| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Eric B(dot) Ridge" <ebr(at)tcdi(dot)com> |
| Cc: | pgsql-general(at)postgresql(dot)org |
| Subject: | Re: "xmin" system column |
| Date: | 2006-01-26 21:44:49 |
| Message-ID: | 24623.1138311889@sss.pgh.pa.us |
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| Lists: | pgsql-general |
"Eric B. Ridge" <ebr(at)tcdi(dot)com> writes:
> Outside of "VACUUM FREEZE", is there any way the "xmin" column in a
> relation can change, assuming of course the tuple is never updated
> again?
If the tuple lives long enough, VACUUM will change it to FrozenTransactionId
eventually, even without the FREEZE option.
> I'm considering using this as a way to identify all tuples
> modified in the same transaction (in an effort to group them
> together), and am wondering if there's any way tuples from different
> transactions could end up with the same xmin value.
This seems OK as long as the transaction was fairly recent. Note that
you will need a fairly restrictive definition of "same transaction"
(no subtransactions).
regards, tom lane
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