BUFFERS - A question

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. 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. 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. thanks. ravi

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 > >