Avg of float inconsistency - sql-server

Question

The select returns right at 23,000 rows
The except will return between 60 to 200 rows (and not the same rows)
The except should return 0 as it is select a except select a
PK: [docSVenum1].[enumID], [docSVenum1].[valueID], [FTSindexWordOnce].[wordID]
[tf] is a float and and I get float is not exact
But I naively thought avg(float) would be repeatable
Avg(float) does appear to be repeatable
What is the solution?
TF is between 0 and 1 and I only need like 5 significant digits
I just need avg(TF) to be the same number run to run
Decimal(9,8) gives me enough precision and if I cast to decimal(9,8) the except properly returns 0
I can change [TF] to decimal(9,8) but it will be bit of work and lot of regression testing as some of the test that use [tf] take over a day to run
Is change [TF] to decimal(9,8) the best solution?
SELECT [docSVenum1].[enumID], [docSVenum1].[valueID], [FTSindexWordOnce].[wordID]
, avg([FTSindexWordOnce].[tf]) AS [avgTFraw]
FROM [docSVenum1]
JOIN [docFieldLock]
ON [docFieldLock].[sID] = [docSVenum1].[sID]
AND [docFieldLock].[fieldID] = [docSVenum1].[enumID]
AND [docFieldLock].[lockID] IN (4, 5) /* secLvl docAdm */
JOIN [FTSindexWordOnce]
ON [FTSindexWordOnce].[sID] = [docSVenum1].[sID]
GROUP BY [docSVenum1].[enumID], [docSVenum1].[valueID], [FTSindexWordOnce].[wordID]
except
SELECT [docSVenum1].[enumID], [docSVenum1].[valueID], [FTSindexWordOnce].[wordID]
, avg([FTSindexWordOnce].[tf]) AS [avgTFraw]
FROM [docSVenum1]
JOIN [docFieldLock]
ON [docFieldLock].[sID] = [docSVenum1].[sID]
AND [docFieldLock].[fieldID] = [docSVenum1].[enumID]
AND [docFieldLock].[lockID] IN (4, 5) /* secLvl docAdm */
JOIN [FTSindexWordOnce]
ON [FTSindexWordOnce].[sID] = [docSVenum1].[sID]
GROUP BY [docSVenum1].[enumID], [docSVenum1].[valueID], [FTSindexWordOnce].[wordID]
order by [docSVenum1].[enumID], [docSVenum1].[valueID], [FTSindexWordOnce].[wordID]
In this case tf is term frequency of tf-idf
tf normalization is subjective and does not require much precision
Avg(tf) needs to be consistent from select to select or the results are not consistent
In a single select with joins I need a consistent avg(tf)
Going with decimal and a low precision for tf got consistent results

Answer 1

This is very similiar to: SELECT SUM(...) is non-deterministic when adding the column-values of datatype float.
The problem is that with inaccurate datatype (FLOAT/REAL) the order of of arithmetic operations on floating point matters. Demo from connect:
DECLARE #fl FLOAT = 100000000000000000000
DECLARE #i SMALLINT = 0
WHILE (#i < 100)
BEGIN
SET #fl = #fl + CONVERT(float, 5000)
SET #i = #i + 1
END
SET #fl = #fl - 100000000000000000000
SELECT CONVERT(NVARCHAR(40), #fl, 2)
-- 0.000000000000000e+000
DECLARE #fl FLOAT = 0
DECLARE #i SMALLINT = 0
WHILE (#i < 100)
BEGIN
SET #fl = #fl + CONVERT(float, 5000)
SET #i = #i + 1
END
SET #fl = #fl + 100000000000000000000
SET #fl = #fl - 100000000000000000000
SELECT #fl
-- 507904
LiveDemo
Possible solutions:
CAST all arguments to accurate datatype like DECIMAL/NUMERIC
alter table and change FLOAT to DECIMAL
you can try to force query optimizer to calculate the sum with the same order.
The good news is that when a stable query result matters to your
application, you can force the order to be the same by preventing
parallelism with OPTION (MAXDOP 1).
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