chore: import upstream snapshot with attribution

testing/go/regression/tests/select_distinct_on.sql

@@ -0,0 +1,19 @@
+--
+-- SELECT_DISTINCT_ON
+--
+
+SELECT DISTINCT ON (string4) string4, two, ten
+   FROM onek
+   ORDER BY string4 using <, two using >, ten using <;
+
+-- this will fail due to conflict of ordering requirements
+SELECT DISTINCT ON (string4, ten) string4, two, ten
+   FROM onek
+   ORDER BY string4 using <, two using <, ten using <;
+
+SELECT DISTINCT ON (string4, ten) string4, ten, two
+   FROM onek
+   ORDER BY string4 using <, ten using >, two using <;
+
+-- bug #5049: early 8.4.x chokes on volatile DISTINCT ON clauses
+select distinct on (1) floor(random()) as r, f1 from int4_tbl order by 1,2;