On 25.01.2011 22:53, Kevin Grittner wrote:
Heikki Linnakangas<heikki.linnakan...@enterprisedb.com> wrote:
On 25.01.2011 05:30, Kevin Grittner wrote:
The readme says this:
4. PostgreSQL supports subtransactions -- an issue not mentioned
in the papers.
But I don't see any mention anywhere else on how subtransactions
are handled. If a subtransaction aborts, are its predicate locks
immediately released?
No. Here's the reasoning. Within a top level transaction, you
might start a subtransaction, read some data, and then decide based
on what you read that the subtransaction should be rolled back. If
the decision as to what is part of the top level transaction can
depend on what is read in the subtransaction, predicate locks taken
by the subtransaction must survive rollback of the subtransaction.
Does that make sense to you?
Yes, that's what I suspected. And I gather that all the data structures
in predicate.c work with top-level xids, not subxids. When looking at an
xid that comes from a tuple's xmin or xmax, for example, you always call
SubTransGetTopmostTransaction() before doing much else with it.
Is there somewhere you would like to
see that argument documented?
README-SSI .
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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