Re: [HACKERS] Re: Trouble with float4 afterupgrading from 6.5.3 to 7.0.2

At 16:12 7/08/00 +0000, Thomas Lockhart wrote:
>> I would have expected the latter to be at worst 10.10000000000000 +/-
>> .00000000000001.
>> Am I missing something?
>
>10.1 can't be represented exactly, so the float8 representation has bits
>set way down at the low end of the mantissa. When converting to float4
>those low bits get rounded up or down into the lowest bit of the float4
>representation. At that point, you have lost knowledge that this ever
>was supposed to be *exactly* 10.1. And when converting back to float8,
>that float4 low bit becomes a middle-range bit in the float8
>representation, with all the bits underneath that zeroed.
>

Now I understand, but it doesn't quite make sense given what was displayed.
The float4 value is *displayed* as 10.1, not 10.1000001, so I had assumed
that there was a level of either accuracy or display rouding happening.
When this value is converted to float8, I hoped that the result would be
the same as:

   Cast( Cast(f4val as varchar(32)) as float8)

Maybe this hope is naieve, but it it a lot more useful than the current
situation. But now that I understand what is happening, I see that (short
of varchar conversions!), it is probably quite hard to do since we can't
tell the 'correct' value.


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