decimal support for agg function

[liukun4515]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
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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[liukun4515]

@realno @alamb I add this issue to track agg function which needs to be implemented decimal type.
@realno You have added some agg functions in #1525, I have tracked them.

[realno]

@liukun4515 I have added more functions and am planning to add a few in the next few days. Here are the list, please add as tasks:

covar(col1, col2): calculate the covariance (sample) of col1 and col2

covar_samp(col1, col2): calculate the covariance (sample) of col1 and col2

covar_pop(col1, col2): calculate the covariance (population) of col1 and col2

corr(col1, col2): calculate the correlation coefficient (sample) of col1 and col2

corr_samp(col1, col2): calculate the correlation coefficient (sample) of col1 and col2

corr_pop(col1, col2): calculate the correlation coefficient (population) of col1 and col2

median(col1): calculate the median of col1

[liukun4515]

e next few da

thanks @realno , I will add them.
You can add the agg functions first, the decimal supporting is not an emergency work.

[liukun4515]

@liukun4515 I have added more functions and am planning to add a few in the next few days. Here are the list, please add as tasks:

covar(col1, col2): calculate the covariance (sample) of col1 and col2

covar_samp(col1, col2): calculate the covariance (sample) of col1 and col2

covar_pop(col1, col2): calculate the covariance (population) of col1 and col2

corr(col1, col2): calculate the correlation coefficient (sample) of col1 and col2

corr_samp(col1, col2): calculate the correlation coefficient (sample) of col1 and col2

corr_pop(col1, col2): calculate the correlation coefficient (population) of col1 and col2

median(col1): calculate the median of col1

@realno please check the document of pg https://www.postgresql.org/docs/14/functions-aggregate.html
Some of agg functions don't need to support decimal type, and just need to support double precision data type(double/float64).