Sampling is often used in statistical analysis to gain an understanding of the distribution, shape and dispersion of a variable or the relationship between variables. Fusion SQL returns a random sample for all basic selects that do not contain an ORDER BY clause. The random sample is designed to return a uniform distribution of samples that match a query. The sample can be used to infer statistical information about the larger result set. The example below returns a random sample of single field: 
select filesize_d from logs where year_i = 2019
If no limit is specified the sample size will be 25000. To increase the sample size add a limit larger then 25000. 
select filesize_d from logs where year_i = 2019 limit 50000
The ability to subset the data with a query and then sample from that subset is called Stratified Random Sampling. Stratified Random Sampling is an important statistical technique used to better understand sub-populations of a larger data set.

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Descriptive statistics

Sub-queries can be used to return random samples for fast, often sub-second, statistical analysis. For example: 
select count(*) as samplesize,
       mean(filesize_d) as mean,
       min(filesize_d) as min,
       max(filesize_d) as max,
       approx_percentile(filesize_d, .50) as median,
       variance(filesize_d) as variance,
       std(filesize_d) as standard_dev,
       skewness(filesize_d) as skewness,
       kurtosis(filesize_d) as kurtosis,
       sum(filesize_d) as sum
           from (select filesize_d from logs where year_i = 2019 limit 50000)
In the example above the sub-query is returning a random sample of 50000 results which is operated on by the main statistical query. The statistical query returns aggregations which describe the distribution, shape and dispersion of the sample set.

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Correlation and covariance

Sub-queries can be used to provide random samples for correlation and covariance: 
select corr(filesize_d, response_d) as correlation,
       covar_samp(filesize_d, response_d) as covariance
            from (select filesize_d, response_d from logs limit 50000)
In the example above the random sample returns two fields to the corr and covar_samp functions in the main query. Correlation and covariance are used to show the strength of the linear relationship between two variables.

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Pushed-down statistical queries

A narrower set of statistical aggregations can be pushed down to the search engine and operate over entire result sets. These functions are: count(*), count(distinct), sum, min, max, avg and approx_percentile. Below is an example of a fully pushed down statistical query: 
select count(*) as cnt, avg(filesize_d) as avg, approx_percentile(filesize_d, .50) as median from logs where year_i = 2018
Statistical queries that contain a mix of the queries above and non-pushdown such as skewness or kurtosis will be operate over a random sample that matches the query. Below is an example of a statisical query that operates over a random sample: 
select count(*) as cnt,  skewness(filesize_d) as skewness from logs where year_i = 2018