Re: Transaction isolation levels

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Michael Fuhr wrote:

>What words in section 12.2.1 are you referring to? I see the
>following (emphasis mine):
>
> In effect, a SELECT query sees a snapshot of the database as of
> the instant that that *query* begins to run. Notice that two
> successive SELECT commands can see different data, even though
> they are within a single transaction, if other transactions commit
> changes during execution of the first SELECT.
>
>
Apologies for the confusion. Indeed the documentation is right. I
confused "start of query" with "start of transaction".

For my curiosity, what is the reason that in read comitted mode MVCC can
guarantee a consistent snapshot durign a query, while in serializable
mode MVCC can guarantee it for a longer interval (the entire
transaction). Are these different MVCC implementations, or is some kind
of locking performed when executing queries in read committed mode?

Thanks,
Geert

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