Unique Elements and Top-K Frequencies

Describe an algorithm to return the first occurrence of each unique element in a list, and then how you would return the k most frequent elements.

Assume the input is stored in a PostgreSQL table with one row per list element and a stable input order. Write a query that returns the first position for each distinct element, then a second result for the k most frequent elements.

Output

1. For first occurrences: one row per distinct element, with the element value and its first position in the list.
2. For top-k frequency: one row per returned element, with the element value and its frequency.
3. Order first-occurrence results by first position ascending, and top-k results by frequency descending, then first position ascending.