Re: Optimization of the alignment padding - Mailing list pgsql-hackers

There is a long TODO about it:
* Merge xmin/xmax/cmin/cmax back into three header fields  Before subtransactions, there used to be only three fields
neededto  store these four values. This was possible because only the current  transaction looks at the cmin/cmax
values.If the current transaction  created and expired the row the fields stored where xmin (same as  xmax), cmin,
cmax,and if the transaction was expiring a row from a  another transaction, the fields stored were xmin (cmin was not
needed),xmax, and cmax. Such a system worked because a transaction  could only see rows from another completed
transaction.However,  subtransactions can see rows from outer transactions, and once the  subtransaction completes, the
outertransaction continues, requiring  the storage of all four fields. With subtransactions, an outer  transaction can
createa row, a subtransaction expire it, and when the  subtransaction completes, the outer transaction still has to
have proper visibility of the row's cmin, for example, for cursors.  One possible solution is to create a phantom cid
whichrepresents a  cmin/cmax pair and is stored in local memory.  Another idea is to  store both cmin and cmax only in
localmemory.
 

As mentioned before, this saves four bytes in all cases.

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Alvaro Herrera wrote:
> ITAGAKI Takahiro wrote:
> 
> > After the subtransaction had been added,
> > the size of HeapTupleHeader became 27 bytes.
> > This consumes extra bytes per tuple for the alignment padding,
> > especially on systems where MAXIMUM_ALIGNOF is 8.
> 
> There was a discussion during the 8.1 devel cycle about shortening the
> HeapTupleHeader struct.  It involved some games with the command Ids.
> Maybe you'll want to look at that, as it could have an impact on what
> you're trying to do here.  It reduced the size of the header by 4 bytes.
> 
> There was even a detailed design posted by Tom, I see you were copied on
> it:
> 
> 
> From: Tom Lane <tgl@sss.pgh.pa.us>
> To: Alvaro Herrera <alvherre@alvh.no-ip.org>
> Cc: "Jim C. Nasby" <jnasby@pervasive.com>,
>         Bruce Momjian <pgman@candle.pha.pa.us>,
>         ITAGAKI Takahiro <itagaki.takahiro@lab.ntt.co.jp>,
>         pgsql-hackers@postgresql.org, mkoi-pg@aon.at
> Date: Wed, 07 Sep 2005 13:38:07 -0400
> Subject: Re: [HACKERS] Remove xmin and cmin from frozen tuples
> 
> -- 
> Alvaro Herrera                                http://www.CommandPrompt.com/
> The PostgreSQL Company - Command Prompt, Inc.
> 
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