Github user fmcquillan99 commented on a diff in the pull request:

    https://github.com/apache/madlib/pull/239#discussion_r172921714
  
    --- Diff: src/ports/postgres/modules/sample/balance_sample.sql_in ---
    @@ -543,6 +544,90 @@ SELECT * FROM output_table ORDER BY mainhue, name;
     (25 rows)
     </pre>
     
    +-# To perform the balance sampling for independent groups, use the 
'grouping_cols'
    +parameter. Note below that each group (zone) has a different count of the 
classes (mainhue),
    +with some groups not containing some class values.
    +<pre class="syntax">
    +DROP TABLE IF EXISTS output_table;
    +SELECT madlib.balance_sample(
    +    'flags',          -- Source table
    +    'output_table',   -- Output table
    +    'mainhue',        -- Class column
    +    NULL,             -- Uniform
    +    NULL,             -- Output table size
    +    'zone'            -- No grouping
    +);
    +SELECT * FROM output_table ORDER BY zone, mainhue;
    +</pre>
    +<pre class="result">
    + __madlib_id__ | id |    name     | landmass | zone | area | population | 
language | colours | mainhue
    
+---------------+----+-------------+----------+------+------+------------+----------+---------+---------
    +             6 |  8 | Greece      |        3 |    1 |  132 |         10 |  
      6 |       2 | blue
    +             5 |  8 | Greece      |        3 |    1 |  132 |         10 |  
      6 |       2 | blue
    +             8 | 17 | Sweden      |        3 |    1 |  450 |          8 |  
      6 |       2 | blue
    +             7 |  8 | Greece      |        3 |    1 |  132 |         10 |  
      6 |       2 | blue
    +             2 |  7 | Denmark     |        3 |    1 |   43 |          5 |  
      6 |       2 | red
    +             1 |  6 | China       |        5 |    1 | 9561 |       1008 |  
      7 |       2 | red
    +             4 | 12 | Luxembourg  |        3 |    1 |    3 |          0 |  
      4 |       3 | red
    +             3 | 18 | Switzerland |        3 |    1 |   41 |          6 |  
      4 |       2 | red
    +             1 |  2 | Australia   |        6 |    2 | 7690 |         15 |  
      1 |       3 | blue
    +             1 |  1 | Argentina   |        2 |    3 | 2777 |         28 |  
      2 |       2 | blue
    +             2 |  4 | Brazil      |        2 |    3 | 8512 |        119 |  
      6 |       4 | green
    +             6 |  9 | Guatemala   |        1 |    4 |  109 |          8 |  
      2 |       2 | blue
    +             5 |  9 | Guatemala   |        1 |    4 |  109 |          8 |  
      2 |       2 | blue
    +             4 |  9 | Guatemala   |        1 |    4 |  109 |          8 |  
      2 |       2 | blue
    +            12 | 13 | Mexico      |        1 |    4 | 1973 |         77 |  
      2 |       4 | green
    +            10 | 13 | Mexico      |        1 |    4 | 1973 |         77 |  
      2 |       4 | green
    +            11 | 13 | Mexico      |        1 |    4 | 1973 |         77 |  
      2 |       4 | green
    +             1 | 19 | UK          |        3 |    4 |  245 |         56 |  
      1 |       3 | red
    +             3 |  5 | Canada      |        1 |    4 | 9976 |         24 |  
      1 |       2 | red
    +             2 | 15 | Portugal    |        3 |    4 |   92 |         10 |  
      6 |       5 | red
    +             8 | 20 | USA         |        1 |    4 | 9363 |        231 |  
      1 |       3 | white
    +             7 | 20 | USA         |        1 |    4 | 9363 |        231 |  
      1 |       3 | white
    +             9 | 10 | Ireland     |        3 |    4 |   70 |          3 |  
      1 |       3 | white
    +(23 rows)
    +</pre>
    +
    +-# Grouping can be used with class size specification as well. Note below 
that
    +'blue=<Integer>' is the only valid class value since 'blue' is the only 
class
    +value that is present in each group.
    +<pre class="syntax">
    +DROP TABLE IF EXISTS output_table;
    +SELECT madlib.balance_sample(
    +    'flags',          -- Source table
    +    'output_table',   -- Output table
    +    'mainhue',        -- Class column
    +    'blue=2',         -- Specified class value size. Rest of the values 
are sampled as is.
    --- End diff --
    
    Do you mean 
    “Rest of the values are left as is.”
    since they are not actually sampled. ?


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