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Message-ID: <486E070D.8010101@enterprisedb.com>
Date: Fri, 04 Jul 2008 14:18:37 +0300
From: "Heikki Linnakangas" <heikki@enterprisedb.com>
Organization: EnterpriseDB
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To: "Zdenek Kotala" <Zdenek.Kotala@Sun.COM>
Cc: "Pavan Deolasee" <pavan.deolasee@gmail.com>,
	"PostgreSQL-patches" <pgsql-patches@postgresql.org>
Subject: Re: page macros cleanup
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	<2e78013d0807030450m4cab60d0jd99d1acce59fae56@mail.gmail.com>
	<486DD1D1.50604@sun.com>
	<2e78013d0807040150w393ad72codaf06a6ac12a83ce@mail.gmail.com>
	<486DF644.4070604@enterprisedb.com> <486E008F.3080304@sun.com>
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Zdenek Kotala wrote:
> By my opinion first place where tuple should be placed is:
> 
> MAXALIGN(SizeOfPageHeaderData + sizeof(ItemIdData))

Yeah, but we don't enforce that directly. We enforce it by MAXALIGNing 
size in PageAddItem, and with the rule that special-size is MAXALIGNed.

To put it another way, it's possible that there's an unaligned amount of 
free space on a page, as returned by PageGetExactFreeSpace. But since 
item size is always MAXALIGNed, it's impossible to use it all.


Come to think of it, why do we require MAXALIGN alignment of tuples? I 
must be missing something, but AFAICS the widest fields in 
HeapTupleHeaderData are 4-bytes wide, so it should be possible to get 
away with 4-byte alignment.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com