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

Answer 2

HashAggregate  (cost=868719.08..868719.24 rows=16 width=10)
HashAggregate  (cost=1448560.83..1448564.99 rows=416 width=22)

Hmm, width from 10 to 22 is a doubling. Perhaps you should join after grouping instead of before?

Answer 3

The following query solves the problem without increasing the query execution time. The question still stands why does the execution time increase significantly with adding a very simple join, but it might be a Postgres specific question and somebody with extensive experience in the area might answer it eventually.

WITH 
  OSES AS (SELECT os_id,os_desc from bi.os_desc) 
SELECT 
  COUNT(impression_id) as imp_count, 
  os_desc FROM bi.impressions imp, 
  OSES os 
WHERE 
  os.os_id=imp.os_id 
GROUP BY os_desc 
ORDER BY imp_count;