[Info-vax] RDB Question

[Arne Vajhøj]

On 12/4/2018 4:19 PM, Arne Vajhøj wrote:
> On 12/4/2018 11:43 AM, Dave Froble wrote:
>> I have to ask, why would Rdb be taking out locks for a query (if I'm 
>> understanding the definition of query)?  If one is going to read, but 
>> never write, then why lock?  While there just may be some reason to 
>> not read locked data, that is not normally the case.  There is great 
>> efficiency in skipping the locking when it is not needed.
> 
> RDBMS'es has a very specific definition of transaction isolation
> level.
> 
> As an example with REPEATABLE READ and SERIALIZEABLE then:
> 
> BEGIN TRANSACTION
> read some data
> do some stuff
> read same data again
> COMMIT or ROLLBACK transaction
> 
> then the two reads has to return the same data.

https://en.wikipedia.org/wiki/Isolation_(database_systems)

has details.

Arne