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The decimal data type has been supported in #122, but in this issue #122 we just support several agg functions.
In this issue we will list some functions which are not implemented for the decimal data type.

• sum support sum/avg agg for decimal, change sum(float32) --> float64 #1408

• min/max Support decimal for min and max aggregate #1407

• count/DistinctCount: don't need to implement

• avg support sum/avg agg for decimal, change sum(float32) --> float64 #1408

• var(col1): calculate the variance (sample) of col1

• var_samp(col1): calculate the variance (sample) of col1

• var_pop(col1): calculate the variance (population) of col1

• stddev(col1): calculate the standard deviation (sample) of col1

• stddev_samp(col1): calculate the standard deviation (sample) of col1

• stddev_pop(col1): calculate the standard deviation (population) of col1

TODO

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