How Oracle process concurrent requests

Hi all

Database: 11g

I'm trying to understand how an Oracle DB handles multiple (simultaneous) requests submitted at the same time? One of my colleagues said that it can process only one instance at a time and not really treat multpile queries at a time. Is this true? An insertion point doesn't seem right to me, but I couldn't find any information on how he manages in fact several requests at once.

Any info, etc. would be greatly appreciated.

Thank you

S

>
I'm trying to understand how an Oracle DB handles multiple (simultaneous) requests submitted at the same time? One of my colleagues said that it can process only one instance at a time and not really treat multpile queries at a time. Is this true? An insertion point doesn't seem right to me, but I couldn't find any information on how he manages in fact several requests at once.

Any info, etc. would be greatly appreciated.
>

In contrast to some databases (they also have changed the behavior now), Oracle has never locks the table for read operations. First, Oracle have always pick up blocks at the moment (compatible mode) for a query and current mode for an LMD. When a session is a select statement, Oracle will read the segment headers and go and get the blocks. Like all queries are made in compatible mode, no matter how many sessions is but they all do the same becomes coherent. If some of these blocks have been changed until the query has begun but not committed or rolled back, Oracle will know that they are back and go and get them. But they are also in the buffer. Now, while the session is ended to retrieve the blocks (while still working Session 2), it would be possible that some session DML has updated some blocks that Session2 would have to pick up. Now 2 Session should also rebuild the undo blocks for blocks of the pre-image. It's why you can see while the meeting has made 1000 becomes coherent, is it possible that Session2 should be 5000 gets consistent because it was to read UNDO blocks so.

To extend the subject on works the Oracle query mechanism, if the blocks are in the buffer cache, it will bring (LIO) and send it back to the PGA TOUR that he will send back to the client in the form of data. If blocks are not in the cache, it will go and read disks (PIO) and before to return data to the PGA, stop and load the buffer cache and then send it back to PGA and then back to the client. So now when Session B comes in next with the same query, Oracle will get from the buffer and return it immediately.

You want to do a trace on sessions 1 and 2 for example (all by doing an update the same table Session 3 prior to the session 2 finishes) and you will see clearly how Oracle worked for your query.

Tags: Database

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