Tom Lane wrote:
Chris Browne <[EMAIL PROTECTED]> writes:
Note that we required that the "provider transaction" have the
attributes IsXactIsoLevelSerializable and XactReadOnly both being
true, so we have the mandates that the resultant backend process:
a) Is in read only mode, and
b) Is in serializable mode.
If XactReadOnly were a "hard" read only constraint, that argument
might be worth the electrons it's written on. I quote TFM:
Now I think someone was looking into a "hard" read only mode for
use in doing read-only queries against a PITR slave; if that
ever happens it might be adaptable to serve this purpose too.
But we haven't got it today.
That would haven been me then ;-)
I think that lazy xid assignment actually got us 90% of the way towards
a "hard" transaction read-only constraint - nearly all data-modfying
operation surely depend on the xact having an xid assigned, no? (The
only exception might be nextval() and friends).
I seem to remember there being some pushback to the idea of changing the
semantics of "set transaction isolation read only" from "soft" to "hard"
semantics though - on the basis that it might break existing
applications. If that has changed (or my memory tricks me ;-) ) I'd
volunteer to create a patch for 8.4 to make "set transaction read only"
a hard constraint.
regards, Florian Pflug
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