Re: "xmin" system column

On Jan 26, 2006, at 5:22 PM, Michael Fuhr wrote:
> I suppose a sequence is out of the question?  Too easy to get it
> wrong?

Well, I just wanted to avoid embedding this idea into my  
application.  Would rather Postgres take care of it for me.

> Not in the standard installation, but I think a C function that
> returns GetTopTransactionId() should work.  It's trivial to write
> and examples have been posted before; search the archives.

Hmm.  I also see GetCurrentTransactionStartTimestamp() in xact.h.   
That could work as a mostly-unique identifier.  Its value could  
survive dumps (assuming clock is set correctly!) and a little wrapper  
around it could be used by triggers or by default column values.

Futher reading in xact.c says:
	/*
	 * This is the value of now(), ie, the transaction start time.
	 * This does not change as we enter and exit subtransactions, so we  
don't
	 * keep it inside the TransactionState stack.
	 */
	static TimestampTz xactStartTimestamp;



hahaha, *blush*.  I could just use "now()", right?  pg8.1 docs say  
that now()/CURRENT_TIMESTAMP "return the start time of the current  
transaction; their values do not change during the transaction".  I  
could use a composite of (now(), GetTopTransctionId()) to assume  
batch uniqueness.

eric