From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Geert Jansen <geert(at)boskant(dot)nl> |
Cc: | Michael Fuhr <mike(at)fuhr(dot)org>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Transaction isolation levels |
Date: | 2005-07-09 15:30:44 |
Message-ID: | 4773.1120923044@sss.pgh.pa.us |
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Lists: | pgsql-general |
Geert Jansen <geert(at)boskant(dot)nl> writes:
> 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?
It's the same implementation in either case; it's just a matter of which
"snapshot" we refer to when deciding whether recently-committed row
versions are visible to our own query. The snapshot is basically a
list of open transactions, which are to be considered not-committed
even if they in fact commit while our query or transaction continues.
regards, tom lane
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