On 12/07/2012 02:53 AM, Tom Lane wrote:
Vlad Arkhipov <arhi...@dc.baikal.ru> writes:
In a BEFORE UPDATE trigger I need to know whether the row was previously
modified by this transaction. Is it safe to use xmin and txid_current()
for this purpose (xmin is 32-bit txid type but txid_current() returns
64-bit bigint).
IF OLD.xmin = txid_current() THEN
Comparing to txid_current() mod 2^32 would probably work, but note this
will not think that subtransactions or parent transactions are "this
transaction", so any use of savepoints or plpgsql exception blocks is
likely to cause headaches. Why do you think you need to know this?
regards, tom lane
The use case is quite simple. I'm trying to rewrite our internal system
versioning extension (SQL feature T180) in more abstract way. Any
temporal versioned table uses its associated history table to store
updated and deleted data rows. For this purpose the extension adds AFTER
UPDATE/DELETE triggers to the table that insert OLD row in the history
table for updated and deleted rows. But if there are multiple changes to
a row in the same transaction the trigger should generate a history row
only for the first change.
On 12/07/2012 06:26 AM, Tom Lane wrote:
It strikes me that the notion of "this row was previously modified by
the current transaction" is squishier than it might look, and we'd do
well to clarify it before we consider exporting anything. I think there
are three ways you might define such a function:
1. xmin is exactly equal to current (sub)transaction's XID.
2. xmin is this (sub)transaction's XID, or the XID of any subcommitted
subtransaction of it.
3. xmin is this (sub)transaction's XID, or the XID of any subcommitted
subtransaction, or the XID of any open parent transaction or
subcommitted subtransaction thereof.
If I understand you correctly, what I'm looking for is described by the
3rd case and I may use TransactionIdIsCurrentTransactionId() for this
purpose?
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