--------------------------------------------------
From: "Gregory Stark" <[EMAIL PROTECTED]>
Sent: Friday, February 01, 2008 10:31 PM
To: "Simon Riggs" <[EMAIL PROTECTED]>
Cc: <[EMAIL PROTECTED]>; <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Backward reading
Is there any example of backward reading tuples into PostgreSQL code?
Don't think so, but we don't always use randomAccess anyway. Sounds like
we might be able to drop the length at the end of each tuple in those
cases...
We already do. We only generate the "frozen" tape when we think it might
be
necessary.
Thanks for your reply. I need to read tuples backward in order to rearrange
runs on tapes in a different way than what Postres does now.
Has that of "frozen tape" something to do with it?
Regards, Manolo.
I think the easiest (possibly only?) way to trigger this case is to run
the
query in a cursor like:
postgres=# set enable_indexscan = off;
SET
postgres=# explain select * from h order by i;
QUERY PLAN
----------------------------------------------------------------
Sort (cost=61772.22..62022.20 rows=99994 width=512)
Sort Key: i
-> Seq Scan on h (cost=0.00..7666.94 rows=99994 width=512)
(3 rows)
postgres=# begin;
BEGIN
postgres=# declare c cursor for select * from h order by i;
DECLARE CURSOR
postgres=# fetch 5 from c;
i | r
---+------
1 | 10352
2 | 15034
3 | 91904
4 | 89058
5 | 87001
(5 rows)
postgres=# fetch backward 5 from c;
i | r
---+------
4 | 89058
3 | 91904
2 | 15034
1 | 10352
(4 rows)
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's RemoteDBA services!
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choose an index scan if your joining column's datatypes do not
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