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http://tracker.firebirdsql.org/browse/CORE-3960?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sean Leyne reopened CORE-3960:
------------------------------
Re-opened based on Dmitry's analysis/comments.
> make it possible to see all 17 significant digits for double precision values
> in isql
> -------------------------------------------------------------------------------------
>
> Key: CORE-3960
> URL: http://tracker.firebirdsql.org/browse/CORE-3960
> Project: Firebird Core
> Issue Type: New Feature
> Components: ISQL
> Environment: 1.5.5/2.5.1 classic on windows/linux
> Reporter: Paulius Pazera
>
> when we have slightly different double precision values in database, e.g.
> 1 (stored as 3ff0 0000 0000 000016 64-bit double precision value, see
> http://en.wikipedia.org/wiki/Double_precision)
> ≈ 1.0000000000000002 (3ff0 0000 0000 000116)
> ≈ 1.0000000000000004 (3ff0 0000 0000 000216, more precise decimal
> representation could be found via http://www.binaryconvert.com)
> in the code they are correctly treated as different values,
> but currently isql does not have a way to display all 17 significant digits
> and they look the same:
> select cast(1.0000000000000000 as double precision) firstValue,
> /* isql incorrectly shows following two values being the same as first
> one */
> cast(1.0000000000000002 as double precision) secondValue,
> cast(1.0000000000000004 as double precision) thirdValue,
> /* following is just to show that those two double precision values
> are really different */
> cast(1.0000000000000004 as double precision)-cast(1.0000000000000002
> as double precision) difference,
> /* as opposed to following two values which have the same 64-bit
> binary representation */
> cast(1.00000000000000021 as double precision)-cast(1.00000000000000024
> as double precision) no_difference,
> /* following is to show that comparison works as expected on those
> values */
> (select 'different' from rdb$database where cast(1.0000000000000004 as
> double precision)<>cast(1.0000000000000002 as double precision)) different,
> (select 'equal' from rdb$database where cast(1.0000000000000002 as
> double precision)=cast(1.0000000000000002 as double precision)) equal
> from rdb$database;
> FIRSTVALUE SECONDVALUE THIRDVALUE
> DIFFERENCE NO_DIFFERENCE DIFFERENT EQUAL
> ======================= ======================= =======================
> ======================= ======================= ========= ======
> 1.000000000000000 1.000000000000000 1.000000000000000
> 2.220446049250313e-16 0.0000000000000000 different equal
> that confuses user, makes him think that values in DB are identical, that the
> code should produce one result, but he sees completely different results, and
> even may start to suspect that code isn't working fine, that comparison
> operator doesn't work well on floating point values. Basically it makes code
> debugging difficult
> if isql had a way to display all 17 significant digits, then user would see
> that all three values are different, e.g.
> FIRSTVALUE SECONDVALUE THIRDVALUE
>
> ======================= ======================= =======================
> 1.0000000000000000 1.0000000000000002 1.0000000000000004
> and then he would see which code blocks are really executed and will
> understand easier why he gets the result he sees
> please note that this request is not about floating point decimal to binary
> conversion issues which introduces some discrepancy (i.e. not about casting
> string to double precision), and not about calculations which could increase
> that discrepancy significantly making e.g. 13th significant digit unreliable
> (i.e. not about how those values were derived, not about if can trust those
> values up to the last bit/digit, not about which digit is unreliable), it's
> just about ability to see what we already have in database (if two values are
> different only by last significant 17th digit, then we want ability to see
> that using isql)
> other users may want to see only 2 or 4 digits after decimal point, so
> probably it would be good idea to extend this request and allow user to
> specify double precision formatting mask for isql, e.g. via command line
> switch/option or something else
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