Re: postgresql shared buffers

one more doubt.in the following example we dont need sort right.But in 
this case also relations were scanned sequentially one by one.So is it the 
case that any relation is accessed only once from database while executing 
a given query?



praveen=# explain select count(*) from a_9000_0,b_9000_0;                                    QUERY PLAN
---------------------------------------------------------------------------------- Aggregate
(cost=537566595.00..537566595.01rows=1 width=0)   ->  Nested Loop  (cost=7616.00..485726595.00 rows=20736000000
width=0)        ->  Seq Scan on a_9000_0  (cost=0.00..6979.00 rows=144000 
 
width=0)         ->  Materialize  (cost=7616.00..9549.00 rows=144000 width=0)               ->  Seq Scan on b_9000_0
(cost=0.00..6979.00rows=144000 
 
width=0)
(5 rows)


Regards,
Praveen


On Fri, 8 Sep 2006, Heikki Linnakangas wrote:

> Date: Fri, 08 Sep 2006 15:30:37 +0100
> From: Heikki Linnakangas <heikki@enterprisedb.com>
> To: Praveen Kumar N <praveen_n@students.iiit.net>
> Cc: pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] postgresql shared buffers
> 
> Praveen Kumar N wrote:
>> On Fri, 8 Sep 2006, Heikki Linnakangas wrote:
>> 
>>> Looks like Alvaro guessed right. It reads both relations in sequence, 
>>> sorts them in temporary storage, outside bufmgr, and then does a merge 
>>> join on the sorted inputs.
>> 
>> could you tell me how can we trace that? I mean which functions shall I 
>> checkout for that.
>
> The sort code is in src/backend/utils/sort/tuplesort.c and logtape.c. Can't 
> remember function names from the top of my head.
>
>

--  N Praveen Kumar Btech-IV CSE IIIT,Hyd AP,India
    Imagination is more important than knowledge...                                --Albert Einstein