Decimal float numbers have an interesting feature - same value may be represented with different precision - compare for example 0.5 and 0.500. DB2 has special function to compare decimal float numbers, and it works correctly with such values, but what about traditional compare (if a=b then ...) treats them equal and it's correct. When sorting such values they should also be treated as equal (see also http://tracker.firebirdsql.org/browse/CORE-3547). And yes, decfloat value can also be '-0'.
That does not cause problems when building index keys but is not ideal for sort keys. The difference between that keys that is important here is that for sort keys we should be able to restore original value from the key. That requirement is already violated for floating point values after fix for CORE-3547 (we loose -0 value) but looks like it does not cause serious troubles here. Unfortunately for decimal floats things look (on my mind) worse. An output of select decFloatValue from table depends upon is it ordered by that value and what is even more worse upon presence of an index - when index is used field's values are extracted from table and are therefore precise. If you restore http://tracker.firebirdsql.org/secure/attachment/13073/decfloat.gbak and try three following statements: select * from dtst; // plain unordered select * from dtst plan (dtst natural) order by x; // plan frces use of sort instead use of index select * from dtst plan order by x; // use index to order data retrieval you will see how 0.500 changes in case of sort. You may also look at behavior of plus/minus zero, very well seen in same selects from QTST table. I've left -0 and +0 different sort keys (like it was before fix of 3547) in order to let you see an effect, this may be changed in single line of code. I wonder - how should we better handle that issues? Ignore like we did for plus/minus 0 in 3547? Or do something better like keeping original values in sort block? Alex. ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot Firebird-Devel mailing list, web interface at https://lists.sourceforge.net/lists/listinfo/firebird-devel
