.

So is there a better query for this, and are inline queries generally a bad idea?

Answer 1

I would use joins as they perform much better then ‘IN’ clause:

select gl.id, gl.site_id, gl.start, gl.'end', gl.duration  from      generator_logs gl     inner join (         select max(start) as start, site_id         from generator_logs          group by site_id     ) gl2         on gl.site_id = gl2.site_id         and gl.start = gl2.start 

Also as Tony pointed out you were missing correlation in your original query