Keeping Your Data Clean: Strategies for Duplicate Removal in PostgreSQL

This method identifies duplicate rows using a subquery and then deletes them in the main query. Here's how it works:

• We define a subquery that assigns a row number to each row based on specific columns. This helps identify duplicates.
• The subquery filters for rows with a row number greater than 1 (since the first occurrence wouldn't be a duplicate).
• The main DELETE statement uses the subquery to target these duplicate rows based on their IDs and deletes them from the original table.

Example:

DELETE FROM your_table
WHERE id IN (
  SELECT id
  FROM (
    SELECT id, ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY id) AS rnum
    FROM your_table
  ) AS t
  WHERE t.rnum > 1
);

Using DISTINCT with INSERT:

This approach involves creating a new table and inserting only distinct rows from the original table.

• We define a new table with the same structure as the original table.
• We use an INSERT statement with the DISTINCT keyword to insert only unique rows from the original table into the new table.
• Once the new table is populated with distinct data, you can drop the original table and rename the new table (if desired).

CREATE TABLE unique_data (
  -- Define columns with same data types as your_table
);

INSERT DISTINCT INTO unique_data
SELECT *
FROM your_table;

DROP TABLE your_table;

RENAME TABLE unique_data TO your_table;

Choosing the right method:

For small tables, both methods work well. The subquery approach might be slightly faster for very small tables. However, if you plan to keep the original table structure, the DISTINCT with INSERT approach is a good option.

Additional considerations:

• These examples assume you want to identify duplicates based on all columns in the table. If you only care about specific columns, modify the PARTITION BY clause in the subquery to reflect those columns.
• It's always a good practice to back up your table before running any data manipulation queries.

DELETE FROM your_table a
USING (
  SELECT MIN(id) AS keep_id, column1, column2
  FROM your_table
  GROUP BY column1, column2
  HAVING COUNT(*) > 1
) AS b
WHERE a.id = b.keep_id AND a.column1 = b.column1 AND a.column2 = b.column2;

This version keeps the first occurrence of each duplicate group by finding the minimum id within each group.

Using DISTINCT with INSERT (Maintaining original table structure):

CREATE TABLE unique_data LIKE your_table;  -- Inherits structure from original table

INSERT DISTINCT INTO unique_data
SELECT *
FROM your_table;

ALTER TABLE your_table DISABLE TRIGGERS;  -- Disable triggers (optional, for efficiency)
TRUNCATE TABLE your_table;               -- Faster than DROP + CREATE

ALTER TABLE your_table ENABLE TRIGGERS;  -- Re-enable triggers (optional)

DROP TABLE IF EXISTS original_your_table;  -- Drop old table (optional)

RENAME TABLE unique_data TO your_table;

The ctid is a system-generated column unique to each row in the table. It represents the physical location of the row on disk. This method leverages ctid to identify and delete duplicates efficiently for small tables.

WITH duplicates AS (
  SELECT *
  FROM your_table
  GROUP BY ctid HAVING COUNT(*) > 1
)
DELETE FROM your_table
USING duplicates;

This approach uses a Common Table Expression (CTE) named duplicates to identify rows with the same ctid (meaning duplicates). The DELETE statement then uses this CTE to target and delete those duplicate rows from the original table.

Important Note: This method relies on the physical layout of the table and might not be ideal for frequently updated tables where row positions change.

Employing ROW_NUMBER() with DELETE FROM ... WHERE EXISTS:

This method utilizes ROW_NUMBER() to assign a row number based on a specific order. Then, it identifies and deletes duplicate rows using a combination of DELETE FROM ... WHERE EXISTS.

DELETE FROM your_table a
WHERE EXISTS (
  SELECT 1
  FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY id) AS rnum
    FROM your_table
  ) AS b
  WHERE a.id = b.id AND b.rnum > 1
);

Here, a subquery assigns a row number (rnum) to each row based on column1 and column2 (modify as needed). The DELETE statement checks if a matching row with rnum greater than 1 exists in the subquery for each row in the original table. If a duplicate is found, it gets deleted.