Writer not stop Reader ?

Writer not stop Reader ?

Post by nikk » Fri, 09 Apr 1999 04:00:00



In Oracle, the Oracle Writer wont' stop the Reader.

If so, when the Data is written to the Database, and the same data is
read from the database, won't it be accurate for the reader ?

 
 
 

Writer not stop Reader ?

Post by Andrew Bab » Fri, 09 Apr 1999 04:00:00


Nikko,

Oracle's Rollback segment is used for maintaining read consistency within
the database.

If a  writer updates a record, but before the writer commits the
transaction, another session reads the same data, they will see the
original data, unless the SELECT statement includes the FOR UPDATE
clause, in which case the reader will wait or fail (if NOWAIT specified)
until the writer commits the transaction.

Hope this helps,
Andrew


> In Oracle, the Oracle Writer wont' stop the Reader.

> If so, when the Data is written to the Database, and the same data is
> read from the database, won't it be accurate for the reader ?


 
 
 

Writer not stop Reader ?

Post by Jonathan Lewi » Fri, 09 Apr 1999 04:00:00


It's a question of scale.

Technically you are correct, a writer can block a reader
for a fraction of a second, and a reader can block a writer
for a fraction of a second (events which are recorded as
buffer busy waits usually).

However, the claim that 'writers don't block readers etc.'
is a comparison with other database systems where a
writer will stop a reader from reading a record (or block)
for the entire duration of the transaction, and a reader
will lock a writer for the entire duration of a transaction -
a time period that could be measured in seconds,
minutes or hours;  rather than Oracle's sub-hundredth
of a second 'pin'.

--

Jonathan Lewis
Yet another Oracle-related web site: www.jlcomp.demon.co.uk


>In Oracle, the Oracle Writer wont' stop the Reader.

>If so, when the Data is written to the Database, and the same data is
>read from the database, won't it be accurate for the reader ?

 
 
 

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