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Re: minimal update

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: minimal update
Date: 2007-12-29 05:40:13
Message-ID: 4775DDBD.4050005@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-hackers

Andrew Dunstan wrote:
>
>
> Tom Lane wrote:
>> Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>>  
>>> How does this look?
>>>     
>>
>>  
>>>     if (newtuple->t_len == oldtuple->t_len &&
>>>         newtuple->t_data->t_hoff == oldtuple->t_data->t_hoff &&
>>>         HeapTupleHeaderGetNatts(newtuple) == 
>>> HeapTupleHeaderGetNatts(oldtuple) &&
>>>         (newtuple->t_data->t_infomask & (HEAP_HASOID|HEAP_HASNULL)) 
>>> == (oldtuple->t_data->t_infomask & (HEAP_HASOID|HEAP_HASNULL)) &&
>>>         memcmp(newtuple->t_data + offsetof(HeapTupleHeaderData, 
>>> t_bits),
>>>                oldtuple->t_data + offsetof(HeapTupleHeaderData, t_bits)
>>>                newtuple->t_len - offsetof(HeapTupleHeaderData, 
>>> t_bits)) == 0)
>>>     
>>
>>  
>>>       rettuple = NULL;
>>>     
>>
>> Looks sane.  It might be even saner if you compare all of the
>> non-visibility-related infomask bits, viz
>>
>>     (newtuple->t_data->t_infomask & ~HEAP_XACT_MASK) ==
>>     (oldtuple->t_data->t_infomask & ~HEAP_XACT_MASK)
>>
>> rather than just HASOID and HASNULL.
>>
>>            
>>   
>
> Sadly, the memcmp is failing on my test ("update foo set bar = bar") 
> on 8.2. Looks like I'm in for weekend with my fave debugger :-(
>
>

Turns out we needed those pointers used in the arguments to memcmp cast 
to char * so the pointer arithmetic would work right.

I'll be suggesting we add a utility function like this for 8.4.

cheers

andrew

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