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Re: column ordering, was Re: [PATCHES] Enums patch v2

From: Russell Smith <mr-russ(at)pws(dot)com(dot)au>
To: Andrew Dunstan <andrew(at)dunslane(dot)net>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Stephen Frost <sfrost(at)snowman(dot)net>, Robert Treat <xzilla(at)users(dot)sourceforge(dot)net>, pgsql-hackers(at)postgresql(dot)org, Martijn van Oosterhout <kleptog(at)svana(dot)org>, Gregory Stark <stark(at)enterprisedb(dot)com>
Subject: Re: column ordering, was Re: [PATCHES] Enums patch v2
Date: 2006-12-20 21:23:49
Message-ID: 4589A9E5.40509@pws.com.au (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Andrew Dunstan wrote:
> Russell Smith wrote:
>> Tom Lane wrote:
>>> Stephen Frost <sfrost(at)snowman(dot)net> writes:
>>>  
>>>> Force references to go through macros which implement the lookup 
>>>> for the
>>>> appropriate type?  ie: LOGICAL_COL(table_oid,2) vs.
>>>> PHYSICAL_COL(table_oid,1)  Perhaps that's too simplistic.
>>>>     
>>>
>>> It doesn't really address the question of how you know which one to
>>> use at any particular line of code; or even more to the point, what
>>> mechanism will warn you if you use the wrong one.
>>>
>>> My gut feeling about this is that we could probably enforce such a
>>> distinction if we were using C++, but while coding in C I have no
>>> confidence in it.  (And no, that's not a vote to move to C++ ...)
>>>   
>> What about a comprimise...
>>
>> The 8.1 documentation for ALTER TABLE states the following.
>>
>> Adding a column with a non-null default or changing the type of an 
>> existing column will require the entire table to be rewritten. This 
>> may take a significant amount of time for a large table; and it will 
>> temporarily require double the disk space.
>>
>>
>> Now, we are rewriting the table from scratch anyway, the on disk 
>> format is changing.  What is stopping us from switching the column 
>> order at the same time.  The only thing I can think is that the 
>> catalogs will need more work to update them.  It's a middle sized 
>> price to pay for being able to reorder the columns in the table.  One 
>> of the problems I have is wanting to add a column in the middle of 
>> the table, but FK constraints stop me dropping the table to do the 
>> reorder.  If ALTER TABLE would let me stick it in the middle and 
>> rewrite the table on disk, I wouldn't care.  It's likely that I would 
>> be rewriting the table anyway.  And by specifying AT POSITION, or 
>> BEFORE/AFTER you know for big tables it's going to take a while.
>>
>
> This isn't really a compromise. Remember that this discussion started 
> with consideration of optimal record layout (minimising space use by 
> reducing or eliminating alignment padding). The above proposal really 
> does nothing for that.
>
> cheers
>
> andrew
>
>
This is partly true.  If you have the ability to rewrite the table and 
put columns in a specific order you can "manually" minimize the 
alignment padding.  However that will probably produce a table that is 
not in the logical order you would like.  I still see plenty of use case 
for both my initial case as the alignment padding case, even without 
logical layout being different to disk layout.

Also there has been a large about of discussion on performance relating 
to having firm numbers for proposals for different compiler options.  Do 
anybody have tested numbers, and known information about where/how you 
can eliminate padding by column ordering?  Tom suggests in this thread 
that lots of types have padding issues, so how much is it really going 
to buy us space wise if we re-order the table in optimal format.  What 
is the optimal ordering to reduce disk usage?

Russell.

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