How can PostgreSQL retrieve the final row in every sequence of identical column values (interchange)?

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There are different ways to retrieve the final row in every sequence of identical column values in PostgreSQL, but one common approach is to use a combination of window functions and subqueries.

Here is an example query that demonstrates this approach:

WITH sequences AS (
  SELECT *,
         row_number() OVER (PARTITION BY column1 ORDER BY column2) AS rn1,
         row_number() OVER (PARTITION BY column1 ORDER BY column2 DESC) AS rn2
  FROM mytable
)
SELECT *
FROM sequences
WHERE rn1 = 1 AND rn2 = 1;

Explanation:

1. We define a Common Table Expression (CTE) named sequences that contains the original table plus two additional columns based on window functions. The first row_number() function assigns a sequential number to each row within a group of identical values in column1, ordered by column2. The second row_number() function does the same, but in descending order of column2.

2. We use the sequences CTE in the main query and filter the results to only include rows where both rn1 and rn2 are equal to 1. This means that the row is the first and last row in its group of identical values in column1. These rows correspond to the final row in every sequence of identical column values or interchange.

By tweaking the ORDER BY clause in the window functions, you can change the criteria for identifying sequences and final rows. Additionally, you may need to adjust the column names and table references in the query to match your specific scenario.