Github user fmcquillan99 commented on a diff in the pull request:
https://github.com/apache/madlib/pull/239#discussion_r172921328
--- 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)
--- End diff --
For zone 4 there are 12 output rows sampled from 9 input rows. I am
assuming this is just rounding due to small numbers in the example? i.e.,
could have been 8 rows or 12 rows in the output table for uniform sampling for
4 groups (blue, green, red, white) but we went with 12?
Which is fine, just confirming.
---
