Re: same query in high number of times

Peter Alban wrote:

duration: 2533.734 ms  statement:

SELECT news.url_text,news.title, comments.name, comments.createdate, comments.user_id, comments.comment FROM news, comments WHERE comments.cid=news.id  AND comments.published='1' GROUP BY news.url_text,news.title comments.name, comments.createdate, comments.user_id, comments.comment ORDER BY comments.createdate DESC LIMIT 3


And here is the query plan :
                                                              QUERY PLAN                                                              
----------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=4313.54..4313.55 rows=3 width=595) (actual time=288.525..288.528 rows=3 loops=1)
   ->  Sort  (cost=4313.54..4347.26 rows=13486 width=595) (actual time=288.523..288.523 rows=3 loops=1)
         Sort Key: comments.createdate
         ->  HashAggregate  (cost=3253.60..3388.46 rows=13486 width=595) (actual time=137.521..148.132 rows=13415 loops=1)
               ->  Hash Join  (cost=1400.73..3051.31 rows=13486 width=595) (actual time=14.298..51.049 rows=13578 loops=1)
                     Hash Cond: ("outer".cid = "inner".id)
                     ->  Seq Scan on comments  (cost=0.00..1178.72 rows=13480 width=522) (actual time=0.012..17.434 rows=13418 loops=1)
                           Filter: (published = 1)
                     ->  Hash  (cost=1391.18..1391.18 rows=3818 width=81) (actual time=14.268..14.268 rows=3818 loops=1)
                           ->  Seq Scan on news  (cost=0.00..1391.18 rows=3818 width=81) (actual time=0.021..10.072 rows=3818 loops=1)

The same is being requested from different sessions . So why is it not being cached .

Because the query results are not cached only the RAW tables are.   The query is rerun every time it is requested.

What is the group by clause accomplishing??? 
The sorting and hash Aggregate is eating up all the time

work_mem = 51024                        # min 64, size in KB

Thats allot memory dedicated to work mem if you have 30 connections open this could eat up 1.5gigs pushing the data out of cache.