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Re: Advice on MyXactMade* flags, MyLastRecPtr, pendingDeletes and lazy XID assignment

From: "Florian G(dot) Pflug" <fgp(at)phlo(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Postgresql-Hackers <pgsql-hackers(at)postgresql(dot)org>, Simon Riggs <simon(at)2ndquadrant(dot)com>
Subject: Re: Advice on MyXactMade* flags, MyLastRecPtr, pendingDeletes and lazy XID assignment
Date: 2007-08-29 19:58:49
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Lists: pgsql-hackers
Tom Lane wrote:
> "Florian G. Pflug" <fgp(at)phlo(dot)org> writes:
>> Tom Lane wrote:
>>> One thought here is that it's not clear that we really need a concept of
>>> transaction-controlled vs not-transaction-controlled xlog records
>>> anymore.
>> I've thinking about keeping XLOG_NO_TRAN, and doing
>> if (!no_tran)
>>    Assert(TransactionIdIsValid(GetCurrentTransactionIdIfAny())
>> in xlog.c as a safety measure.
> Why do you think this is a safety measure?  All that it is checking
> is whether the caller has preserved an entirely useless distinction.
> The real correctness property is that you can't write your XID
> into a heap tuple or XLOG record if you haven't acquired an XID,
> but that seems nearly tautological.

I was confused. I wanted to protect against the case the an XID hits
the disk, but doesn't show up in any xl_xid field, and therefore might
be reused after crash recovery. But of course, to make that happen
you'd have to actually *store* the XID into the data you pass to
XLogInsert, which is kind of hard if you haven't asked for it first.

So, I now agree, XLOG_NO_TRAN should be buried.

greetings, Florian Pflug

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