If it's of any interest, I can say something about the hint bits in the
index tuple header. In my implementation, my decision was to use only one
hint bit. It went into the unused 13th bit of the IndexTuple header. When
the hint bit is set, it means that
(xmin is committed OR xmin = InvalidTransactionId)
AND (xmax is committed OR xmax = InvalidTransactionId)
Then there are 12 bytes for xmin/xmax/cid. I did sweat something over this
decision... but maybe it was a wasted effort if the 12 bytes end up
occupying 16 bytes anyway.
On Thu, Feb 25, 2010 at 1:25 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Greg Stark <gsstark(at)mit(dot)edu> writes:
> > In indexes we currently get away with a reduced header which has few
> > of the 6 bytes of info bits. However the only reason we can do is
> > because we impose arbitrary limitations that work for indexes but
> > wouldn't be reasonable for tables. Such as a lower maximum number of
> > columns, inability to add new columns or drop columns later, etc.
> Wait a second, which idea are we currently talking about? No heap at
> all, or just the ability to check visibility without visiting the heap?
> If it's a genuine IOT (ie no separate heap), then you are not going to
> be able to get away without a full heap tuple header. We've sweated
> blood to get that struct down to where it is; there's no way to make it
> smaller without giving up some really fundamental things, for example
> the ability to do UPDATE :-(
> If you just want to avoid a heap visit for visibility checks, I think
> you'd only need to add xmin/xmax/cmin plus the hint bits for same.
> This is going to end up costing 16 bytes in practice --- you might
> think you could squeeze into 12 but on 64-bit machines (MAXALIGN 8)
> you'll save nothing. So that's effectively a doubling of index size
> for common cases such as a single int4 or int8 index column. The other
> problem is the extra write load created by needing to update the index's
> copies of the hint bits; not to mention extra writes to freeze the xids
> when they get old enough.
> regards, tom lane
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