Why does the following join increase the query time significantly?

Question

I have a star schema here and I am querying the fact table and would like to join one very small dimension table. I can\'t really explain the following:

EXPL         


        

Answer 1

Rewritten with (recommended) explicit ANSI JOIN syntax:

SELECT COUNT(impression_id), imp.os_id, os.os_desc 
FROM   bi.impressions imp
JOIN   bi.os_desc os ON os.os_id = imp.os_id
GROUP  BY imp.os_id, os.os_desc;

First of all, your second query might be wrong, if more or less than exactly one match are found in os_desc for every row in impressions.
This can be ruled out if you have a foreign key constraint on os_id in place, that guarantees referential integrity, plus a NOT NULL constraint on bi.impressions.os_id. If so, in a first step, simplify to:

SELECT COUNT(*) AS ct, imp.os_id, os.os_desc 
FROM   bi.impressions imp
JOIN   bi.os_desc     os USING (os_id)
GROUP  BY imp.os_id, os.os_desc;

count(*) is faster than count(column) and equivalent here if the column is NOT NULL. And add a column alias for the count.

Faster, yet:

SELECT os_id, os.os_desc, sub.ct
FROM  (
   SELECT os_id, COUNT(*) AS ct
   FROM   bi.impressions
   GROUP  BY 1
   ) sub
JOIN   bi.os_desc os USING (os_id)

Aggregate first, join later. More here:

• Aggregate a single column in query with many columns
• PostgreSQL - order by an array