Fix for aggregate functions applied to empty sets #964
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@@ Coverage Diff @@
## develop #964 +/- ##
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+ Coverage 68.09% 68.11% +0.01%
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Files 1 1
Lines 9918 9924 +6
Branches 2966 2967 +1
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+ Hits 6754 6760 +6
Misses 3164 3164
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jacqt
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Jan 1, 2019
* Return undefined from aggregate functions when the accumulator is undefined (fixes AlaSQL#917) * Build from src
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Current behaviour
When you call most of the built in aggregate functions (
STDEV,VARetc.) on an empty results set (e.g.SELECT STDDEV(col) FROM table WHERE 1=0) an error is thrown, as detailed in #917.Expected behaviour
No error should be thrown and
undefined(ornull?) should be returned.Cause
An aggregate function has 3 stages (as detailed here), stage 1 for the first value, stage 2 for each subsequent value, and stage 3 to perform any last transforms on the accumulator. The problem is that with an empty set, we skip straight to stage 3 - but the accumulator will always be
undefined, because it hasn't been set to anything in stages 1/2 (which won't have run, since there's no first value or subsequent values) which usually leads to errors.Fix
I propose we can wrap all the aggregate functions in an outer functions which checks whether the accumulator is☺️ .
undefinedat the post-processing state and then just short-circuits (by returningundefined). An alternative would be to add that condition to all the affected aggregate functions, but figured this approach might be simplerThe assumption is that user-defined aggregators would handle this case themselves (might be worth mentioning in the docs?).