How Can I Retrieve Old Column Values During an SQL UPDATE Operation?

Retrieving Old Column Values During an UPDATE Using Pure SQL

Introduction

When updating rows in a database, it can be useful to access the original values of the updated columns. While SQL offers the RETURNING clause to retrieve post-update values, it does not support directly fetching the old column values. This article explores a workaround to retrieve both new and old column values during an UPDATE operation.

Solution Using a Self-Join

Despite the RETURNING clause limitation, it is possible to access the original column values by joining the table to itself. This technique eliminates the need for triggers or stored procedures.

UPDATE my_table
SET processing_by = our_id_info  -- unique to this worker
FROM my_table AS old_my_table
WHERE trans_nbr IN (
  SELECT trans_nbr
  FROM my_table
  GROUP BY trans_nbr
  HAVING COUNT(*) > 1
  LIMIT our_limit_to_have_single_process_grab
)
AND my_table.row_id = old_my_table.row_id
RETURNING my_table.row_id, my_table.processing_by, old_my_table.processing_by AS old_processing_by;

This query performs the following steps:

1. Filters and identifies rows to be updated using a subquery.
2. Joins the table to itself using an alias (old_my_table) to access the original column values.
3. Sets processing_by to our_id_info and joins only on matching row IDs.
4. Returns the row ID, new processing_by value, and old processing_by value.

Avoiding Concurrent Write Issues

While the self-join solution works well without concurrent write load, it introduces concurrency issues if multiple workers attempt to update the same rows. To address this:

Serializable Isolation Level

Serializing transactions with the SERIALIZABLE isolation level ensures that no other transactions can access data during an operation. However, this can impact performance.

Explicit Locking

Locking the specific row to be updated using the FOR UPDATE clause prevents other transactions from modifying it until the operation completes. This ensures data integrity while maintaining performance.

UPDATE tbl x
SET tbl_id = 23
, name = 'New Guy'
FROM (SELECT tbl_id, name FROM tbl WHERE tbl_id = 3 FOR UPDATE) y
WHERE x.tbl_id = y.tbl_id
RETURNING y.tbl_id AS old_id, y.name AS old_name
, x.tbl_id, x.name;

Conclusion

By using a self-join and applying concurrency control mechanisms, it is possible to retrieve both old and new column values during an UPDATE operation using pure SQL. This approach provides flexibility and control in handling data modifications in database systems.

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