Tom Lane wrote:

My recollection is that other people (perhaps Peter?) were the ones objecting before. However I'd be somewhat unhappy with the proposal as given:## Advertising

Option 'SHORT' would be default and produce the standard sprintf(ascii,... Option 'LONG' would produce sprintf(ascii, "%25.18g", num).

>

since this seems to me to hardwire inappropriate assumptions about the number of significant digits in a double. (Yes, I know practically everyone uses IEEE floats these days. But it's inappropriate for PG to assume that.)

I understand this. Unfortunately I only have IEEE compliant stuff.

In fact, for some numbers I have been testing with, the double representation can distinguish up to DBL_BIG+2.AFAICT the real issue here is that binary float representations will have a fractional decimal digit of precision beyond what DBL_DIG claims.

I think I could support adding an option that switches between the current output format: sprintf(ascii, "%.*g", DBL_DIG, num); and: sprintf(ascii, "%.*g", DBL_DIG+1, num);

Easy to find numbers with double representation which would need DBL_BIG+2.

and similarly for float4. Given carefully written float I/O routines, reading the latter output should reproduce the originally stored value.

For some numbers it does not. Not true as I said above.

Tests like a==b will fail for some numbers with DBL_BIG+1.(And if the I/O routines are not carefully written, you probably lose anyway.) I don't see a need for allowing more flexibility than that.

Its like I said before, the guys from matlab (in x86 IEEE float) go to DBL_BIG+3 to have 'maximum precision'.

Yes. I think there are several options.Comments?

I checked the sprintf(ascii, "%A", num) output format and all the numbers that would fail because of DBL_DIG=15 are ok. After insertion on a table and conversion to double after a query, comparison a==b holds.

AFAICT "%A" is system independent.

I would (if I may) propose the following:

Have two parameters, say DOUBLE_OUTPUT and EXTRA_DIGITS. DOUBLE_OUTPUT would select from decimal output or normalized output. EXTRA_DIGITS would add the required extra digits, from 0 (default) to 3, when output is decimal.

EXTRA_DIGITS:

in the range [0:3]. 0 as defualt.

DOUBLE_OUTPUT:

'DECIMAL': sprintf(ascii, "%.*g", DBL_DIG+EXTRA_DIGITS, num); (default)

'NORMALIZED': sprintf(ascii, "%A", num);

The same could be done for floats (float4).

This way PG does not assume anything (DOUBLE_OUTPUT as 'NORMALIZED'), it does not hardwire 'inappropriate' assumptions about the number of significant digits in a double (default EXTRA_DIGITS=0), and it gives flexibility (EXTRA_DIGITS!=0) if needed.

I think this is functional and reasonable.

Regards,

Pedro M. Ferreira

regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])

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