IDS Forum

Re: BUFFERS - A question

Posted By: Jack Parker
Date: Wednesday, 22 January 2003, at 5:08 p.m.

In Response To: BUFFERS - A question (rkusenet )

Answers within.

----- Original Message -----
From: "rkusenet " <rkusenet@sympatico.ca>
To: <ids@iiug.org>
Sent: Wednesday, January 22, 2003 10:49 AM
Subject: BUFFERS - A question [82]


> I apologize if this is too stupid a question.
>
> If I issue this command:-
>
> unload to ...
> select * from ...
>
> does the server transfer the data from the table to the BUFFER.
> AFAIK the server first reads the data into the BUFFER and then
> performs any operations on it, even if it is SELECT only.
> The only exception is HPL light read which I believe reads
> directly from the extents bypassing BUFFERS.

It depends. Most probably yes - unless you happen to get a light scan -
which may occur if
you are in dirty read or RR/CR with a shared lock
your table is larger than the buffers
you don't have any varchars

Easy to check - onstat -g lsc (scn for XPS unless I have it backwards) will
show you light scans while they are in progress. I would not expect light
scans with an unload.

>
> I need to understand this correctly for the following scenario.
>
> We are debating whether to download few tables during peak hours
> or off peak hours. These tables are large tables and are downloaded
> to pipe delimited format using the above mentioned command in
> dbaccess. My concern is that this should not fill out BUFFERS,
> which will then force other data from BUFFERS to be swapped
> out, degrading performance.

Absolutely. Your unload will most likely flood the buffers and piss other
folks off.

>
> While we are at it, if while scanning data from a table , the
> server comes across empty pages, does the empty page get loaded
> in BUFFERS. I believe Oracle loads it into the BUFFERS and only
> then it is determined that it is empty.

Can't determine if it's an empty page without reading it - where do things
go when they get read? Into the buffers.

>
> thanks.
>
> ravi
>
>

Messages In This Thread

• BUFFERS - A question
  rkusenet -- Wednesday, 22 January 2003, at 10:48 a.m.
  • Re: BUFFERS - A question
    Jack Parker -- Wednesday, 22 January 2003, at 5:08 p.m.

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